Saturday, 2 November 2019

Dem Bones, Dem Bones...

Well a tenuous link to Halloween I guess, but this week, I'm looking at the 'dog bone' roundabout which is a bit of a curiosity.

The dog bone junction is a development of the dumbbell junction which is a grade separated (roads on different levels) arrangement consisting of a pair of roundabouts connected by a short road which goes above or below a main road with slip roads connecting the roundabout to the main road, like this junction on the A13 in East London;

The reason for the layout is to keep the high-speed main road flowing without delays and safety risks by having junctions at grade (on the same level). The photo below is a view of this type of junction where the main road is carried on a flyover with the roundabouts and local roads below. The arrangement can be reversed with the main road at a lower level.

The layout means that there's less road and structures work than might be the case with a larger junction such as a single roundabout under or over the main road. The example above is slightly unusual because of the length of the main road bridge, but local levels mean it's on a long viaduct. Here's a schematic of the local roads over the main road.

The problem with the arrangement comes when we want to take people walking and cycling across the junction because we are going to have to get people over the slip roads. In my example from the A13, I've sketched the shared walking and cycling routes which run through the junction in red;

This junction isn't too bad as the slip roads have toucan crossings, the usual UK problem though is that they take ages to get a green man/ cycle and so people tend to chance a gap in the traffic to get across.

If you look closely at the aerial images of the roundabouts, you'll notice the marks on the road from vehicle tyres which show the heaviest movements, highlighted here;

Clearly there are very few movements all the way round the roundabouts - in other words, people coming along the local roads, performing a U-turn at a roundabout and going back the same way - this gives a hint of how the dog bone roundabout works.

At the next junction to the west, this little-used area has been filled in (to form two "tear drops") and the roundabout is now one loop - welcome to the dog bone roundabout!

The junction was signalised and converted into a dog bone roundabout in order to provide a cycle route around it and to increase the junction capacity (because the arrangement gives more stacking space). It's arranged so that drivers getting into the correct lane at the approach will pop out at the correct exit. The cycle route taken over the slip roads and the access to the business park from the northern roundabout by toucan crossings again;

It's OK to cycle through , but the stop start of toucan crossings does make it hard work sometimes, although at peak time, there's no way you can find a gap in traffic.

So far, I've given some urban/ industrial area examples. There is a debate on whether we should be building urban roads like this at all, but if the infrastructure is already there, some retrofitting is more likely than a rebuild and of course, signalising gives a great opportunity for walking and cycling space. 

In theory one could take the walking and cycling route through the centre of the junction as shown below;

In this example, those not needing to access the industrial estate bypass the toucan so it's a bit more efficient and there's no dog-leg on the slip road crossings. However, the signalling needs careful thought in terms of where stop lines go for general traffic. 

Where the idea comes into its own is when we can grade separate the walking and cycling route and that is going to be on the edge of town or interurban locations where signals are less favoured; with the increasing use of "free flow" slip roads (known as Segregated Left Turn Lanes - SLTLs) it's even harder to get people through the junctions on foot and cycle (and I'll cover SLTLs in another post soon).

For junctions which are being changed to a dumbbell arrangement being added, I think they should be developed as dog bone layouts with grade separation for walking and cycling, assuming walking and cycling cannot be completely unbundled (and in many cases, the local roads are the desire line). The drawing below has the main road at a lower level than the local roads;

In this arrangement, the walking and cycling route (red) runs alongside the local roads and crosses into the centre of the tear drops before crossing over the main road in the centre of the bridge which carries the central road. This bypasses the slip roads completely and maintains a good desire line. Here's a variation with an SLTL on one quadrant of the junction which shows the scalability of the layout;

Grade separation means that drivers and people walking/ cycling are not held up when using the arrangement, but the key to the layout is the dog bone arrangement couple with taking the walking and cycling route through the centre of the junction - it might be a little less comfortable between traffic lanes passing either side, but with some offset for vehicle restraint barriers, I'd argue the loss of comfort it exceeded by the improvement in directness.

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