2. Network Management

Introduction

Whatever happens it will be one of the side effects of traffic calming that drivers try to divert from the traffic calmed roads. It is therefore essential that this is understood in the context of the network within which traffic calming is proposed. Bexley Council and no doubt many others, define clear areas (small networks) of roads within which they would expect traffic calming to have an effect and beyond which there would be no significant effect. This approach ensures that consultation can be defined to particular areas with little risk of the scheme having an adverse effect on those who have not been consulted. It ensures that the effects of traffic calming are identified and can then be acted upon as necessary.

Speed

But before going any further I believe that you should first define, perhaps in a subjective way, what you consider to be the maximum desirable speed for each length of road on your whole network. This may seem a huge task but once started it is not difficult and this will form a basis for consultation with the emergency services, bus operators, the public and the local authorities. I tried this first at Gillingham (Kent), and defined the speeds initially at 20, 30 and 40mph max. But this proved unsatisfactory, so I decided on 5mph increments and this seemed to work well; so six speed ranges were initially defined as illustrated in Table 2 with the last two added later.

Table 2 Speed profile by class of road
> 40 Main roads and dual carriageways
35-40 Some main roads and principal district distributers
30-35 Most principal and some local distributers
25-30 Urban roads including some local distributers
20-25 Local roads in residential areas, mixed use streets
15-20 Local roads outside schools, playing fields, busy shopping streets
10-15 Certain minor roads outside schools, busy shopping streets
<10 Home Zones

It is very important when considering the maximum speed profile not to base this on the speed that drivers can "make it" around the sharpest bend for example. This will fail; the speed must be below any such threshold.

It surprises me that I have not seen this process done very much; it seems a vital step in developing traffic calming for any road or area. If you do not define the maximum speed that you think drivers should comply with, how can you possibly know what device to install to bring about those maximum speeds? Cornwall CC have developed a policy which states that urban main roads and roads through villages shall be driven at speeds not exceeding 30mph and all roads in residential areas and some shopping centres shall not be driven at speeds above 20mph. They are installing these speed limits, and where drivers do not comply within a small tolerance traffic calming devices are to be installed to ensure that they do.

Flows

As part of your network management you will need to examine the critical junctions and establish the limits of the effects of the measures that you wish to undertake. Avoid being put off making a junction improvement on the grounds that it may "attract extra traffic through the road now better connected to the remainder of the network". If you do not wish the adverse effects of additional traffic on such a road you may need to consider traffic calming as part of the measures on that link rather than leave an inefficient or dangerous junction (node) in operation. The use of mini-roundabouts, well designed in accordance with my recommendations, will have the effect of reducing speeds at the critical points in the network where turning movements need to take place safely and allowing traffic movements to take place according to demand. Speeds between the nodes in the network will be controlled by the other measures outlined on these pages. Roundabouts at key junctions on the main network will help it to operate and may be a very important part of your strategy. In Wokingham, Berks, we were able to reduce rat-running traffic in the town centre so much by improving the junctions on the main roads that the (unpopular) measures for the town centre proved unnecessary.

HGVs

Controlling the speed and volume of HGVs relates to network management and the type of traffic calming features. Where HGVs continue to disturb villages detrimentally, it is important to look first at the network which serves their origins and destinations. Then, in the absence of any alternative, give some consideration to the use of deflections that can work in controlling their speeds such as long ramp speed tables, cushions or H-humps. Details of these are set out in the next section on vertical deflections.