Introduction

I have a basic mistrust of lateral deflections as I feel that they can be dangerous in themselves. We all know of bends where drivers fail to cope with the sharpness of the curve and the result can be loss of control often with serious results. The lateral deflection at roundabouts is linked up with the need for drivers to be prepared to give way and this very important fact is what tends to make roundabouts so effective. But physical features on their own can be risky.

There are several types of lateral deflection in use and these are essentially chicanes. For a vehicle path to be forced into a curve there need to be at least three points restraining the vehicle like bending a beam. The chicane is best operated for each direction of traffic (assuming 2-way) and there will need to be a feature in the middle of the road at some point to achieve the desired curve.

So far all of the devices that I have seen use kerbed build-outs and central islands for the restraining features. But I would like to see some build-outs, which can be inconspicuous at the best of times, constructed in raised overrun so that the slightly errant driver does not immediately run the danger of hitting a kerb, rather the vehicle suffers a little vertical deflection. Mini-roundabouts have set a standard for this over a long period so why this has not been tried much in traffic calming schemes remains a mystery.

In rural areas where the roads are often used by agricultural machinery it is quite out of the question to use kerbed build-outs or central islands to constrain speeds in this way as these would obstruct such traffic, so this is where I believe that there is good scope for using raised areas designed to be safely overrun. Drivers then have the choice: go over or around.

The lateral deflection to constrain speeds to 30mph has always been taken as 100m radius on level ground representing a sideways acceleration of 0.2g as the comfort limit. This figure is itself speed dependent. It is known that drivers find such sideways forces quite unacceptable at higher speeds but at speeds below 30mph they will accept higher sideways g-forces. This brings me to the all-important point about super-elevation. Many new roads are built with this even in urban areas presumably to "assist" drivers. The result of such design is inevitably higher speed so that is yet another example of the kind of muddle-thinking that I keep coming across in traffic engineering. There is a need not to assist drivers to keep their speed up and this applies to roundabouts too where I am against draining the circulating carriageway towards the central island for all the reasons set out in my book on mini-roundabouts, but in particular to keep speeds down.

Defining the "track" and single lane dualling

I think this is one of the most exciting prospects in traffic calming. It is in essence what has been done at Borehamwood for almost all of Shenley Road, the main shopping street. Here a central reservation has been constructed and in effect this provides a (nearly) continuous refuge for pedestrians. The running track is just one lane wide so there is no room for overtaking. With frequent speed tables, which give some focus to pedestrian movements, crossing the road is easy and is done with virtually no delay.

There are many high streets in the UK and elsewhere where this principle could be used and it can also be done on the narrower one-way streets where so often each element of the road is just the wrong width: the carriageway is too narrow for parking or loading but the footways are too narrow for pedestrians but the running track is too wide. In so many of these streets the overall width varies so a running track can be set for 3-3.5m with parking or loading bays added where there is sufficient width over and above the chosen width for pedestrians.

Single track one-way working

Fig a.1 illustrates the type of arrangement so common in towns and a method of dealing with it. Moving traffic can be kept to a minimum path and if the path is not straight there will need to be some widening to accommodate long vehicles. In this arrangement a kerbface should be retained but perhaps to a low height such as 50mm with drops or speed tables provided where pedestrian movements focus. Church St Falmouth, where the carriageway has been narrowed to a single running track. The area is subject to a loading prohibition at times.

Two-way traffic

Shenley Road, Borehamwood stands out, where there are two distinct running tracks and speed tables which are provided close to the main pedestrian desire lines. This scheme won the Urban Street Environment award 1994 and is a superb example of what can be achieved in an area where there is intense competition between motor traffic, pedestrians and cyclists. There are slight flaws in the detailing of the ramps onto and off the humps which will require maintenance and perhaps there could have been more soft landscaing of the central reservation instead of the continuous brickwork. Nevertheless, an important scheme, essential viewing, and most interesting, not a single traffic signal pole or Belisha Beacon in sight!

Single track - two way working

This is essentially a pinch-point and these are looked at in my page on them where I recommend keeping them very short, but there are many circumstances, often due to close proximity of buildings where there is a need to narrow the carriageway down and force shuttle working and if this is done over a significant length it can help pedestrians cross. This two-way bus lane in High Street, Exeter, has been in operation for many years and is easy to cross. Where traffic flows are higher it may be necessary to introduce traffic signals as are commonplace on narrow roads over or under bridges.

Every time the traffic flow reverses pedestrians can cross. This might be a good way of dealing with a narrow section of a small town or village high street where one-way flow is not possible but there is a need for wider footways so reducing a section to single track working.

Single track working (two-way, one-way or single lane dualling) presents an opportunity to identify the wheel tracks in specific materials. In any event the need for good structural integrity of the surface is paramount.

Getting drivers to slow down at entry points to villages and side-roads is my next page.