[sustran] Traffic Impact of Highway Capacity Reductions

[SUSTRAN Resource Centre]

In case sustran-discussers have not yet digested news of the ground-breaking research on 'Traffic Impact of Highway Capacity Reductions: Assessment of the evidence', from Phil Goodwin and team at University College London,   here is the synopsis which can be seen at:   

http://www.ucl.ac.uk/transport-studies/sc1.htm


'Traffic Impact of Highway Capacity Reductions: Assessment of the evidence'
by Cairns, Hass-Klau and Goodwin 1998, available from Landor Publishing, London.

In 1994, a government committee report showed that building roads can generate traffic. Since then, there has been a lot of interest in whether the opposite is true - can reducing road space for cars cut traffic ? This could be particularly important when introducing policies like bus lanes, which could provide a cheap and effective way to improve the attractiveness of public transport, but which would be untenable if displaced traffic brought neighbouring roads to a standstill. The same issue is often raised during plans to introduce street-running light rail systems, cycle lanes, wider footpaths or pedestrianisation schemes.

Therefore London Transport and the Department of Environment, Transport and the Regions (DETR) commissioned research to investigate the question, and employed a team at University College London (UCL) to look at the evidence (as reported here), and the consultancy MVA to look at the modelling implications (as reported elsewhere). Consequently the UCL team examined nearly 60 locations where road space had been taken away from cars and put to other use. Examples were studied from the UK, Germany, Switzerland, Italy, The Netherlands, Sweden, Norway, the USA, Canada, Tasmania and Japan. In some cases, road space for cars had been reduced because of deliberate policies like bus lanes or pedestrianisation, in others it was because of problems like roadworks. Irrespective of the cause, in such circumstances, there are often predictions of major traffic chaos.

Examination of the evidence suggested that these predictions rarely, if ever, prove accurate. Prolonged, long-term gridlock is simply not reported, although there can be short-term disruption, and some increase in problems on particular local roads. In many cases, there were actually significant reductions in the total amount of traffic on the networks studied. On average, 14-25% of the traffic that used to use the affected route, could not be found on the neighbouring streets. However, the results varied substantially, depending on the context. For example, where schemes made public transport more attractive, they were more likely to encourage people to change mode than those which did not.

In explaining what was happening to the traffic, the following model of behavioural response emerged. Initially, when road space for cars is reduced, drivers simply change their driving styles in ways which pack more vehicles in, for example, by driving closer together. As conditions deteriorate, they then take the next easiest options - swapping to neighbouring streets, or changing their time of travel, leaving a bit earlier or later to avoid the worst of the traffic. As such adjustments also become problematic, a whole variety of responses is triggered, ranging from people altering how they travel, or where they carry out activities, through to people moving house or moving job, where the change in travelling conditions 'tips the balance' in a decision that was being made for other reasons anyway. Taken together, this third set of responses accounts for the measurable 'disappearance' of a proportion of traffic from the networks studied.

The project also highlighted the amount of variability which underlies apparently stable traffic flows, and which enables people to change their travel habits. Specifically, individuals make adjustments to their travel behaviour on a fairly regular basis anyway, either because of minor factors (like the occasional decision to work from home, or to carry out one activity on the way to another), or because of more important decisions (like changes in car ownership or job location or house location), or because of longer-term, life-cycle events (like changes in household composition). Hence, when road space is reduced, some people are forced to alter a repeated, habitual pattern of behaviour, but other people are spontaneously reconsidering their travel options anyway, and can take account of changes in the network conditions as part of this process. It is this flexibility which enables surprisingly large changes in traffic flows to result from a particular change to road conditions. 

The findings from the project were welcomed by Gavin Strang, Minister for Transport, and the DETR aims to commission a good practice guide, to enable local authorities to take account of the results.