Disappearing traffic

Disappearing traffic, also sometimes referred to as suppressed traffic or traffic evaporation, relates to the observation that when highway capacity is reduced (typically due to provision of lanes for buses, street-running trams or bicycles, wider pavements (sidewalks), pedestrianisation, closures for road maintenance, or natural disasters), some proportion of the traffic disappears, resulting in fewer problems of congestion than had been expected.

Studies

In 1994, the UK advisory committee SACTRA carried out a major review of the effect of increasing road capacity, and reported that the evidence suggested such increases often resulted in substantial increases in the volume of traffic.^[1] Following this, London Transport and the Department of the Environment, Transport and the Regions commissioned a study to see if the reverse also occurred, namely that when road capacity was reduced, there would be a reduction in traffic. This follow-up study was carried out by Sally Cairns, Carmen Hass-Klau and Phil Goodwin, with an Annex by Ryuichi Kitamura, Toshiyuki Yamamoto and Satoshi Fujii, and published as a book in 1998.^[2] A third study was carried out by Sally Cairns, Steve Atkins and Phil Goodwin, and published in the journal Municipal Engineer in 2002;^[3] this paper was awarded the George Stephenson Gold Medal by the Institution of Mechanical Engineers in 2003.

The 1998 study referred to about 150 sources of evidence, of which the most important were about 60 case studies in the UK, Germany, Austria, Switzerland, Italy, The Netherlands, Sweden, Norway, the US, Canada, Tasmania and Japan. They included major town centre traffic schemes to make pedestrian areas closed to traffic, bus priority measures (especially bus lanes), bridge and road closures for maintenance, and closures due to natural disasters, mostly earthquakes. The 2002 study added some extra case studies, including some involving cycle lanes. The Annex by Kitamura and his colleagues reported a detailed study of the effects of the Hanshin-Awaji earthquake in Japan.

Taking the results as a whole, there was an average reduction of 41% of the traffic flows on the roads whose capacity had been reduced, of which rather less than half could be detected as reappearing on alternative routes. Thus, on average, about 25% of the traffic disappeared. Analysis of surveys and traffic counts indicated that the disappearance was accounted for by between 15 and 20 different behavioural responses, including changing to other modes of transport, changing to other destinations, a reduction in the frequency of trips, and car-sharing. There was a large variation around these average results, with the biggest effects seen in large-scale pedestrianisation in German town centres, and the smallest seen in small-scale temporary closures with good alternative routes, and small reductions in capacity in uncongested streets. In a few cases, there was actually an increase in the volume of traffic, notably in towns which had closed some town centre roads at the same time as opening a new by-pass.

Cairns et al. concluded that:

...the findings reinforce the overall conclusion of the original study—namely, that well-designed and well-implemented schemes to reallocate roadspace away from general traffic can help to improve conditions for pedestrians, cyclists or public transport users, without significantly increasing congestion or other related problems.^[3]

References

1. Wood, Derek; Standing Advisory Committee on Trunk Road Assessment (1994). Trunk Roads and the Generation of Traffic. London: HMSO. p. 242. ISBN 0-11-551613-1.
2. Cairns, Sally; Hass-Klau, Carmen; Goodwin, Phil (1998). Traffic Impact of Highway Capacity Reductions: Assessment of the Evidence. London: Landor Publishing. p. 261. ISBN 1-899650-10-5.
3. Cairns, Sally; Atkins, Stephen; Goodwin, Phil (2002). "Disappearing traffic? The story so far". Municipal Engineer 151 (1): 13–22.