2.8 Traffic signal-control shall be used on the through route conflict points on a through-about (as illustrated in Figure 2.8a) or double-through-about (as illustrated in Figure 2.8b).

NOTE 1 Traffic signal-control on the through route conflict points on a through-about or double-through-about is the minimum requirement, additional signal-control can be added.

NOTE 2 A through-about junction is less efficient in handling turning movements than a roundabout.

NOTE 3 The benefit of a through-about junction is that major traffic movements are removed from some of the conflicts on the circulatory carriageway and this can provide increased capacity.

NOTE 4 A double-through-about is useful if the dominant flows are the two straight ahead movements, reducing conflict on the circulatory carriageway.

NOTE 5 Through routes on a through-about can have signal-controlled junction, highway link and roundabout design elements, as well as advanced signal technology, installed to control approach speeds and optimise capacity. Therefore, an ‘aspect not covered’ departure is necessary to be submitted to the Overseeing Organisation for any proposed through route so that all design elements can be considered holistically.

2.8.1 Through-about and double-through-about junctions should have clear directional signage, that includes directions that right turning traffic needs to be in the left-hand lane.

NOTE 1 Map type signing helps illustrate correct routing through through-about junctions.

NOTE 2 Map type signing for a through-about junction necessitates non-prescribed sign authorisation by the relevant national authority. Information regarding this process can be obtained from the Overseeing Organisation.

3.8 A roundabout shall provide space for the turning movements of the design vehicle in accordance with Table 3.8.

NOTE The design vehicle for Table 3.8 and Figure 3.8.1N1 is an articulated vehicle with a single axle at the rear of the trailer, of length 15.5 metres.

3.8.1 An overrun area may be necessary (Figure 3.8.1N1) to provide sufficient entry deflection for vehicles at compact or smaller normal roundabouts while still allowing large vehicles to circulate.

NOTE 1 An overrun area for a compact or smaller normal roundabout is illustrated in Figure 3.8.1N1, where:

1. a, is the main central island;
2. b, is the central overrun area (where provided);
3. c, is the remaining circulatory carriageway width (1.0 to 1.2 times the maximum entry width);
4. d, is the vehicle;
5. e, is the 1 metre minimum clearance from the edge of kerbing (provided on both the inside and the outside of the circulatory carriageway);
6. f, is the ICD;
7. R1, is the radius from the centre of the roundabout to the outside of the inner 1 metre clearance (e) (values for R1 can be found in Table 3.8.1N1); and
8. R2, is the radius from the centre of the roundabout to the inside of the outer 1 metre clearance (e) (values for R2 can be found in Table 3.8.1N1).

NOTE 2 In urban areas, providing adequate turning space for long vehicles can be an important consideration.

3.8.2 The swept path for the design vehicle for Figure 3.8.1N1 may impinge by up to 0.3 metres into either the inner or outer 1-metre clearance allowance ('e', as shown on Figure 3.8.1N1) of the central island where there are constraints.

NOTE Given the anticipated low frequency of the design vehicle for Figure 3.8.1N1, the impingement into the inner and outer clearance is not particularly significant and the dimensions in Table 3.8 need not be increased accordingly.

3.9 The profile dimensions for an overrun area must be in accordance with The Highways (Traffic Calming) Regulations UKSI 1999/1026 [Ref 8.N].

NOTE Advice on the use of overrun areas is provided in TAL 12/93 [Ref 10.N].

3.9.1 The compact and normal roundabout overrun area should be capable of being mounted by the trailers of a HGV, but be unattractive to cars e.g. by having a slope and/or a textured surface.

3.9.2 Overrun areas should not be used adjacent to walkers, cyclists and horse-riders (WCHR) crossings.

NOTE WCHR waiting within an overrun area puts them at risk of being struck by passing vehicles.

3.9.3 Where an overrun area is used adjacent to a pedestrian crossing, the overrun areas should not resemble footways or refuges in order to discourage pedestrians utilising it to cross the carriageway.

3.11 The entry width shall be measured from point A at the right-hand end of the give way line along the normal to the nearside kerb where there are no road markings or hatching alongside the kerbs, as shown in Figure 3.11.

NOTE 1 The entry width is the width of the carriageway at the point of entry.

NOTE 2 For capacity assessment, the measurement is taken as the total width of the lanes which drivers are likely to use.

NOTE 3 Entry width and sharpness of flare are the most important determinants of capacity, whereas entry deflection is the most important factor for safety as it governs the speed of vehicles through the roundabout.

NOTE 4 Advice on calculating the capacity of the roundabout is provided in Appendix B.

3.11.1 Where there is white edge lining or hatching the measurement should be taken between the edges of the markings closest to the running lanes rather than kerb to kerb.

3.12 On a single carriageway approach to a normal roundabout, the entry width shall not exceed 10.5 metres.

3.12.1 On a single-carriageway road, where predicted flows are low and increased lane width is not operationally necessary, a compact roundabout with single lane entries should be used.

NOTE The use of single lane entries can result in entry closures during planned maintenance and therefore be subject to an agreed traffic management plan with the Overseeing Organisation.

3.13 On a dual carriageway approach to a normal roundabout, the entry width shall not exceed 15 metres.

3.14 Lane widths at the give way line for normal and compact roundabouts shall be no less than 3 metres and no greater than 4.5 metres.

3.14.1 At the give way line, a lane width value of 4.5 metres should be used at single lane entries.

3.14.2 At the give way line, lane width values of between 3 metres and 3.5 metres should be used at multi-lane entries.

NOTE The use of lane bifurcation where one lane widens into two can maximise the use of the entry width and can reduce the impact of drivers having a tendency to use the nearside lane when entering roundabouts.

3.14.3 Vehicle swept paths should be assessed using the largest vehicle that is anticipated to use each entry lane.

3.14.4 Vehicle swept paths should be assessed on multi-lane entries to ensure sufficient width is provided for each entry lane.

3.14.5 No more than two lanes should be added to the number of upstream lanes on the entry to a roundabout.

3.14.6 No entry should be more than four lanes wide.

3.14.7 Where entry flaring is provided, lane markings indicating the tapered lanes should start from a point where both lanes have a minimum width of 3 metres.

3.14.8 Where an existing approach arm is being modified and heavy goods vehicles and bus usage is infrequent, the minimum width of the lanes at the start of the tapered lanes at entry flaring may be reduced to 2.5 metres.

NOTE The choice of minimum width influences the length of segregated lanes on the approach to the roundabout and this can potentially impact the operational performance of the approach arm at peak times.

3.14.9 When developing additional entry lanes, short offside lanes should not be used.

NOTE Road users tend to infrequently use short offside lanes. This can result in debris collecting on the road surface, which forms a safety hazard, particularly for two-wheeled vehicles.

3.15 Hatching on the entry to reduce the entry width shall not be used in the controlled area of a zebra or signal-controlled crossing.

3.15.1 Hatching should not be used to reduce the entry width in areas adjacent to pedestrian crossing points.

3.16 The approach half width shall be measured as the width of the approach carriageway, excluding any hatching, in advance of any entry flare, as shown in Figure 3.16.

NOTE The approach half width is the shortest distance between the median line, or the edge of the central reserve on dual carriageway roads, and the nearside edge of the road.

3.16.1 Where there is white edge lining or hatching, the measurement of approach half width should be taken between markings rather than kerb to kerb.

3.17 Entry flaring shall be measured using the average effective flare length, l' as shown on Figure 3.17.

NOTE 1 The average effective flare length, l', is the average length over which the entry widens, and is the length of the curve CF'.

NOTE 2 Normal roundabouts usually have flared entries to accommodate one or two additional lanes at the give way line to increase capacity.

NOTE 3 The average effective flare length, l' (as illustrated in Figure 3.17) is determined following the below methodology:

NOTE 4 The total length of the entry widening (BG) can be about twice the average effective flare length, l'.

NOTE 5 The approach outlined in 3.17 follows that used in TRL LR 942 [Ref 15.I].

3.17.1 A minimum average effective flare length of 5 metres in urban areas and 25 metres in rural areas should be used, but capacity can be the determining factor for the actual length.

NOTE 1 Average effective flare lengths greater than 25 metres can improve the geometric layout but have little effect in increasing capacity.

NOTE 2 Where the average effective flare length exceeds 100 metres, the design becomes one of link widening. Requirements and advice for link design are provided in CD 109 [Ref 4.N].

NOTE 3 Single-lane entries e.g. those at compact roundabouts, can be slightly flared to accommodate HGVs - even a small increase in entry width can increase capacity.

3.17.2 Where the design speed is high, entry widening should be developed gradually with no sudden changes in direction.

NOTE 1 The sharpness of flare, S, is a measure of the rate at which extra width is developed in the entry flare.

NOTE 2 The sharpness of flare, S, is defined by the relationship:

NOTE 3 Values of S greater than unity (S > 1) correspond to sharp flares and smaller values (0 ≤ S ≤ 1) to gradual flares.

3.18 The entry angle for roundabouts shall be measured as the conflict angle, , between:

NOTE 1 Further advice on the methods of measuring entry angle is provided in Appendix A.

NOTE 2 For a normal roundabout, where the arms are well separated (as illustrated in Figure 3.18N2), the entry angle is determined as the angle between the projected path of an entering vehicle and the path of a circulating vehicle, using the below methodology:

1. construct the curve EF as the locus of the midpoint between the nearside kerb and the median line (or the edge of any traffic island or central reserve);
2. construct BC as the tangent to EF at the give way line;
3. construct the curve AD as the locus of the midpoint of (the used section of) the circulatory carriageway (a proxy for the average direction of travel for traffic circulating past the arm);
4. the entry angle, , is measured as the acute angle between BC and the tangent to AD.

NOTE 3 For a compact roundabout or normal roundabout, where the arms are close together (as illustrated in Figure 3.18N3), the entry angle is determined as the angle between the projected path of an entering vehicle and the projected path of an exiting vehicle, using the methodology below:

1. construct line BC as in Figure 3.18N3;
2. construct the curve JK in the next exit as the locus of points midway between the nearside kerb and the median line (or the edge of any traffic island or central reserve);
3. construct the line GH as the equivalent of line BC i.e. the tangent to the curve JK at the point where JK intersects the border of the inscribed circle;
4. the lines BC and GH intersect at L;
5. the entry angle, , is half of angle HLB. ( = [angle HLB]/2 , Note that if angle GLB exceeds 180 degrees, is defined as zero.)

NOTE 4 On a compact roundabout or normal roundabout, where the arms are close together, there can be insufficient separation between entry and adjacent exit to be able to define the path of the circulating vehicle clearly. In this case, circulating traffic which leaves at the following exit can be influenced by the angle at which that arm joins the roundabout.

3.18.1 The entry angle should be no less than 20 degrees and no greater than 60 degrees for normal and compact roundabouts.

NOTE 1 Small entry angles force drivers to look over their shoulders or use their mirrors to gauge a suitable gap to allow them to join the circulating traffic.

NOTE 2 Large entry angles tend to have lower capacity and can produce excessive entry deflection which can lead to sharp braking at entries, accompanied by shunt accidents, especially when approach speeds are high.

3.18.2 Except on compact roundabouts in urban areas, the kerb line of the traffic island (or central reserve in the case of a dual carriageway) should lie on an arc which, when projected forward, meets the central island tangentially (see Figure 3.18.2).

NOTE Aligning the kerb line with the central island as shown on Figure 3.18.2 reduces the likelihood of vehicle paths overlapping.

3.19 The entry kerb radius shall be measured as the minimum radius of curvature of the nearside kerb line over the distance from 25 metres upstream of the give way line to 10 metres downstream of it (see Figure 3.19).

NOTE The entry kerb radius is the radius of the best fit circular curve measured over a length of 25 metres within a 35- metre zone.

3.19.1 The entry kerb radius should not be less than 10 metres.

3.19.2 The entry kerb radius should not be greater than 100 metres.

NOTE 1 An entry kerb radius of greater than 100 metres tends to result in inadequate entry deflection.

NOTE 2 Although entry capacity can be increased by increasing the entry kerb radius, once its value reaches 20 metres, further increases only result in very small capacity improvements. Reducing the entry kerb radius below 15 metre reduces capacity.

3.19.3 Except at compact roundabouts, if the approach is intended for regular use by HGVs, the entry kerb radii should not be less than 20 metres.

3.20 The entry path radius for an ahead movement at a 4-arm roundabout shall be determined as shown on Figure 3.20.

NOTE 1 The entry path radius (shown as 'a' on Figures 3.20, 3.22 and 3.23) is measured as the smallest best fit circular curve over a distance of 25 metres occurring along the approach entry path in the vicinity of the give way line, but not more than 50 metres in advance of it.

NOTE 2 The commencement point (shown as 'b' on Figures 3.20, 3.22 and 3.23) is 50 metres in advance of the give way line and at least 1 metre from the nearside kerb or centre line (or edge of central reserve).

NOTE 3 Further advice on constructing the entry path radius is provided in Appendix A.

3.21 The entry path radius (or its inverse, entry path curvature) shall be measured for all turning movements at a roundabout, except at a through-about approach arm for the through route.

NOTE The entry path radius is a measure of the deflection to the left imposed on vehicles entering a roundabout. It is the most important determinant of safety at roundabouts because it governs the speed of vehicles through the junction and whether drivers are likely to give way to circulating vehicles.

3.21.1 Where there is a turning movement at a through-about approach arm for the through route, entry deflection should be provided to turning vehicles, such that the entry angle is no less than 20 degrees and no greater than 60 degrees.

3.22 The entry path radius for the left-turn movement where the approach to the roundabout curves to the left shall be determined as shown on Figure 3.22.

3.23 The entry path radius for the left-turn movement where the approach to the roundabout curves to the right shall be determined as shown on Figure 3.23.

3.24 At compact roundabouts in urban areas, where the speed limit is 40 mph or less within 100 metres of the give way line on any approach, the entry path radius shall not exceed 70 metres.

NOTE On roads with a speed limit of 40 mph or less within 100 metres of the give way line on all approaches, compact roundabouts can have low values of entry and exit radii in conjunction with high values of entry deflection.

3.25 At compact roundabouts where the speed limit is 50 mph or greater within 100 metres of the give way line on any approach, the entry path radius shall not exceed 100 metres.

3.26 At normal roundabouts, the entry path radius shall not exceed 100 metres.

3.26.1 In order to ensure that the entry path radius provides suitable deflection, the arms may be staggered as shown in Figure 3.26.1.

NOTE 1 Staggering the arms as shown in Figure 3.26.1 can:

NOTE 2 In advance of the entry flare, approach curvature follows CD 109 [Ref 4.N] requirements on horizontal radii.

3.26.2 On normal and compact roundabouts, where sufficient entry deflection cannot be achieved by means of the central island alone, deflection should be generated by enlarging traffic islands or by providing a central overrun area for HGVs.

3.26.3 On existing normal and compact roundabouts, subsidiary deflection islands (SDI) may be used:

NOTE Requirements and advice on the provision of a subsidiary deflection are provided in Section 7, "Design of subsidiary deflection islands".

3.26.4 On normal and compact roundabouts, where an overrun area is provided, the entry path radius should be measured relative to the perimeter of this area rather than that of the central island.

3.27 Right pointing arrows on lane dedication signs or as markings on the road shall not be used on normal and compact roundabout approaches.

NOTE 1 Preventing the use of right pointing arrows on lane dedication signs or as road markings is to avoid confusing drivers, particularly those from overseas, over which way to proceed around the roundabout.

NOTE 2 Further guidance on the use of road markings is provided in Appendix D.

3.27.1 Where a right-hand lane is dedicated to a specific destination, it should be associated with an ahead arrow on the approach.

3.27.2 A right pointing arrow may be used on the circulatory carriageway.

3.27.3 Where any particular lane is dedicated to a specific destination the other lanes should also have arrow markings.

3.27.4 Where any particular lane is dedicated to a specific destination the road markings should be accompanied by direction signing indicating lane dedication.

3.27.5 Left turn arrows should be avoided on the circulatory carriageway.

NOTE Left turn arrows can sometimes lead to drivers mistakenly turning into roundabout entries.

3.27.6 Where lane direction markings have been, or are to be used on the approaches of a particular arm, then the direction markings within the entry lanes should be an extension of those markings in a logical and consistent manner, using the same designation system as those upstream (Figures 3.27.6a and 3.27.6b).

3.27.7 Where no approach markings have been provided, then the entry markings should be designed to give an even balance of any queuing traffic over the entry lanes whilst providing a smooth path onto the roundabout.

NOTE Arrow markings and route destinations can be particularly beneficial for larger, more complex roundabouts, especially those that have more than four entry/exit arms.

3.27.8 Approach lane markings should be positioned in advance of the give way line in a location where they are not obscured by queuing vehicles, and in a manner which balances the traffic between the approach lanes.

NOTE 1 Approach lane markings near to the give way line can result in drivers switching lanes too close to the junction.

NOTE 2 Further guidance is provided in TSM Chapter 5 [Ref 14.N].

3.27.9 Dedicated lane signs and associated road markings should be used on the approach to a signal-controlled roundabout where a single lane divides into separate lanes.

NOTE Dedicated lane signs are not appropriate for use where a lane is shared use, for instance where a lane is used for ahead and turning traffic.

3.28 On normal and compact roundabouts, the exit width shall be measured as the distance between the nearside kerb and the edge of the traffic island (or central reserve of a dual carriageway) where it intersects with the outer edge of the circulatory carriageway, as shown on Figure 3.28.

NOTE As with entry width, exit width is measured normal to the nearside kerb. Values are typically similar to or slightly less than entry widths (exits have less flaring).

3.28.1 The exit width for normal roundabouts should accommodate one more traffic lane than is present on the link downstream.

3.28.2 At a normal roundabout, if the downstream link is a single carriageway road, the exit width should be between 7 metres and 7.5 metres and the exit should taper down to a minimum of 6 metres.

NOTE The additional width allows traffic to pass a broken down vehicle.

3.28.3 Where the downstream link is a single carriageway road, the exit width should reduce at a taper of 1:15 to 1:20, starting at the end of the exit from the roundabout, ensuring 6 metres at end of traffic island, to avoid exiting vehicles encroaching onto the opposing lane at the end of the traffic island.

NOTE The exit width taper of 1:15 to 1:20 is for use on tapers on exit from the roundabout, not at locations where a differential acceleration lane (DAL) has been added.

3.28.4 At a normal roundabout, if the downstream link is an all-purpose two-lane dual carriageway, the exit width should be between 10 metres and 11 metres, with the exit tapering down to two lanes wide.

3.28.5 For non-signal-controlled roundabouts, vehicle swept paths should be assessed using the largest vehicle anticipated to use each exit lane.

3.28.6 Where traffic is required to merge after exiting, sufficient distance should be provided from the exit to allow the merging manoeuvre to take place in a safe and efficient manner.

3.28.7 Any exit line markings associated with the concentric-spiral, or spiral type of markings should be designed so as to provide a smooth exit from the circulatory carriageway.

NOTE 1 For smaller roundabouts, the use of lane direction arrows, route numbers and destinations at exits can confuse drivers and clutter the circulatory carriageway.

NOTE 2 For larger roundabouts, markings can be useful where a driver in a circulatory lane is presented with the choice of either exiting the roundabout, or continuing to circulate.

NOTE 3 Further advice on concentric-spiral and spiral markings is provided in Appendix D.

3.28.8 Where the peak exit volume approaches the capacity of the downstream link, tapers longer than 1:20 may be provided.

NOTE A taper longer than 1:20 can help merge the traffic where the density in each lane is high.

3.28.9 Where the exit is on an up gradient, the exit width may be maintained for a short distance before tapering in.

3.28.10 Where the exit road is on an up gradient combined with an alignment which bends to the left, the exit width may be maintained over a longer distance.

NOTE Maintaining the exit width can help drivers to overtake slower moving vehicles and Differential Acceleration Lanes (DALs) can provide further overtaking opportunities on exit arms with higher traffic flows.

3.28.11 At a compact roundabout, the exit width should be similar to the entry width.

3.29 The exit kerb radius shall be measured as shown in Figure 3.28.

3.29.1 At normal roundabouts the exit kerb radius should exceed the largest entry radius.

3.29.2 At normal roundabouts, the exit kerb radius should be 40 metres.

3.29.3 Where an exit kerb radius of 40 metres cannot be achieved, the exit kerb radius should be no less than 20 metres and no greater than 100 metres.

NOTE The shortest distance possible between an entry arm and the next exit is governed by the minimum entry radius and the minimum exit radius for the type of roundabout in question.

3.29.4 A higher exit kerb radius may be used on normal roundabouts with larger ICDs on high speed roads to suit the overall junction geometry.

NOTE A compound curve starting with a 40 metre radius and developing to a larger radius, of up to 100 metres, can be used.

3.29.5 At compact roundabouts the exit kerb radius should be equal to the largest entry radius.

3.29.6 At a compact roundabout, the value of the exit kerb radius should be no less than 15 metres and no greater than 20 metres.

3.29.7 Larger values of exit radius, which lead to high exit speeds, should not be located where there are significant numbers of cyclists using the junction or where pedestrian crossing facilities are located immediately downstream.

3.29.8 Exits should be checked to ensure that vehicle paths are smooth and vehicles are not directed towards traffic islands.

NOTE Sharp turns into exits can increase the likelihood of load shedding by HGVs and decrease the traffic capacity of the junction.

3.29.9 On an exit, traffic islands should end at a tangent (or, at least, parallel) to the centre line and be long enough to prevent an exiting vehicle from crossing the centre line into oncoming traffic.

3.43 On a 7.3 metres wide dual carriageway, SSD shall be measured to the position of an object at the give way line (5.5 metres from the traffic island) as shown on Figure 3.43.

3.44 On a 10 metre wide single carriageway, SSD shall be measured to the position of an object at the give way line (5.5 metres from the edge of the traffic island) as shown on Figure 3.44.

3.45 On a 7.3 metre wide single carriageway, SSD shall be measured to the position of an object at the give way line (3.65 metres from the edge of the traffic island) as shown on Figure 3.45.

3.46 Visibility on the approach, 'a', to the roundabout shall conform to CD 109 [Ref 4.N].

NOTE 1 On high speed dual carriageway approaches, the provision of transverse yellow bar markings can reduce rear shunt and overshoot accidents by helping to alert the driver to the presence of the roundabout. On high speed single carriageway roads on which drivers fail to adjust their speed in time to negotiate the roundabout safely or to stop, the provision of ‘Reduce Speed Now’ signs can have a similar effect. Transverse yellow bar markings are only to be used in certain circumstances, refer to TSM Chapter 5 [Ref 14.N] and TRL LR 1010 [Ref 21.I] for further guidance.

NOTE 2 Visibility on the approach, 'a' shown on Figures 3.43, 3.44 and 3.45, is the desirable minimum SSD for the design speed of the road.

NOTE 3 The visibility on the approach is measured from a vehicle position in the centre of the nearside lane, measured from the centre of the lane as shown on Figures 3.43, 3.44 and 3.45.

3.47 Where chevron signs are located on the central island, they shall be visible to approaching drivers in all lanes from a distance equal to the desirable minimum SSD measured back along the approach lanes from the give way line.

NOTE The desirable minimum SSD is measured back from the give way line as this is the point at or before which road users need to be able to reduce speed.

3.48 Where chevron signs are used, the signs shall not be stacked.

3.48.1 Where the chevron signs are inconspicuous, yellow backing boards or larger signs should be used.

3.48.2 Where the approach to the roundabout is over a crest, a higher sign mounting height may be used.

NOTE Chevron signs sometimes can impinge on circulatory visibility but the effects can be minimised by positioning the signs 2 metres back from the central island kerb line (further guidance regarding the positioning of signs is provided in TSM Chapter 4 [Ref 13.N]).

3.49 Drivers of all vehicles approaching the roundabout shall be able to see objects of height between 0.26 metres and 2 metres on the full width of the circulatory carriageway, from the centre of the nearside lane at a distance of 15 metres back from the give way line, for the visibility distance as shown in Table 3.49.

NOTE The visibility distance given in Table 3.49 is measured along the centre of the circulatory carriageway as shown in Figure 3.49N.

3.50 For visibility to the right, the envelope of visibility shall be obtainable from a driver’s eye height of between 1.05 metres and 2 metres to an object height of between 1.05 metres and 2 metres.

3.51 Drivers of all vehicles approaching the roundabout shall be able to see the full width of the circulatory carriageway to their right, from the centre of the offside lane at the give way line, for the visibility distance provided in Table 3.49 and as shown in Figure 3.51.

NOTE 1 The requirement for visibility to the right includes roundabouts with bridge parapets on either side of the circulatory carriageway.

NOTE 2 Where entry problems are caused by poor visibility to the right, visibility can be improved by extending the traffic island to narrow the circulatory carriageway and moving the give way line forward.

3.51.1 To reduce excessive approach speeds on dual carriageway approaches, visibility to the right may be limited by screening the vehicle until it is within 15 metres of the give way line.

NOTE Excessive visibility to the right can result in high entry speeds, potentially leading to overshoot accidents and to accidents for single vehicles and PTWs. Further accident mitigation can be provided by signing and marking, and by ensuring that the layout guides drivers around the central island.

3.51.2 Screening provided to reduce the visibility to the right should be at least 2 metres high in order to block the view of all road users.

NOTE Screening can be used on flared approaches on high speed single carriageway roads where there is a long traffic island.

3.52 Visibility to the right shall conform to Table 3.49 and be measured from the centre of the offside lane at a distance of 15 metres back from the give way line, as shown in Figure 3.52.

3.53 For circulatory visibility on non-signal-controlled and part-time signal-controlled roundabouts, the envelope of visibility shall be obtainable from a driver’s eye height of between 1.05 metres and 2 metres to an object height of between 1.05 metres and 2 metres.

3.54 This visibility shall be checked at a distance of 2 metres in from the central island, as shown in Figure 3.54.

3.55 Drivers on the circulatory carriageway of non-signal-controlled and part-time signal-controlled roundabouts shall be able to see the full width of the circulatory carriageway ahead of them for the visibility distance given in Table 3.49.

NOTE It is often useful to improve the conspicuousness of central islands by landscaping, as long as circulatory visibility is not obstructed.

3.55.1 At least the outer 2 metres of the central island should be hard standing or planted with grass or similar low-level vegetation to prevent visibility issues occurring.

NOTE 1 Grass or similar low-level vegetation can potentially cause visibility issues if not regularly maintained and road worker risk needs to be considered when using this type of landscaping.

NOTE 2 Further requirements and advice for the landscape design of the central island are provided in LD 117 [Ref 8.I].

3.55.2 On a full-time signal-controlled roundabout, the circulatory visibility requirements for a non-signal-controlled roundabout should be applied.

NOTE Applying the same circulatory visibility for a full-time signal-controlled roundabout to that of a non-signal-controlled roundabout can help the signal-controlled roundabout to operate safely when the signals are not in use.

3.56 On the circulatory carriageway, the exit visibility shall conform to Table 3.49.

NOTE CD 109 [Ref 4.N]provides requirements and advice regarding the SSD once a vehicle has crossed the inscribed circle at the exit from the roundabout.

3.57 Drivers approaching a roundabout with a zebra crossing across the entry, shall be able to see the full width of the crossing from a distance at least equal to the desirable minimum SSD for the design speed of the roundabout approach.

3.58 Drivers approaching a roundabout with a signal-controlled crossing shall be able to see at least one signal head for desirable minimum SSD for the design speed of the roundabout approach.

3.59 At the give way line, drivers shall be able to see the full width of a pedestrian crossing (whether signal-controlled, zebra or informal) across the next exit if it is within 20 metres of the give way line on that arm, as shown on Figure 3.59.

3.59.1 Crossings should not be sited between 20 metres and 60 metres from the give way line.

NOTE Requirements and advice for visibility to crossings for a signalised roundabout are given in CD 123 [Ref 3.N].

4.2 Signal-controlled roundabouts shall be designed incorporating direct and/or indirect signal configuration.

4.2.1 When direct signal-control is used at a roundabout, as illustrated in Figure 4.2.1 for a large signalised roundabout, the signal control may be partial where traffic flows on the minor roads (Arms B and C in the figure) are light and continue to operate in a self-regulating manner under normal priority control.

4.2.2 Where direct signal-control is applied to a roundabout, the effect of the internal queues on the roundabout should be assessed as part of the performance analysis of the roundabout during design.

NOTE 1 Under direct signal-control (as illustrated on Figure 4.2.1), the internal queuing capacity on the circulatory carriageway to the right of Arm E is limited by the width of the central reserve on the dual carriageway approach. Similarly, to the right of Arm F the internal queuing capacity is severely limited by the size of the deflection island.

NOTE 2 Internal approach lanes less than 15 metres in length can result in the blockage of the exit arm, particularly where HGVs are present.

4.2.3 Where a roundabout uses direct signal-control, the capacity on the internal approaches to Arms E and F (as illustrated on Figure 4.2.1) may be improved by geometric modifications to these external approaches by significantly reducing the entry radii.

NOTE Where a roundabout uses direct signal-control, the scope for geometric modifications is reduced due to the requirements for the design to be in accordance with Section 3.

4.2.4 Indirect signal-control may be used where an entry arm with a very heavy traffic flow is preceded by (that is, is to the left of) an entry arm with a very light traffic flow.

NOTE A heavy flow on a single roundabout arm can often proceed virtually uninterrupted, possibly with high circulatory speeds. This causes an unbalanced traffic flow which adversely affects the capacity of the roundabout and can lead to excessive queue lengths (and substantial delays) to other external approaches which in turn can result in congestion at preceding junctions or on slip roads.

4.2.5 The installation of indirect signal-control (as illustrated in Figure 4.2.5) may be introduced to control entry flows on one or more approaches and can provide a gap in the circulatory traffic, to favour those entry arms that were previously subjected to excessive delays and queues.

NOTE Indirect signal-control can balance the capacity of the entry arms, however increases in vehicle gap distances can be detrimental to cyclists and pedestrians crossing the arms.

4.3 When an assessment demonstrates that additional approach lane capacity is needed on a signal-controlled roundabout, the additional lane(s) shall be provided adjacent to either the offside or nearside carriageway edge.

4.3.1 On a signalised roundabout, the provision of an additional offside external approach lane, may be used where:

NOTE Figure 4.3.1N illustrates the provision of an additional offside approach lane.

4.3.2 On signal-controlled roundabout, the provision of an additional nearside external approach lane may be used where:

NOTE 1 Figure 4.3.2N1 illustrates the provision of an additional nearside approach lane.

NOTE 2 CD 123 [Ref 3.N] provides requirements and advice for direct tapers, where they are provided, and storage lengths for additional approach lanes.

4.4 On signal-controlled roundabouts, where dedicated left turn lanes are provided, traffic islands shall be used to separate uncontrolled traffic from controlled traffic, as shown in Figure 4.4.

4.4.1 Traffic islands may be provided in other situations to separate two independently controlled lanes of traffic on the same roundabout entry or on the circulatory carriageway of a grade separated roundabout.

4.5 Give way markings shall not be provided at any entries with direct full-time signal-control.

NOTE Give way markings can be used at any entries where there is indirect part-time or full-time signal-control.

4.6 The design of a signal-controlled roundabout shall allow for the swept paths of the design vehicle on all entry, circulatory and exit lanes, including all through lanes across the central island of a through-about.

NOTE The nature of the signal-controlled roundabout and its associated traffic islands and pedestrian refuges can restrict the movement of vehicles, particularly HGVs.

4.9 All internal and external approaches on roundabouts that operate under signal-control at any time shall be provided with an intervisibility zone which extends across the full carriageway width of each arm from a distance of 2.5 metres back from each stop line.

4.9.1 For signal-controlled roundabouts the junction intervisibility zone on the circulatory carriageway should be measured to a point 2.5 metres beyond the secondary signal, as illustrated on Figure 4.9.1.

4.10 Where an advance stop line (ASL) is provided on a roundabout approach, the intervisibility zone shall be measured from a point 2.5 metres behind the cycle stop line.

NOTE The intervisibility zone is measured from a point 2.5 metres behind the cyclists’ stop line because the cycle reservoir behind the ASL does not create any physical impediment to intervisibility.

4.10.1 Where there is a pedestrian crossing adjacent to a stop line, the junction intervisibility should be extended to ensure that drivers of all vehicles on each entry lane are able to see the full extent of the pedestrian crossing (and its approach).

4.10.2 The junction intervisibility at a pedestrian crossing should include the full width of the strip of tactile paving laid parallel to the edge of carriageway.

NOTE Further requirements and advice on the provision and intervisibility of pedestrian crossings adjacent to a signal-controlled junction are provided in CD 123 [Ref 3.N].

4.11 No substantial fixed obstructions shall be located within the intervisibility zone of new roundabouts.

NOTE Details of what constitutes a substantial fixed obstruction are provided in CD 109 [Ref 4.N].

4.11.1 No substantial fixed obstructions should be located within the intervisibility zone of existing roundabouts.

5.2 The white circle of a mini-roundabout shall have a maximum of 4 metres diameter and positioned using the inside of the swept path of cars.

NOTE A mini-roundabout does not have a kerbed central island. In its place is a flush or domed circular solid white road marking capable of being driven over where unavoidable by large vehicles or where the layout of the junction makes it impractical to do so. The circular marking can be edged with kerbs provided the maximum height above the road surface at the perimeter does not exceed 6 mm.

5.2.1 Where a white circle with a full diameter of 4 metres is not achievable on a mini-roundabout, a white circle with a diameter as large as possible between 1 metre and 4 metres should be provided.

NOTE A larger diameter up to the maximum 4 metres can improve conspicuousness of the central marking.

5.3 Additional circular rings shall not be added around the white circle of a mini-roundabout.

5.4 The centre of the design vehicle path shall be at least 1 metre from kerbs, the perimeter of the white circle, and from any road marking separating opposing traffic.

NOTE Figures 5.4Na and 5.4Nb provide examples of how the design vehicle path and white circle location of a mini-roundabout are determined using swept paths.

5.4.1 The white circle of a mini-roundabout should be sized and located so that drivers of cars are not encouraged to drive on it or pass on the wrong side of it when negotiating the junction.

5.5 For a right-turn design vehicle path on a mini-roundabout, a minimum design vehicle path radius of 6 metres, at the centre of the path, shall be used.

5.5.1 For the right turn minimum design vehicle path radius of 6 metres, the vehicle path should be widened to 3 metres at the apex of the turn.

5.6 The height of the dome of the white circle above the adjacent carriageway must be no greater than 125 mm at its highest point (including construction tolerance) (in accordance with UKSI 2016/362 (TSRGD) [Ref 9.N]Schedule 9 Part 8 Paragraph 4).

NOTE A domed white circle marking can be used to deter light vehicles from overrunning and improve its conspicuousness. The dome can normally be formed from bituminous material, concrete or block paving.

5.6.1 The white circle for a 4-metre diameter marking should be domed to a recommended height at the centre of 100 mm.

5.6.2 For smaller diameter markings the height of the dome should be reduced pro rata (i.e. by 25 mm per metre width of the white circle diameter).

5.6.3 A domed white circle should be avoided for mini-roundabouts regularly overrun by heavy goods vehicles or buses in residential areas.

NOTE The use of a domed white circle can lead to the perception of vibration by residents and discomfort to bus drivers and passengers.

5.6.4 Fire and ambulance services should be consulted about any proposal to introduce a mini-roundabout with a domed white circle.

5.7 The height of the white dome at its perimeter shall not exceed 6mm.

5.8 The design of overrun areas must be in accordance with UKSI 1999/1026 [Ref 8.N].

5.9 The diameter of a mini-roundabout overrun area shall not exceed 7.5 metres, including the white circle.

NOTE An example of a mini-roundabout overrun area is shown in Figure 5.9N.

5.9.1 A concentric overrun area may be used on a mini-roundabout to increase the deflection and it's conspicuousness.

NOTE Light vehicles are not legally obliged to avoid overrun areas in the same way as the white circle of a mini-roundabout and therefore concentric overrun areas cannot be relied upon for the purposes of achieving deflection.

5.10 Additional road markings shall not be placed on or around the edges of a concentric overrun area.

5.10.1 The circulatory arrow markings of a mini-roundabout should be placed on the surrounding circulating area and not on the overrun area.

5.11 Where vehicles can pass on the wrong side of the white circle on a mini-roundabout, a kerbed traffic island shall be provided on the arms of a junction.

5.11.1 Traffic islands may be provided to separate opposing streams of traffic and, where appropriate, to serve one or more of the following purposes:

5.11.2 Islands for separating opposing streams of traffic or deflecting approaching vehicles may be kerbed physical islands or created using road markings prescribed in UKSI 2016/362 (TSRGD) [Ref 9.N].

5.11.3 A kerbed island may be used at an entry to accommodate bollards and supplementary signs.

NOTE Requirements and guidance on the appropriate signage for a mini-roundabout are provided in UKSI 2016/362 (TSRGD) [Ref 9.N] and TSM Chapter 3 [Ref 12.N].

5.11.4 Any sign on a kerbed island should not restrict visibility to the right.

5.11.5 Kerbed islands designed to narrow the carriageway within 40 metres of the give way line of a mini-roundabout may be used as a calming feature to control the speed of approaching traffic.

NOTE Guidance on the use of islands to narrow the carriageway is contained in TAL 7/95 [Ref 18.I].

5.12 A kerbed island shall be positioned at least 0.5 metres clear of any vehicle swept path.

5.13 Solid or raised areas of markings shall not be used at mini-roundabouts, other than for the white circle.

5.14 For a single lane approach on a mini-roundabout, the lane width at the give way line shall be no less than 3 metres and no greater than 4 metres.

5.14.1 For a two lane approach on a mini-roundabout, the minimum lane width at the give way line may be reduced to 2.5 metres, provided heavy goods vehicles and buses do not frequently use the entry.

5.15 At an entry with multiple lanes on a mini-roundabout, no more than one lane shall be marked as being for a given exit arm.

NOTE Markings are provided such that traffic going ahead or turning proceeds in single file for each movement.

5.15.1 Three lane entries should not be used for mini-roundabouts.

NOTE 1 The presence of two or more approach lanes encourages two abreast flow through the mini-roundabout, increasing the number of potential conflicts. Additional signing and marking can be used where entries are divided into multiple lanes to ensure safe and efficient operation.

NOTE 2 Where a three-arm mini-roundabout with single lane approaches replaces a major/minor priority junction, the junction becomes easier to negotiate, as drivers only have to concentrate on one stream of traffic circulating at low speed from their right. However, as the number of arms and/or traffic lanes to the mini-roundabout increases, so does the potential for conflict.

5.16 No more than two lanes shall be provided at an intermediate give way line between double mini-roundabouts.

5.16.1 On a double mini-roundabout, the short link between the two roundabouts should provide space for vehicles waiting at the intermediate give way lines, as illustrated in Figure 5.16.1.

NOTE 1 Where the link between the two roundabouts is not adequately sized, large opposing right-turning movements can lead to gridlock at double mini-roundabouts, particularly if the network is congested.

NOTE 2 The capacity at an intermediate give way line between double mini-roundabouts can be reduced by the effect of the first junction, and a queue at the intermediate give way line can interact with the first junction. Double junctions with short links of only one or two car lengths can be more susceptible to queuing than those with greater separation.

5.17 On mini-roundabouts, the exit width shall be measured as the distance between the nearside kerb and exit median (or the edge of any traffic island) where it intersects with the outer edge of the circulatory carriageway.

5.18 Deflection or other means of slowing vehicles on approach to the give way stop line shall be provided on a mini-roundabout.

NOTE 1 Other means of slowing vehicles include additional signage or narrowing of approach.

NOTE 2 Both the speed and path of a vehicle through a mini-roundabout are important factors in accident causation. Adequate deflection allows the approaching drivers to be aware of the circulatory nature of the junction ahead. Drivers need to be ready to stop if necessary on the approach so it is essential for entry (and circulatory) speeds to be managed by careful design.

NOTE 3 Where vehicle speeds are already low on mini-roundabouts, full entry deflection as required for normal or compact roundabouts is not essential.

NOTE 4 The introduction of some entry deflection on entry to the mini-roundabout helps to induce gyratory movement and increase efficiency.

5.18.1 A lateral shift (see Figure 5.18.3) of 0.8 metres minimum should be provided at entry.

NOTE The value of 0.8 metres for lateral shift corresponds to the minimum width required to accommodate hatched road marking to UKSI 2016/362 (TSRGD) [Ref 9.N] diagram 1040 (Schedule 11 Part 4 Item 23). These markings are used to separate opposing traffic flows and further details can be found in UKSI 2016/362 (TSRGD) [Ref 9.N] (Schedule 11 Part 3) and TSM Chapter 5 [Ref 14.N].

5.18.2 Deflection (or lateral shift) should be introduced on the offside of the approach arm.

5.18.3 For offside shift, the lateral shift should be measured from the centre of the approach road, developed at a rate 1 in 7.5 as shown in Figure 5.18.3 Example A.

5.18.4 Where there are constraints (for example, land restrictions, structural obstructions, environmental features) at a mini-roundabout, an alternative method, known as 'nearside shift' and illustrated in Figure 5.18.3 example B, may be used to develop shift along the nearside carriageway edge.

NOTE Nearside shift can be an effective way of introducing deflection in order to encourage low entry speeds. However, nearside shift can have the effect of deflecting traffic to the right, towards the central island, and is therefore often less effective in inducing a gyratory movement than offside shift. For this reason, nearside shift is deemed to be less desirable than offside shift.

5.18.5 For nearside shift, the lateral shift should be measured from the nearside edge of the approach road, developed at a rate 1 in 12.5, as illustrated in Figure 5.18.3 Example B.

5.18.6 Mandatory give way signs and markings to UKSI 2016/362 (TSRGD) [Ref 9.N] diagram 602 (Schedule 9 Part 2 Item 2), diagram 1003 (Schedule 9 Part 6 Items 3 and 9) and diagram 1023A (Schedule 9 Part 6 Item 4) should only be used on the approach to a three-arm mini-roundabout where there is another entry to the right but none to the left as shown in Figure 5.21 and in accordance with TSM Chapter 3 [Ref 12.N] and TSM Chapter 5 [Ref 14.N].

NOTE The use of give way signs and markings in other situations can confuse drivers as to who has priority and undermines the priority rule established for mini-roundabouts.

5.18.7 Where the lateral shift cannot be achieved or visibility to the right is limited, mandatory give way signs and markings to UKSI 2016/362 (TSRGD) [Ref 9.N] diagram 602 (Schedule 9 Part 2 Item 2), diagram 1003 (Schedule 9 Part 6 Items 3 and 9) and diagram 1023A (Schedule 9 Part 6 Item 4) may be used on the approach.

5.18.8 Where the give way sign is co-located with the mini-roundabout regulatory sign, the give way sign should be uppermost.

NOTE Further guidance on the classification of signs is provided in TSM Chapter 1 [Ref 11.N].

5.18.9 On a mini-roundabout where sufficient entry deflection of vehicle paths is not achieved by road markings, islands and existing kerbs, a reduction in vehicle speeds may be achieved by narrowing the approach.

5.18.10 On a mini-roundabout, overrun areas may be utilised instead of narrowing the approach, if narrowing the approach arm affects the swept path of long vehicles on the nearside of an entry.

5.19 Any vertical deflection for traffic calming at a mini-roundabout shall take the form of a speed table with the following requirements:

5.20 The design of crossfalls and gradients at mini-roundabouts shall not result in ponding of surface water within the roundabout carriageway including on and around the central white circle.

5.20.1 Gullies should not be installed adjacent to the white circle to drain ponding or accumulated run-off.

5.20.2 Where a mini-roundabout is constructed at the location of a former priority junction, channels, which can give the impression of a former priority junction layout, should be eliminated.

NOTE Mini-roundabouts have often been superimposed on the existing carriageway profile with little or no change in level.

5.20.3 Where the carriageway levels are re-profiled, crossfall should be outward sloping to avoid ponding and improve junction conspicuousness.

5.21 A minimum visibility distance ‘D’, as shown in Figure 5.21 and in accordance with Table 5.21, shall be the minimum sight distance required at a distance ‘F’ from the give way line in relation to the approach speed of the arm.

Table 5.21 Minimum visibility distance to the right

85th percentile speed of arm to the right (mph)	'D' distance (metres)
	For a gap acceptance time of two seconds	For a gap acceptance time of three seconds
35	40	55
30	35	50
25	25	40

NOTE 1 ‘D’ is measured from the centre of the offside approach lane to the nearside carriageway edge of the arm to the right.

NOTE 2 Distance ‘D’ varies with the 85th percentile approach speed 70 metres before the give way line on the arm to the right and the ‘gap acceptance time’.

NOTE 3 The 'gap acceptance time' is dependent on the size of the roundabout, it is two seconds when the distance from the give way line to the centre of the white circle is 7.0 metres or less, otherwise it is three seconds.

5.22 The visibility distance ‘D’ shall be unobstructed between driver’s eye heights of 1.05 metres and 2.0 metres at the centre of the offside approach lane to object heights between 0.26 metres and 2.0 metres at the nearside edge of the arm to the right.

5.23 The SSD on the approach to a mini-roundabout (illustrated as ‘E’ in Figure 5.21) shall be provided in accordance with Table 5.23.

5.24 The SSD on the approach to a mini-roundabout shall be provided within the whole of an envelope between eye heights of 1.05 metres and 2.0 metres at the centre of the path of an approaching vehicle to object heights of 0.26 metres to 2.0 metres at the give way line.

5.25 The minimum ‘F’ distance in Figure 5.21 shall be 9.0 metres, except in the following circumstances:

NOTE 1 A minimum 'F' distance of 9.0 metres is provided so that the first two vehicles in the approach queue have visibility of traffic coming from the arm on the right.

NOTE 2 'F' distances significantly greater than 9.0 metres can result in high approach speeds. Consider limiting the visibility to the right of adjacent entries to a maximum ‘F’ distance of 15 metres back on the approach and to no more than the ‘D’ distance.

NOTE 3 Excessive visibility between adjacent entries can result in approach and entry speeds greater than desirable for the junction geometry, with a tendency for approaching drivers to take a decision too early about whether to give way, particularly in locations with low turning movements. Road users approaching a mini-roundabout need to be able to stop if vehicles are circulating or if there is an obstruction on the junction. There is little or no advantage in increasing the ‘D’ distance as this could lead to excessive approach speeds.

NOTE 4 An 'F' dimension of 2.4 metres enables a road user who has reached the give way line to see approaching vehicles without encroaching past the give way line.

NOTE 5 An 'F' dimension of 2.4 metres can, however, allow only one vehicle at a time to enter safely and requires following drivers to be prepared to stop and look.

5.26 Where 'F' dimension of 2.4 metres is used, the mandatory give way markings and upright sign must be in accordance with UKSI 2016/362 (TSRGD) [Ref 9.N] diagram 1003 (Schedule 9 Part 6 Items 3 and 9), diagram 1023A (Schedule 9, Part 6 Item 4) and diagram 602 (Schedule 9 Part 2 Item 2) to require road users to give way to circulating traffic at the give way line.

6.3 Accesses, junctions, lay-bys and bus stops shall not be located within SLTLs.

6.4 Uncontrolled crossing points shall not be provided across SLTLs.

NOTE Advice on provisions for pedestrians and cyclists is given in Section 8 of this document.

6.4.1 Where there is demand for WCHR facilities, SLTLs should only be provided if WCHRs can be catered for safely within the junction design.

6.5 At signalised roundabouts any SLTL shall be physically separated from the circulatory carriageway and adjacent entry and exit arms with kerbed islands.

6.6 Physical SLTLs shall only be provided at street lit junctions.

6.7 Physical SLTLs shall be used where vehicles using the SLTL have to give way at the exit.

6.8 Where the SLTL is situated on an abnormal load route, the use of a physical segregated left turn island shall be assessed to demonstrate sufficient swept path width is provided for the abnormal load vehicle anticipated to use the SLTL.

6.8.1 Where sufficient carriageway width cannot be provided between kerbs, a non-physical SLTL island should be assessed for implementation providing that other conditions laid down in Figure C.1 and requirements elsewhere within this document are met.

6.9 Non-physical SLTLs shall only be used on sections of highway / road where crossing facilities for WCHRs are provided away from the SLTL or where WCHR facilities are not required.

6.9.1 A physical island should be provided in preference to a non-physical SLTL.

NOTE 1 Non-physical SLTLs can be subject to abuse by drivers crossing the hatched island area resulting in conflicts in the vicinity of the SLTL.

NOTE 2 The effectiveness of a non-physical island incorporated in a SLTL layout can be enhanced through the use of continuous, rather than broken, road markings (to UKSI 2016/362 (TSRGD) [Ref 9.N] diagram 1042 (Schedule 9 Part 6 Item 22)), coloured surfacing and enhanced road marking materials that provide improved performance during the hours of darkness and in inclement weather.

6.9.2 Raised or domed surfaces, flush kerbs or infilling with marking material to reinforce road markings should not be used (with the exception of coloured infill surfacing) on a non-physical left turn lane.

6.11 On an SLTL, nearside curve radii shall be 10 metres or greater.

NOTE The curve radius used for the SLTL is dependent on both the design speed of the approach road and site constraints.

6.11.1 The need for speed reduction measures on the approach to an SLTL should be based on the minimum curve radii used in relation to the approach speed.

NOTE The driver’s perception of the approach and SLTL radii is a determining factor in their approach speed.

6.12 On an SLTL, the exit radius used shall be greater than, or equal to, the entry radius used in the design of the SLTL.

6.13 The carriageway widths given in Table 6.13 shall be used on an SLTL based on the minimum curve radius on the entry or exit.

NOTE Carriageway widths in Table 6.13 are inclusive of the 0.3 metres marking offset from kerbs.

6.14 A maximum value of 5% superelevation shall be applied along an SLTL.

NOTE Further requirements and advice on superelevation are provided in CD 109 [Ref 4.N].

6.15 Hatched road markings shall be provided on the nearside of the curve to retain a marked lane width of a minimum of 3.5 metres as shown in the indicative cross-section in Figure 6.15 (for a an SLTL island less than 50 metres in length and with a nearside kerb radius of 20 metres).

NOTE Refer to Table 6.13 for different carriageway width requirements based on SLTL island length and the nearside kerb radius.

6.16 Hard strips shall be terminated at the start of the entry taper and commence at the end of the exit taper as shown on Figure 6.16.

6.17 Where hatched road markings are provided on the inside of the SLTL, the hard strip shall form the start of the hatching.

6.18 Physical islands shall be a minimum width of 1.6 metres.

NOTE Requirements for pedestrian and cycle traffic at physical islands are provided in Section 8 of this document.

6.19 Where road markings are used to create the lane segregation, the overall width of the marked island shall be a minimum of 1 metre.

6.20 Where WCHRs are prohibited from crossing the SLTL the physical island shall extend a minimum of 1.5 metres and 6 metres into the entry and exit roads respectively beyond the traffic islands, as shown on Figure 6.20.

6.21 Where WCHRs are permitted and therefore crossing facilities are provided adjacent to the roundabout entry or exit, the physical island shall extend a minimum length of 2.5 metres on both the entry and exit beyond the crossing point as shown on Figure 6.21.

6.22 Non-physical islands shall start and finish at the entry and exit road limits respectively as shown on Figure 6.22.

NOTE Further requirements and guidance on the use of lane markings is provided in UKSI 2016/362 (TSRGD) [Ref 9.N].

6.23 On the approach and exit to the non-physical island hatched road markings shall be provided.

6.24 The desirable minimum SSD for the SLTL shall be the lesser of:

6.25 The desirable minimum SSD for an SLTL shall be applied to the section of SLTL between the end of the entry taper and the start of exit taper.

6.26 The desirable minimum SSD on the SLTL up to the end of the entry taper and after the start of the exit taper shall be calculated in accordance with CD 109 [Ref 4.N].

NOTE Requirements and advice for the visibility on the approach to an SLTL are provided in CD 109 [Ref 4.N]. The SSD is measured from the centre of the approach or exit lane to the centre of the SLTL lane.

6.27 The maximum curve radius used to determine the SSD from Table 6.27, shall be the greater of either the entry or exit radius of the SLTL.

NOTE 1 The entry or exit radius of the SLTL is the nearside kerb radius that occurs immediately after the entry taper and immediately before the exit taper in the direction of travel.

NOTE 2 See Figures 6.27N2a and 6.27N2b for details of entry and exit tapers and nearside kerb radii.

6.28 SLTLs shall not be used at junctions where the approach road gradient is in excess of 4%.

6.28.1 The gradient of the approach should be measured back along the mainline from the start of the entry taper.

NOTE Further requirements and advice for the measurement of the gradient are provided in CD 109 [Ref 4.N].

6.29 The longitudinal gradient along the SLTL shall not exceed 4%.

6.30 The approach arrangement of SLTLs shall consist of either a dedicated lane (refer to Figure 6.27N2a) or a diverge on the approach arm (refer to Figure 6.27N2b).

6.31 Where a diverge is proposed, the entry arrangements shall consist of an approach taper, in accordance with Table 6.31, and as shown on Figure 6.27N2b and Figure 6.31.

Table 6.31 SLTL approach treatments

Approach type	Approach taper	Entry taper
Dedicated approach lane	No	Yes
Diverge	Yes	Yes

NOTE Further requirements and advice on road markings and signs are provided in UKSI 2016/362 (TSRGD) [Ref 9.N], TSM Chapter 3 [Ref 12.N], TSM Chapter 4 [Ref 13.N] and TSM Chapter 5 [Ref 14.N].

6.32 The approach taper is related to the design speed and minimum taper ratio values contained in Table 6.32 shall be applied.

6.33 The entry taper length for the SLTL shall be provided in accordance with Table 6.33.

NOTE The entry / exit taper length factors are utilised in calculating the entry / exit taper lengths as illustrated in Figure 6.37.1 below.

6.34 The SLTL width shall be a minimum of 3.5 metres at the start of the entry taper, as shown on Figure 6.27N2a.

6.35 Any widening required to accommodate the swept path of HGVs shall be developed along the length of the entry taper.

NOTE The length of the entry taper is dependent on the widening required to accommodate either the segregated left turn lane island width, or the widening required to accommodate the swept path of HGVs (see Table 6.33).

6.36 The length of the entry taper shall be subject to a minimum width of 1 metre for a non-physical island.

6.37 The length of the entry taper shall be subject to a minimum width of 2.2 metres (1.6 metre island width plus 0.3 metre marking offset on each side) for a physical island, except in the following circumstance:

NOTE Guidance on offset and markings is provided in TSM Chapter 5 [Ref 14.N].

6.37.1 The taper for the hatching should be developed asymmetrically on the SLTL side of the entry taper as shown on Figure 6.37.1.

NOTE Example calculation of entry / exit taper length, based on requirements in Section 6:

6.37.2 The taper for the hatching should terminate in a position offset 0.3 metres from the edge of a physical island as shown on Figure 6.37.2.

6.38 The larger of the two values (that is, the widening required to accommodate either the SLTL island width, or the widening required to accommodate the swept path of heavy goods vehicles (HGVs)) shall be used to calculate the entry taper length using the factors contained in Table 6.33.

6.39 Where only one exit lane has been provided from the roundabout, a give way arrangement shall be provided from the SLTL (see Figure 6.39).

NOTE There are three basic types of SLTL exit layout:

6.40 The exit taper for the SLTL shall be provided in accordance with Table 6.33.

6.40.1 As with the entry taper, the length of the exit taper should be calculated by using the larger value when comparing the width of the SLTL island with the lane width reduction required between the start and end of the exit taper, as shown on Figure 6.37.1.

6.40.2 The taper for the hatching should be developed asymmetrically on the SLTL side of the exit taper as shown on Figure 6.37.1.

6.40.3 The taper for the hatching should terminate in a position offset from the edge of a physical island in accordance with the method described for approach layouts in Section 6.

6.41 Any widening required to accommodate the swept paths of HGVs through the SLTL shall be removed along the length of the exit taper.

6.42 Where an SLTL is present, the exit width reduction shall be completed before the SLTL exit taper.

6.42.1 As a result of the reduction in exit width, the SLTL exit taper may be extended to accommodate the roundabout exit width reduction.

NOTE Further geometric requirements on roundabout exit layouts are provided in Section 3.

6.43 The SLTL width shall be a minimum of 3.5 metres at the end of the exit taper, as shown on Figure 6.27N2a.

6.44 The end taper is related to design speeds and the minimum taper values contained in Table 6.44 shall be provided.

6.45 A give way exit from a SLTL shall be at a minimum entry angle of 20°.

6.45.1 An SLTL give way exit should be located close to the roundabout.

6.46 To calculate the entry angle from the SLTL on to the mainline, the position of the vehicle at the give way shall be determined by carrying out a swept path analysis.

NOTE The entry angle is defined as the angle between the line of the give way marking in accordance with UKSI 2016/362 (TSRGD) [Ref 9.N] diagram 1003A (Schedule 9 Part 6 Item 3) and the centre line of the vehicle at the give way as shown on Figure 6.39.

6.47 Where traffic signs and street furniture are placed on the physical island in the vicinity of the exit, they shall be located so as not to obstruct road user visibility between the SLTL exit and adjacent roundabout exit lane.

8.4 On an SLTL, where a pedestrian refuge or physical SLTL is provided, it shall be a minimum width of 2 metres.

8.5 On an SLTL, where a pedestrian refuge or physical SLTL is provided, the hatching shall be a minimum width of 2.6 metres (or 2.3 metres where the speed limit is 40mph or less) adjacent to the refuge to allow for the offset, as illustrated in Figure 6.37.2.

NOTE Guidance on the interface of the hatching and island are provided in TSM Chapter 5 [Ref 14.N].

8.6 Where pedestrians are expected to cross an SLTL, controlled crossings or grade separated facilities shall be provided.

8.7 On a junction where either an SLTL or an SDI is present, a combination of both controlled and uncontrolled facilities shall not be used on the same arm of a junction.

8.7.1 Where a controlled crossing is used on the SLTL or SDI, other crossing facilities across the whole junction should be consistent.

NOTE Further requirements and guidance on provision of crossings on SLTLs and SDIs are provided in Sections 6 and 7.

8.8 SDIs shall be physical islands where they are intended to be used by pedestrians.

8.8.1 Where SDIs are intended to be used by pedestrians, measures should be incorporated into the design to direct pedestrians to use them as crossing points.

8.8.2 Where the islands are not intended for pedestrian use, measures should be incorporated into the design to discourage pedestrians from using them and instead to direct pedestrians to the designated crossing points.

NOTE SDIs can appear to create refuges that encourage pedestrians to cross at the location of the islands even if the islands are not intended as a pedestrian facility.

8.9 When designing for cycle traffic at new SLTLs, cycle tracks shall be provided.

NOTE 1 Cycle tracks are provided to minimise road safety related risks at the start and end of the SLTL.

NOTE 2 Requirements and advice on provisions for cycle tracks are given in CD 195 [Ref 3.I].

NOTE 3 Both dedicated lanes and merge/diverge arrangements on approach and exit can create difficulties for cycle traffic.

NOTE 4 Requirements and guidance on signing to both instruct cycle traffic and warn approaching motorists regarding cycle tracks at SLTLs is provided in UKSI 2016/362 (TSRGD) [Ref 9.N] and TSM Chapter 4 [Ref 13.N].

8.10 Where cycle traffic is expected to cross physical SLTL islands (as shown on Figure 8.10), or a physical SDI, a minimum refuge island width of 3.0 metres shall be provided.

8.10.1 For all types of segregated lane, if it runs independently to the other approach lanes, then the crossing on the segregated lane island should be staggered.

8.11 Staggered crossings shall be designed so that central refuge island can accommodate the design parameters for the cycle design vehicle and two-way cycle track.

NOTE 1 Where appropriate, pedestrian and equestrian facilities can be incorporated in a staggered crossing.

NOTE 2 Additional examples of cycle track layout and signage on an SLTL are illustrated in Figures 8.11N2a and 8.11N2b. The requirements and advice for the SLTL crossing facilities are provided in Section 6.

There are two different methods for entry angle measurement, depending on the size of the roundabout as shown in Section 3. Where it is not clear which of the two methods should be used, the following should clarify the situation. All three vehicle paths (entry, exit and circulatory carriageway medians) should be constructed, and the entry and exit paths projected towards the roundabout centre. The choice of construction for depends on where these projections meet: if the meeting point is closer to the centre of the roundabout than the arc of the circulatory carriageway median, then the construction shown in Figure 3.18N2 in Section 3 should be used; if they meet outside that area, then the construction illustrated in Figure 3.18N3 should be used. In the limiting case where all three medians intersect at a point, it is common for the circulatory carriageway median approximately to bisect the angle between the other two medians, so that the two methods become equivalent.

To determine the entry path radius, the fastest path allowed by the geometry is drawn. This is the smoothest, flattest path that a vehicle can take through the entry, round the central island and through the exit (in the absence of other traffic) (see Figures 3.20, 3.22 and 3.23 and Figure A.1).

The path is assumed to be 2 metres wide so that the vehicle following it could maintain a distance of at least one metre between its centre line and any kerb or edge marking. The path starts 50 metres in advance of the give way line.

The construction of the path is a matter of personal judgement. Results should be checked by more than one designer for comparison.

The smallest radius of this path on entry that occurs as it bends to the left before joining the circulatory carriageway is called the entry path radius. Note that this is different to, and should not be confused with, the entry kerb radius. The entry path radius can be measured by applying the curve in the vicinity of the give-way line (see Figures 3.20, 3.22 and 3.23 and Figure A.1). It is the radius of the best fit circular curve over a length of 25 metres.

The best predictive equation for the capacity of any roundabout entry (except those at grade-separated junctions and mini-roundabouts, see below) found by research to date is as follows:-

The value of Q_E will be:

At grade separated junctions, there are differences of operation and the "F" term in the above equation becomes "1.11F" and the "f_c" term becomes "1.4f_c". These differences are incorporated in the ARCADY program.

The ranges of the geometric parameters input to the ARCADY database were as follows in Table B.1 (see Table B.2 in section B1.1.1 below for the recommended limits to be used in new design):-

Research for the original calculation above is contained in TRL SR 721 [Ref 13.I], this research was further expanded on in TRL RR 142 [Ref 12.I] to improve capacity modelling for large roundabouts and grade separated junctions using ARCADY.

Additionally, current ARCADY software has a queue simulation mode that considerably improves specific limitations of the original software. This queue simulation mode allows lane starvation to be evaluated so that it can be avoided in further roundabout design stages.

Guidance on the calculation for mini-roundabouts can be found in TRL AG 72 [Ref 7.I].

The methods of measuring the entry angle at conventional large or small normal roundabouts are given in Section 3. For roundabouts, where the distance between the offside of an entry and the next exit is more than 30 metres and is approximately straight, the construction of the entry angle, , is illustrated in Figure B.1.

AD is parallel to the straight circulatory carriageway where A is the point of maximum entry deflection at the right hand end of the give-way line, and D is the point nearest to A on the median island (or marking) of the following entry.

The line BC is a tangent to the line EF, which is midway between the nearside kerb line and the median line or the edge of any median island on the offside, where this line intersects the "give way" line.

is measured as the acute angle between the lines BC and AD in Figure B.1.

The flow chart in Figure D.1 shows the series of concerns or operational difficulties that may lead to road markings being used at a particular existing roundabout.

The flow chart shown in Figure D.2 below outlines the main features of the design of road markings at roundabouts. Many of the features are dealt with separately, but it should be noted that the features are closely related and that the design of road markings at roundabouts is an iterative process.