Sunday, 2 June 2019

A-Road Engineering

I am probably stating the bleeding obvious, but the type of highway design one sees on trunk roads has no business going anywhere near a town centre.

For some reason, though, we see high-speed layouts plucked from a rural A-road and dumped in our urban places. The approach is partly a legacy from how people used to design urban roads, partly how casualty-reduction schemes are designed, partly because the wrong people are working on the wrong schemes and partly because of the reference materials for design.

Today, my family and I headed off to the coast and as usual, I couldn't switch my brain off from engineering (I don't even bother trying) and the A133 between Colchester and Clacton-on-Sea got me thinking. Look at this layout;


There is nothing unremarkable about this layout, it's standard. A National speed limit and sensibly, right turning drivers are separated out from those who will be travelling faster. Right turners complete their turn in two parts in a controlled way - move into the right turn lane and then when there is a gap, turn into the side road. The layout is popularly known as a "right turn pocket".

Guess what, we are not the only ones to do this, here's a similar type of road in the Netherlands;


It's a very similar layout to what we see in the UK and it follows the same principle of separating people moving at different speeds. The A133 goes right into Clacton, but we still see hints of the big A-road design approach;


Anyone familiar with the Design Manual for Roads and Bridges and Chapter 5 of the Traffic Signs Manual (below) will be familiar with this type of treatment;


The town section of the A133 shown above has been like it for years and while it echoes the rural A-road design, it's very much in an urban place and my gut feeling it is less about separating right turners from a high speed road design context, but probably more likely as a response to addressing right turning collisions.

This type of standard treatment is still taught on "road safety" courses and it is used everywhere. When someone relatively inexperienced has been put through a standard road safety course and been shown Chapter 5, I think it is pretty certain that this layout will be picked off the shelf to deal with right turning collisions. It is also used for traffic flow reasons to get the right turners out of the way of other drivers.

In the urban context, we have people walking and cycling. Notwithstanding the fact that if we have urban streets with high levels of traffic, we should be separating cycling; the right turn pocket makes it harder for cycling. For example, the lane width is being squeezed and with a right turning vehicle in the pocket or with refuge/ traffic islands, we get a pinch point for cyclists;


As you can see, when the right turn pocket is in use, a cyclist has to get through a long pinch point. The pedestrian crossing ahead is single stage and doesn't necessarily need the island.

We often do find pedestrian refuges associated with right turn pockets as they help define the space for the various traffic lanes, but they help people cross the road in two halves which creates a conflict between walking and cycling.


Turning right from side streets is also made harder as one has to now cross three lanes of traffic and in my example, right turns have been banned - probably because the right turn is is busy and right turns out would block the main road and there is actually a signalised alternative on a parallel street.

So what is my answer? Well, by all means, keep the A-road engineering out on the rural A-roads, but when we get into the urban realm, we need to change our approach. Putting walking and cycling to one side, a very good way of managing right turning conflicts is to use a combination of filtered permeability and traffic signals.

If we manage right turns into a filtered area with traffic signals, we can let drivers out of the same filtered area without controls. This works because losing the rat-running traffic actually means fewer people turning into the filtered area in the first place (so less chance of holding up other drivers) and with fewer drivers leaving the same area, less interactions with main road traffic;


The image above shows a couple of urban A-roads in green and residential streets in grey. The junction to the bottom right is a filtered cycles only access (to stop drivers avoiding the roundabout). The middle junction on the north-south road is signalised to manage right turns into the filtered estate and the other two junctions are no entry from the north-south road (with a cycles exception). As with many of our urban problems, we need to look at how the network is operating first and then design from there.

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