We are looking at two-way working here where traffic from opposite directions must share the same carriageway space for a given length of road. Shuttle working is well understood in the context of a temporary situation at road-works but here there is usually an implied priority for one traffic stream over the other or there are temporary traffic signals to control traffic. While signals could be and are used in the permanent situation let’s look first at the situation where traffic streams are uncontrolled and there is no give-way system.

The drawing illustrates the type of arrangement I have used in conjunction with single speed cushions. In this case the feature was repeated along the road at about 90m intervals subject to driveway access positions to prevent reported speeds of over 50mph. The arrangement was chosen partly because the route is used by emergency vehicles, the road is not extensively parked and vertical deflections were to be avoided (unless they were emergency vehicle friendly). Special features here include also the cycle bypass something I always recommend when building out the kerbline in this way. The scheme is operating successfully with speeds controlled and emergency vehicles suffering minimal loss of time.

But shuttle working will not guarantee steady traffic speeds because of the need for drivers occasionally to give way to each other. Good drivers will "read" the situation and adjust to minimise speed changes while negotiating the road but it could be said that it is not traffic calming at its best. However there is one strong plus and that is that there is often more than one reason to give way:

• to negotiate the physical nature of the obstruction, and
• to give way to opposing traffic.

All this keeps drivers alert and out of auto-pilot, a good plus for road safety.

Priority working

So what about priorities at shuttles? I feel that highway authorities like to "cover" themselves and so allocate a priority system often unnecessarily, despite the public demanding some means of knowing who has right of way. So think very carefully about this. There will be special cases where priority signs are needed as there may be a junction nearby which must be kept clear or perhaps where traffic would otherwise lock up. Marlow Bridge over the Thames has a priority system has worked well for many years, necessitated by the narrow arches under the bridge towers. Vehicles on the bridge have priority to pass off it; the capacity is quite high. The danger of allocating a priority especially at an isolated shuttle is that some drivers become over-confident of their priority and if this happens while another driver is hesitant there may be a conflict and a risk of accident.

The pinch points at shuttle working with a priority system should not be of any significant length otherwise traffic can get stuck in a ‘follow my leader’ mode when two or more drivers follow one another because the opposing stream cannot break in even if they have priority often resulting in lock-up. Photo c illustrates this problem on Lower Rainham Road Gillingham where parking bays have been provided which effectively extend each shuttle such that the pinch-point has now become a pinch-length. Try to avoid this if possible. An otherwise worthwhile scheme will probably be modified because of this error of concept. The strict UK definition of ‘give way’ has been altered to allow the use of the give-way markings at shuttle working to supplement the current signs in the regulations which are in themselves frequently misunderstood. If your priority working does not have give-way markings and a triangle then do install them. But, try to make your give-way line follow the line of the off side of the priority vehicle path so that it is much clearer who has right of way.

A practical system of alternate priorities has been installed in Somerset at Stratton-on-the-Fosse. However I regret the failure to provide facilities for cycles to bypass the pinch-points.

A scheme south of Tavistock on a minor road parallel to the A386 has reduced flows and constrained speeds, by reducing the carriageway width using temporary bunding. Initially the scheme was introduced just using soil to create verges but these were easily overrun. The layout has in effect provided passing bays very obviously on alternating sides of the road and single track operation. ALthough a somewhat hopeful 30mph speed limit was installed along the route, experience suggests that this is reasonably complied with especially when it is needed most and that is when traffic demand is at its highest. Designers of such schemes will need to accept that during off-peak periods speeds may increase alarmingly without opposing traffic, vertical or lateral deflections.

Shuttle working using signals

Priority working with traffic signals can be an excellent way of dealing with quite a number of problems. Signals were installed in Broad Lane Bracknell to deal with a rather difficult problem of relatively low vans striking an arch railway bridge and head on collisions. The additional footway provided dealt with a pedestrian problem and speeds were to some extent contained by the overall scheme.

In general signal control of shuttles will have some traffic calming effect as some drivers are forced to stop. If they were driving faster than they appreciate they will usually re-start at lower speed at least for a time. Also the feature represents an additional meaningful "node" in the network which usually means reduced speed at the location at least.