Effective road and traffic management

Supplementary evidence from the Chartered Institute of Logistics and Transport in the UK (ETM 36a)

1. At its oral session on 29th March, the Select Committee asked whether bus lanes reduce congestion. The short answer is that bus lanes generally reduce congestion in terms of the aggregate delays to people (not necessarily vehicles) using a road but may well increase the length of traffic queues, depending on the circumstances. It may be helpful to the Committee if we set out in more detail why this is so.

2. The basic mechanism by which bus lanes can reduce congestion is by making bus services more attractive and bringing about a mode shift from car use. As a well-occupied bus can carry far more people than a car, buses can transport more people into city centres and other key locations than cars, a key policy consideration when vehicular traffic demand exceeds the capacity of the local roads. To illustrate this point, if all the passengers now travelling by bus into city centres were to switch to cars, there would in most cases be a large increase in congestion and in many places the roads would not be able to cope. Conversely, if people now travelling by car switched to bus, there would be a fall in the vehicular demand for road space.

3. But there are some very important provisos to this:

a. A bus lane may reduce capacity for other traffic on the section of road containing it. Delays to other traffic are often minimised by stopping the bus lane short of busy junctions (the main constraint on capacity in cities). For the bus lane to have a positive economic impact there needs to be a regular flow of well–patronised buses whose passengers benefit by more than the disbenefit suffered by car users. However, there are also likely to be environmental benefits resulting from any significant switch from car to bus.

b. In some cases (of which examples were referred to in paragraphs 12 and 15 of our initial memorandum) the bus lane will reduce congestion for other road users as well. For example instances have been recorded where the discipline imposed by the bus lane (principally through stopping lane switching) actually improves the flow of other traffic through saturated junctions.

c. But in some of the cases we cite, the initial drop in congestion led to a growth in non-bus traffic leaving the local congestion the same as before. Much of this extra traffic was found to have switched from other congested routes, but it is also possible that some traffic was generated rather than transferred, a familiar consequence of creating (actually releasing in this case) additional highway capacity.

d. The detailed design of bus priority measures is important. If the demand for non-bus traffic is great enough, bus lanes can cause traffic to block back to junctions further back, which prevents the buses from getting to the bus lane in the first place. This can be mitigated by using techniques like traffic signal priority for approaching buses or segregated "bus advance" lanes giving buses earlier green signals than general traffic at the preceding junctions – such techniques have been used successfully in London, York and elsewhere.

4. As a general observation, bus lanes (or other traffic management measures for that matter) implemented in isolation are rarely as effective as packages of complementary traffic management measures implemented together on a route or corridor basis. A good example of this is the London Bus Priority Network implemented by the London Boroughs in partnership with Transport for London.

5. The way to avoid the problems of generated or transferred traffic, referred to in paragraph 3c above, and to reinforce the mode switch from car to bus, is to use demand management measures to discourage car use in congested areas and keep overall traffic levels comfortably within the capacity of the network– for example by restricting the supply of all-day parking and/or increasing the price. The most successful park and ride schemes do this:

April 2011