Design details for more ‘cycle-friendly’ roundabouts are presented in the following sub sections:
Compact roundabouts are single-lane roundabouts with inscribed circle diameters in the range of 30–40m. The associated tighter entry and exit radii, narrower lane widths, and a higher entry deflection angle, all contribute to a speed environment that is low enough to be suitable for all users in urban situations. The consequences of achieving this level of safety are increased delay to motor vehicles and reduced capacity.
While Austroads Guide to Road Design Part 4B Roundabouts(external link) suggests that where cyclists and pedestrians are present, design should be based on entry speeds of 25–30 km/h, and provides some principles, it does not provide any detailed guidance for designs that demonstrably achieve these lower speeds. However, the principles applied by the Austroads Guide are consistent with the design philosophy for compact roundabouts.
The interim guidance below, suggests some desirable design values for compact roundabout design. It is not intended to be prescriptive, but provides a starting point for developing cycling and walking friendly compact designs. It is based on the principles and notes in the Austroads guide, with adaptation of the details sourced from the official UK and German design guides. In most respects the guidance from all these sources is consistent. The UK design advice for compact roundabouts is available in the UK Design Manual for Roads and Bridges Volume 6, Section 2 part 3(external link).
After initial layout based on the recommendations below, each design will then need to be refined after considering the tracking needs of appropriate design vehicles. The design vehicles are those that require the most space, and are likely to use the intersection on a sufficiently regular basis, and will vary depending on the location. So selection of the design vehicle is important. The tracking curves for design vehicles should use a buffer of 0.5m between the wheel tracks and kerbs, and also consider where the vehicle body may encroach – especially separation to paths on the periphery.
The concept and individual dimension descriptions are shown in the figure below.
Overall Size: The inscribed circle diameter, the width of the circulatory carriageway and the central island diameter are interdependent. Once any two of these are established, the remaining measurement is determined automatically.
The inscribed circle diameter (ICD) is the dimension across the outer kerbs of the circulating carriageway. The desirable range is from 30–35m. It is difficult to accommodate the larger design vehicles with an ICD less than 30m, hence it may be necessary to review whether the design vehicle is appropriate or increase the ICD. For smaller spaces see Small roundabouts.
Central island radius (Ri): While all three guides have different ways of arriving at the central island size, in practice they all recommend a central island radius of about 10–12m. The central island dimension should include any apron or encroachment area.
Circulating lane widths: aim for 4–6m. Design swept paths of heavy vehicles are accommodated by an encroachment area or apron around the central island. The encroachment area is included in the central island size. Austroads uses the design vehicle to determine circulating roadway widths, and an encroachment area for vehicles larger than the design vehicle, but acknowledges that larger encroachment areas may be necessary to achieve small enough entry radii.
Aprons and encroachment areas: Interim guidance is to use a fully mountable kerb design of height between 25mm and 50mm.
Note: The design of these encroachment areas requires finding a careful balance between making them an unattractive option to negotiate with a smaller vehicle, a suitable option for larger vehicles to drive over them, and not posing a hazard to motorcycles. While Austroads permits the use of semi-mountable kerbs for all features, their use on aprons around central islands on compact roundabouts is not desirable. As compact roundabouts are designed with narrower circulating widths, larger vehicles are expected to use the encroachment area or apron more often, so any upstand needs to be more forgiving.
Further research on the detailed design of encroachment areas is desirable.
Entry details: The entry detail is just as important as the roundabout size for achieving the desired speed profile. Cycle lanes and road shoulders should be terminated prior to the entry and sharrows may be marked to indicate that cyclists share the lane. The entry path is formed between the entry kerb and the approach splitter island. Compared with conventional roundabouts, the entry may form more of a T-junction, with the centre splitter island situated close to the axis of the roundabout, (ie radial design) and the edge deviating to the left only to the extent required to accommodate vehicle tracking. (This contrasts with conventional roundabouts designed for higher speeds where the entry splitter island edge is aimed tangentially to the central island, and a lower entry angle is used to reduce the differential between speeds of entering and circulating vehicles to below 50km/h.)
It is suggested that designers start with a perpendicular or radial approach and, as necessary, adjust it, whilst balancing considerations of all the design users and vehicle types. Over-designing for large vehicles may compromise achieving low design speeds by increasing the entry path curve. This may be mitigated by adjusting the approach to aim slightly right of centre of the island. The use of overrun areas for larger vehicles therefore helps to provide the space they need while still slowing everyone to safe speeds.
Splitter islands: The axis of the splitter island should be radial to the centre of the central island, and not be wider than necessary, with a minimum width at the pedestrian crossing point of 2.0m, or 2.4m if providing for bicycles. The pedestrian crossing point should be set back from the limit line by about one car length or about 6m. The kerbed edges of the splitter island follow the entry and exit radii near the circulating lane. In some circumstances, it can be beneficial to offset the entry kerb and splitter island slightly to the right to increase the approach deflection and ease the departure curvature. Note: Austroads Guide to Road Design Part 4A states splitter islands may 'be aimed at a point in the central island; applicable where the roundabout is used primarily by cars and cyclists and it is desired to further reduce entry speeds so that drivers have a better opportunity to scan the roundabout for cyclists'.
Entry lane widths (We): Aim for 3.2m. Note; Normally Austroads Guide to Road Design Part 4B sets widths by the tracking of the design vehicle or at least 5 m to allow traffic to get past a broken down vehicle, however it also suggests 3 m maximum where cyclists share the roadway to discourage overtaking by motorists. The narrower widths are preferable, and can be achieved at the beginning of the entry provided splitter islands and/or kerb extensions are designed with semi-mountable kerbs, heights and layouts that permit occasional encroachment.
Entry path curvature: Entry path curvature is based on the easiest path a car driver can use to enter the roundabout and transition to the circulating around the central island. It is fully described in the Austroads Guide to Road Design Part 4B, and the UK guidance. For normal roundabouts, Austroads requires a maximum entry path curve radius of 55m. This is based on keeping motor vehicle relative conflict speeds to below 50 km/h. It notes that a tighter curve would help reduce speeds further for cyclists but does not recommend a design value for them. If the entry path radius is much greater than the desirable, the differential with the radius on the circulating path around the central island is more likely to cause vehicles to lose control around the roundabout. Kerb radii, narrow widths and radial entry design values described hear help ensure an entry path radius of about 15m, which is similar to the circulating path radius.
Entry kerb radius (Re): Aim for 8–15 m. From the approach, the entry kerb is usually offset to the right by a kerb extension.
Exit path curvature: Exit speeds should not be constrained by geometry to be slower than entry speeds. The Exit path curvature limits the rate at which driver speed up as they leave the roundabout. This is important where there is pedestrian and cyclist activity across or near the exit. In which case exit speeds should be limited to 30km/h.
Exit width (Wx): Aim for 4 m. Note: Austroads just uses design vehicle tracking curve. UK uses exit width equal to entry width (angled at limit line - 4.5m) and Germany recommends 3.5–4m.
Exit kerb radius (Rx), Aim for 15–20m. This may need to be increased to accommodate the swept path of a larger design vehicle. Check that resulting exit speeds are appropriate for the situation.
Small roundabouts are practicable for urban environments where there is a lesser need to accommodate large heavy vehicles. It may be acceptable to allow certain vehicles that rarely use the roundabout to track over the central island. Small roundabouts have smaller dimensions than compact roundabouts, so compromises are inevitable in their design, but all the design considerations for compact roundabouts are still relevant. Their horizontal geometry alone may not be sufficient to reduce speeds to be safe for cyclists, pedestrians and motorcyclists. They may need other traffic calming treatments to reduce speeds, such as raised platforms across the approach/departure legs.
For many small urban roundabouts, typical corner splays mean that fences and buildings tend to limit visibility to as little as 5 metres back from the limit line. In the absence of sufficient deflection, this visibility restriction can be the main determinant of entry speed. However, if one approach has better visibility than the rest, this results in a higher crash rate at that entry involving circulating cyclists and motorcyclists, and reduced safety for all users at the next entry due to higher speeds from the right. Thus, where deflection is not sufficient on its own to control speeds, it is imperative that visibility restrictions are equal for all approaches.
Small roundabouts require aprons or encroachment areas so that they are negotiable by the design (heavy) vehicle.
There are a large number of existing small urban roundabouts in New Zealand that have greater than desirable entry speeds, and would require improvements to make them cycling-friendly. If any RCAs are considering transforming existing roundabouts to be more cycling-friendly, the Transport Agency would be very interested to assist and advise on evaluating the changes in operation, cyclist perceptions and safety.
Controlling speeds through multi-lane roundabouts is a particular challenge as, during times of low volume, some drivers will straighten their alignment through the roundabout by cutting across the lane lines. The higher speeds, increased number of conflict points and more complex operation make multi-lane roundabouts less safe and more difficult for all users to negotiate. Hence where less-confident cyclists are to be accommodated at multi-lane roundabouts, they should be able to divert to a path and to cross the entries and exits. See Off-road alternatives.
The safety principles for single lane arrangements apply to dual-lane roundabouts - reducing vehicle speeds, stopping shoulders or marked cycle lanes on approaches, and facilitating lane-claiming by cyclists.
While single-lane roundabouts are preferable in terms of comfort and safety for cycling, a more cycle-friendly roundabout design for two-lane approaches may be achieved using the principles developed in the Auckland region as part of a Land Transport NZ research project. The design concept and evaluation are documented in NZ Transport Agency research reports 287 and 510. The primary aim of the project was to improve the safety of cyclists at two lane roundabouts, and thus increase people's willingness to cycle through them.
This concept for designing more cycle-friendly dual lane roundabouts has two key elements:
Although New Zealand legislation permits large vehicles to straddle two lanes, research found that drivers of large vehicles needed to be advised, by way of signage, to straddle both lanes on the approach to and through the roundabout. This sign has never been gazetted and road controlling authorities wanting to adopt this design philosophy are encouraged to work with the Transport Agency with regards to this non-standard traffic control device.
It was shown that the cycle-friendly approach design resulted in safety improvements for all modes.