This paper reports on a trial of a cyclist roundabout design or c-roundabout undertaken in Auckland, New Zealand by the New Zealand Transport Agency. The cyclist roundabout design was installed at the Palomino Dr/Sturges Rd roundabout in Auckland, New Zealand. The operation of the C-roundabout was evaluated in terms of its safety, capacity, and the opinions of cyclists,pedestrians and car drivers.
The full paper can be download from my Dropbox and the executive summary has been reproduced below. I am no engineer but I did find this paper interesting and it is good to see New Zealand transport authorities being pro-active on the design of roundabouts.
The purpose of this research project was to evaluate the on-site operation of the C-roundabout design (at a site in Auckland, New Zealand). The C-roundabout (cyclist roundabout) is a new multi-lane roundabout design that was developed by D Campbell, I Jurisich and R Dunn as part of a 2006 Land Transport NZ research project titled Improved multi-lane roundabout designs for cyclists. The primary aim of the C-roundabout is to improve the safety of cyclists at multi-lane roundabouts and make multi-lane roundabouts more cyclist-friendly. The concept of the design is to decrease vehicle speeds through the roundabout to around 30 km/hr by increasing the deflection of the roundabout, and to reduce the widths of approach lanes and circulating lanes so that cyclists are required to travel in the centre of the lanes, like other vehicles. This research project was carried out from 2008 to 2011.
There were two parts to this research:
- evaluation of the C-roundabout (Palomino Dr/Sturges Rd)
- evaluation of a single-lane roundabout that had been changed to have two narrow lanes with no reduction in vehicle speeds (Margan Ave/Hutchinson Ave).
1. Evaluation of the C-roundabout (Palomino Dr/Sturges Rd roundabout)
The C-roundabout design was installed at the Palomino Dr/Sturges Rd roundabout in Auckland. The operation of the C-roundabout was evaluated in terms of its safety, capacity, and the opinions of cyclists, pedestrians and car drivers.
The C-roundabout was successful in achieving a lower-speed environment, with the 85th percentile through speeds being reduced to around 30km/hr. These vehicle speeds were close to the speed a cyclist would be travelling through the roundabout, which made it safer for cyclists. The chances of a cyclist surviving a crash decreases significantly above speeds of 20–30km/hr (Austroads 2009a). The lower speeds are also considered safer for other road users (pedestrians and motorists), and should result in less-severe crashes.
In the 2.5 years following the roundabout’s reconstruction, there was no significant change in the crash rate. There were no cyclist crashes either before or after the reconstruction. There have been no injury crashes since the installation of the C-roundabout design; prior to the reconstruction there were 0.32 injury crashes/year. With the lower design speed we would expect the overall injury crash rate to drop. Further crash analysis in one to two years will provide enough data to conclusively assess the safety of the C-roundabout.
The installation of the C-roundabout at this uncongested site (converting a standard multi-lane roundabout to a C-roundabout) had little impact on capacity (based on SIDRA and on-site measurements).
SIDRA modelling indicates that for an uncongested roundabout, conversion to a C-roundabout has very little impact on the average delay (7.5sec/veh before, 8.1sec/veh after), the degree of saturation (0.579 before, 0.568 after) and the design life (12 years before, 12 years after). However, SIDRA calculates a small increase in the critical gap and follow-up headway due to the geometry of the C-roundabout (decreased negotiation speed through the roundabout, narrow approach lane width, and increased roundabout diameter). This could not be confirmed by on-site measurements (gap and follow-up headway) because of insufficient congested periods (total of 13 minutes congested periods in both peak periods).
Delays measurements indicated that converting the multi-lane roundabout to a C-roundabout had no significant impact on the delays.
Regression analysis suggested a drop in capacity for the C-roundabout design when the circulating flows were less than 878veh/hr and an increase in capacity when circulating flows were greater than 878veh/hr. However, as the roundabout had so little congestion there was insufficient survey site data to statistically conclude whether or not this was the case. Further research would be required at more congested sites to confirm this.
The capacity of the Palomino Dr/Sturges Rd C-roundabout was not impacted by trucks/buses as the numbers were very low. Trucks/buses may impact on C-roundabout approach capacity as they are required to straddle both lanes (thus blocking access to one lane)1. However, this impact is unlikely to be significant, as heavy-vehicle numbers are usually low in peak periods. The amount of impact that trucks/buses have on the capacity of a C-roundabout is expected to depend on the following factors:
- Queues and lane utilisation on the approach – the greater the difference between approach lane queue lengths, the greater the impact of trucks/buses on the capacity of the approach. Further research is recommended to confirm this.
- Proportion of trucks/buses – the higher the proportion of trucks/buses, the more likely they negatively impact on the capacity of the approach.
The capacity implications of converting an existing multi-lane roundabout into a C-roundabout were difficult to assess at the Palomino Dr/Sturges Rd site because of the low traffic flows, very little queuing and stop-line delay, and very few congested periods (only 13 minutes in both peak periods).
Road users’ impressions of the C-roundabout (cyclists, pedestrians and car drivers)
The C-roundabout design at the Palomino Dr/Sturges Rd intersection drew positive feedback from cyclists and pedestrians, but a less positive reaction from the car drivers – about half were not in favour of it.
- Cyclists were positive about the C-roundabout and would like to see more of them installed. They found it easier to use, safer, and better for cyclists. They also noticed that car speeds were slower.
- Pedestrians were positive about it and in general found it safe and better for pedestrians. They found it easy to cross, and noticed that the crossing distance was narrower and that car speeds were slower.
- Car drivers noticed that car speeds were slower and the lanes were narrower (the intention of the C- roundabout design), but about half of them did not like the C-roundabout and did not want to see more of them installed. About half the drivers found the C-roundabout more difficult to use and indicated a preference for a standard roundabout.
More than half of the drivers thought that the C-roundabout was ‘safe’ or ‘the same as other roundabouts’, but a significant proportion (41%) thought it was unsafe. The results of the crash analysis did not confirm this perception of the C-roundabout being unsafe, and as vehicle speeds had dropped, it was unlikely to be less safe.
The majority of the drivers thought that the roundabout was actually less safe for cyclists, and were unable to recognise the benefits of the C-roundabout for cyclists.
The drivers’ impression of the roundabout may improve if they are informed of the benefits of the C- roundabout for cyclists and of the cyclists’ support for the C-roundabout. Also, their impression may improve over time as they become accustomed to using the tight, slower roundabout. Further monitoring of car drivers’ impressions of the C-roundabout should be undertaken.
2. Evaluation of single-lane roundabout changed to have two narrow lanes (Margan Ave/Hutchinson Ave roundabout)
This site involved changing two approaches from wide, single lanes to two narrow lanes, without altering the kerbs. The intention of studying this site was to assess whether or not it would be possible to fit a small double-lane roundabout within the same road reserve as a single-lane roundabout, thus improving the capacity at low cost.
It should be noted that the reconstruction of the Margan Avenue/Hutchinson Avenue roundabout did not comply with all of the C-roundabout design principles as it did not involve increasing the vehicle deflection to slow vehicle speeds through the roundabout to around 30km/hr (the design speed for the roundabout was approximately 50km/hr). Thus, the differential speed between vehicles and cyclists was not expected to decrease – ie cyclist safety was not specifically being addressed at this site.
The evaluation showed that the capacity of a single-lane roundabout can be improved (almost doubled, depending on the lane utilisation of two-lane approaches) at a very low cost by converting to the two narrow lanes design.
The crash analysis of the Margan Ave/Hutchinson Ave roundabout showed that the safety of the roundabout decreased after it was changed from a single-lane roundabout. The overall crash rate increased significantly and the injury crash rate also increased. It may not be advisable to install narrow double-lane roundabouts without decreased vehicle speeds at other sites until the safety concerns have been addressed or confirmed at this trial site. To improve the safety of this roundabout, it is recommended that the vehicles speeds should be decreased to around 30km/hr by increasing the vehicle deflection. The safety of the roundabout should be reassessed one year following the above changes to determine whether the increased vehicle deflection has addressed the safety concerns. If the increased vehicle deflection is successful in reducing the crash rate, then it will prove the principle of the C- roundabout – that roundabout speeds should be reduced for safety.
The principles applied to the C-roundabouts in order to reduce approach and circulating speeds should be considered in other locations where a more cyclist-friendly layout is desirable – ie where cyclists form a significant proportion of the local network traffic. However, it is important to note that an important contribution to the safe and efficient operation of a roundabout is that of a similar treatment on all approach legs, producing a well-balanced intersection. Further research into the key features of the C-roundabout design should be undertaken.
Single-lane roundabouts can be converted to narrow two-lane roundabouts for capacity reasons at low cost compared with that of the standard design. Further research on the safety implication of this conversion is needed.
The full reference for this paper is:
Asmus, D., Jurisich, I., Campbell, D. & Dunn, R. (2012). Evaluation of the C-roundabout – an improved multi-lane roundabout design for cyclists. New Zealand Transport Agency Research Report 510.