Friday, 12 June 2015

The Slow-Turbo Roundabout. A promising new Dutch roundabout design and why you should NOT copy it

A few days ago a new Dutch "slow turbo" roundabout design appeared in images on Twitter. Ordinarily, turbo roundabouts are used in the Netherlands to deal with large volumes of high speed motor vehicles in locations such as motorway junctions, which are rarely visible to cyclists and pedestrians, but this new design is intended to be used in areas with high volumes of cyclists and pedestrians:
A promising design, but with unproven safety record so don't copy this yet.
This roundabout is interesting because it combines the safe layout of the proven safest Dutch roundabout design with priority for cyclists at the crossings, when similar layouts have previously only been used where drivers are given priority at the crossings. Extra care has been taken in the design to try to ensure cyclist safety at these priority crossings - a factor which has been proven in the past to lead to a sharp reduction in safety.

A particularly poor "with priority"
design. No sight-lines, no reaction time
Frequent injuries at this location.
This design attempts to reduce the safety problem for cyclists with priority at roundabout arm crossings by a number of measures:

  1. It slows both cyclists (by tight corner radii) and drivers (by use of narrow lanes with raised tables), giving both more time to react at the crossing.
  2. Good sight lines are ensured because cyclists and drivers cross at right angles to one another.
  3. While turbo-roundabouts require multiple lanes, this design keeps the cycle crossings far enough from the roundabout that cyclists are required to cross only one lane of motor vehicles at a time.
  4. A generously sized safe refuge is provided between the two streams of traffic.
  5. The crossings being placed at a considerable distance from the roundabout itself means that the moment in time when drivers negotiate with cyclists on the crossing is distinctly different from that when they have to negotiate with other drivers on the roundabout.

Why it may not be so safe as is hoped
I think there is great promise in this design but I must sound a note of caution.

Raised tables slow motor vehicles where
cyclists have priority at the crossings, but
they resemble less successful crossings.
I am enthusiastic about the possibilities for this type of roundabout because it follows the good design principles of the proven safe Dutch roundabout design as I discussed in the previous section and that it also gives cyclists priority (everyone wants priority), but I also see reasons why it may fail to be safe. For instance:
  1. The distance between the crossings and the roundabout is quite large, but for drivers entering the roundabout the distance between the crossing and the point where they need to choose a lane is very short. This may distract drivers such that they are less likely to notice cyclists.
  2. Raised tables on straight roads have only a slight effect of slowing drivers. When I looked at three different priority crossings a few days ago, the crossings where drivers were effectively slowed had more measures than just a raised table and the example which barely slowed drivers at all looked very much like these crossings.
  3. This isn't the first time that priority has been combined with a layout which provides better sight-lines. For example, one attempt to design a roundabout in Eindhoven combining good sight-lines with cyclist priority resulted in some injuries.
I remain hopeful about this design, but we must keep in mind that it is an experiment.

Experiments are not always successful
Without experimentation we can never discover new and perhaps better ways of doing things. The Netherlands leads the world in cycling infrastructure design, and experimentation continues here. This is a good thing. But ideas should only be adopted more widely if they have been proven to work safely and efficiently.

Experiments which didn't work out so well include an experimental cycle-path surface which was installed near Assen in 2009, but then replaced again in 2013 after it had proven not to provide an adequately comfortable surface for cycling.

There was also the Zwolle "fietsrotonde" for which bold claims were made in advance, but where poor design caused injuries within months of opening.

As of right now, there is precisely one roundabout of this new design in the world. It should be viewed as a brave and worthwhile experiment and it may eventually form the basis for yet safer conditions for cyclists in the Netherlands, but it should not yet form the basis for other experimentation elsewhere. We do not yet have long term accident statistics for this design and it is premature to make any claim about whether or not this new design is in fact safe.

Note also that the new design requires a very large footprint in order to provide the required sight-lines and space for the lanes of the turbo roundabout. As such, it's unlikely to be able to be applied everywhere.

What to adopt in other countries
For experimentation elsewhere it makes most sense to copy the proven safest Dutch designs. So far as roundabouts are concerned, this is it:
This design provides the best basis for emulation elsewhere. Such roundabouts have the the best safety record within the Netherlands and are likely to remain safe even if aspects of the design are compromised.
The safest design currently in use in the Netherlands, shown in this photo, places the emphasis on cyclists to look after their own safety rather than relying upon perfect driver behaviour to keep cyclists safe. This is one of the reasons why this design is successful. The same principles have also been shown to work on similarly designed roundabouts with considerably smaller footprints.

Awful roundabout design from London
widely described as "Dutch", but
actually nothing of the sort. This is
one of many awful recent proposals
from London, none of which are
similar to Dutch designs which they
claim to have copied.
The details of any design are important. Distance between roundabouts and crossings, heights of raised tables, lane widths, road camber, geometry are all important and none of these should be taken as being the sole reason why Dutch roundabout designs are safe for cyclists. It should also be noted that roundabouts are only used by cyclists in the Netherlands where traffic volumes and speeds are relatively low and are controlled. Unfortunately, it is not uncommon for designers elsewhere to completely misunderstand how Dutch roundabouts achieve their safety. "Cargo-cult" style copies of Dutch roundabouts built in the UK in Bedford, Cambridge and York without the safety features of the Dutch roundabouts which provided inspiration and as a result they are not so safe.

In order to try to assist planners and campaigners alike to make the right choices, we offer cycling study tours which take a unique, independent, view of Dutch cycling infrastructure and which explain everything in plain (native) English. Book a place to discover more about what works well and should inspire new infrastructure design elsewhere as well as what works less well and should not be copied.

Find out how how things really work in the Netherlands before trying to copy anything.


ScottRAB said...

"Reduce the safety problem"? Don't you mean 'increase safety'?

Rob said...

I agree with your concerns, especially the first. Car drivers don't seem to expect a bicyle priority crossing so far from the roundabout. I have used it a few times now, and have seen several near misses already. Today I decided to record the situation and experienced a near miss myself. You can hear the car stopping with screeching tires. I had full eye contact with both him and the driver of the car in front of him. In the video I ride from south-east to north-west. Also yes, vertical video. I couldn't mount my phone sideways.

Rob said...

Correction, the video shows south-west to north-east.

Another thing, I think the actual problem on this crossing is not caused by the cycle lanes, but by the seemingly large amount of car infrastructure. I don't really understand why a big turbo roundabout is needed at this location, I've never seen it much more busy than on the video (it's around 17:30).

I'm pretty sure this roundabout for example sees much more traffic (not just cars, also lots of cyclists and pedestrians, there are 4 schools nearby and several bus stops), yet it only needs a single (wide) lane: The original layout can still be seen on Google maps: Google Streetview shows the new situation. The original crossing was terrible, the roundabout seems to work better. There are no sharp corners for any of the users, and the speed of cars is limited by the relatively small radius.

Would be interesting to find information about why a turbo roundabout was needed at all, and why a typical small roundabout works in my example.