Saturday, March 24, 2012

Perne Road roundabout: room for improvements

A county council consultation on a proposed “continental” redesign of the Perne Road/ Radegund Road roundabout has just closed. The current layout of this roundabout allows vehicles to use the fastest and most direct line through the roundabout, which means that speeds are high and it can be hostile for cyclists and pedestrians. This roundabout lies on a main cycle route that is used by both commuters and school children crossing from Birdwood Road to Radegund Road and it is a known accident site for cyclists.

The proposed new layout (see below) significantly reduces the space for traffic around the roundabout and tightens both the exits and entries into the roundabout. The design follows guidance from the Department for Transport on making roundabouts more cycle-friendly. The change in geometry will ensure that drivers keep their speeds low, and this should improve safety for both pedestrians and cyclists, particularly for on-road cyclists.


The proposed design has a good safety record in Europe, where it is usually combined with a peripheral cycle path around the outside of the roundabout. But there are some complicating factors here: this location has relatively high levels of traffic and the proposed design does not include any off-road cycle paths. There are good reasons for excluding the off-road cycle paths from the design: firstly, all of the existing cycle routes are on-road cycle lanes; and secondly adding any off-road paths would significantly increase the construction costs.

We met with the cycling officers on 16 March to discuss the proposed design and our CTC response to the consultation. We broadly welcome the new design because the core design objective of reducing the vehicle speeds should make this a safer junction for pedestrians and cyclists. But neither CTC, nor Cambridge Cycling Campaign are confident that the proposed design is the best option. CTC asked for two “modest” changes to the design: firstly to modify the new build outs so that cyclists can ride over these build outs to bypass stationary traffic at peak times (we think there is a risk of new “pinch points” at the entry and exits of the roundabout). Secondly, to include “desire-line” pedestrian and cycle crossing points on both Perne Road arms (currently, crossings are only proposed for the Birdwood Road and Radegund Road arms).

During the meeting we were shown some alternative designs that had been considered and rejected, including some designs with various off-road cycle path elements. These off-road options have some attractive elements, but there are also some reservations with any of the off-road alternatives. For example, the transition from the on-road cycle lane onto a peripheral off-road cycle path would introduce inconvenient sharp bends at the entry and exit points. Also, a peripheral cycle path would require two separate cycle crossing points on the Perne Road arms. As a result, we prefer the on-road option (keeping cyclists on the main carriageway). This preference is in line with general CTC policy, and the reduced vehicle speeds should make this option even safer.

The conclusion of the meeting was to look again the proposed design, both to look at some possible improvements and also to consider a possible temporary trial of the design (where part of the design is implemented with concrete blocks rather than by permanent construction). We see benefits if this trial can be achieved at low cost: this will allow the key elements of the design to be tested with real traffic to find out if it works as expected (and what doesn’t work!). We intend to stay involved in this project and to continue to work with the council to search for a good design. Hopefully, the lessons learned from this trial can then be applied to other roundabouts in Cambridge.

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