This post is part of this series on learnings from the Dutch CROW design manual for bicycle traffic.

The modern roundabout may have been invented in the UK but they have been perfected in the Netherlands. The roundabout has a lot of plus points above a standard or controlled intersection; the lower number of potential conflict points, the natural reduction in vehicle speed through the intersection, combined with a higher average vehicle speed. The down side is that they take up more space than other types of intersection.

Over the last 30 years, the Dutch have been systematically replacing right of way and light controlled intersections with roundabouts where possible due to the increased safety aspects of roundabouts. In the process, they have distilled the essence of the roundabout into 3 distinct types; single lane, turbo, and signal controlled turbo. In this post, we will look at the first two types as well as looking at how the UK favourite, the mini-roundabout can be adjusted to fit in with the principles of Sustainable Safety (there are no mini-roundabouts in Holland).

If we go back to our diagram of junction treatments, we can see that roundabouts can be used in a large number of situations, the type of roundabout for the given situation depends on the type of roads leading to the intersection and the volume of traffic.

Single lane roundabout

For junctions between low volume distributor roads, and junctions that connect distributor roads and access roads, a single lane roundabout is suitable.

Looking at our road style chart we can see that distributor roads should always have a cycle lane, cycle track or a parallel access road, and so the roundabout needs to have provision for cyclists.

If you look at the potential conflicts of a roundabout (the diagram on the left below), you will see that a roundabout is very good at minimising conflicts, producing just one per arm of the junction. However if we add an on-road cycle lane around the roundabout (the diagram on the right below) we increase the number of potential conflicts by 200%, to 3 per junction arm, and with 66% of these conflicts being between motors and cycles, this is not a good statistic for the subjective safety required to make cycling attractive.

To fix this, the Dutch feed a cycle track around the outside of the roundabout, they never mix cycle and car traffic or have on roundabout cycle lanes. In this way, the motor/cycle conflicts are moved away from the junction itself creating space to reduce help the effect of the conflicts.

To do this, we need to create separate cycleways in each direction on each arm of the junction prior to the roundabout. If the arm is an access road without cycle provision or a distributor or access road with cycle lanes, we need to smoothly transition cycle traffic onto a cycleway a little way before the junction.

So if we apply this to a single lane UK style roundabout, this is the sort of thing that we might come up with for the urban environment:

As you can see, cycles have a dedicated cycleway around the roundabout, here’s the important points:

• Single entrance and single exit lane on each arm, with tight entrance and exit radii. Central island with tactile surface for long vehicle turning circle.
• Separate cycleway and pavements on each junction arm.
• Pedestrian and cycle crossing combined (belisha beacon with bicycle route warning sign) into a single right of way over motor traffic.
• Cycles and pedestrians have priority across the roundabout arms. If motor, bicycle, and pedestrian traffic is too great for this to work, then an urban roundabout is not suitable and a traffic light junction should be considered instead.

For out in the countryside, the priority is switched due to the lower volume of bicycle and pedestrian traffic and the higher speed of motor traffic.

• Cycleway curves away from the junction to cross arms at a cars length distance from the junction to allow vehicle waiting without blocking the roundabout.
• Central islands on pedestrian and cycle crossing used to increase crossing safety on high volume routes.
• Depending on the amount of space available, the curve of the cycleway and pavement can be smoothed to improve pedestrian and cyclist desire lines.
• Cycles and pedestrians have to give way to motor traffic at crossings.

Turbo roundabout

When a single lane roundabout won’t cut it due to traffic volume, the Dutch shun the standard multi-lane roundabout we tend to use all over the UK for the so-called turbo roundabout, a multi-lane roundabout with a forced spiralling of traffic throughout the junction.

These have the advantage of removing the lane changing conflicts that occur on multi-lane roundabouts by forcing traffic to chose an exit direction and selecting the correct lane for that exit before entering the roundabout. Once on the roundabout, vehicles have no choice of direction other than to follow their chosen lane to the predefined exit arm.

This removes the possibility of confrontation between motor vehicles that can occur when two vehicles going straight across in the main “through” direction arrive at the roundabout at the same time in adjacent lanes. This makes the roundabout simpler to navigate for motor vehicles and thus improves the overall safety of the junction.

Turbo roundabouts take up much more space than a single lane roundabout, but a similar amount to a standard 2 lane roundabout.

In the diagram above, we have given priority to motor vehicles over bicycles and pedestrians since turbo roundabouts are mainly used in rural settings and are not usually suitable on urban roads. The consistency principle of Sustainable Safety which states that roads should be familiar to their users, and it is usual to give priority to pedestrians and cyclists in the urban environment where motor traffic speed is low and pedestrian and cyclist volumes are high, while giving motor traffic priority in the rural environment where the opposite is true. 

Traffic signal controlled turbo roundabout

The third type of roundabout the Dutch build is the signalled turbo. These are large junctions that merge the benefits of the turbo roundabout with the control and throughput of a traffic control system (TCS) junction. Since these have more in common with TCS junctions than roundabouts, we’ll leave these for another day.

I’ve also looked at large roundabouts with bi-directional cycle tracks which fit somewhere in between.

Mini-roundabout

In the UK we have one other type of roundabout that is pretty unique to our roads, the mini-roundabout. They are designed to apply roundabout like behaviour (give way to the right) to any three or four way junction.

If included at a junction between access roads, then traffic volume will be low and no special treatment is required. But we often see minis on distributor roads where a single lane roundabout would perhaps be more suitable simply because an existing right of way junction needed to have its priorities changed.

If the mini-roundabout is on a distributor road, then we should expect the distributor to have cycle tracks and pavements and as such it should be treated in a similar way to the single lane roundabout, just taking up less road space. The important design principle is to keep the cyclist/motor conflict points away from the junction itself and to make them clear and obvious.

So given those three roundabout types, we can fulfil any junction where the roads have a traffic volume of between 500 and 1750 PCU/hr. For larger volumes, the TCS junction or TCS turbo roundabout which we’ll talk about next.