Traffic evaporation

A lot of the discussion about the downside of closing Hammersmith Bridge has centred around the problem of traffic being diverted to neighbouring areas, such as the Upper Richmond Road, Putney High Street and Putney and Chiswick bridges.

There was certainly an increase in the amount of rush-hour traffic in these areas in the period after the bridge was closed to motorised vehicles. Further changes have taken place, such as redirection of bus routes and the closure of back streets in East Sheen, that have also affected traffic flow.

I'm not aware of any official figures on the amount of traffic before and after the closure to motors. However, many people expect the traffic to revert to its previous levels after a period of time, because of a phenomenon known as traffic evaporation.

This can be summarised as follows: when the road capacity is reduced (in this case, by closing Hammersmith Bridge), the total amount of traffic is also reduced - so it doesn't necessarily all get diverted to other areas. Some of it simply disappears.

A study by Cairns, Atkins and Goodwin entitled "Disappearing traffic? The story so far" explains this much better than I can. The introduction goes as follows:

Reallocating roadspace from general traffic, to improve conditions for pedestrians or cyclists or buses or on-street light rail or other high-occupancy vehicles, is often predicted to cause major traffic problems on neighbouring streets. This paper reports on two phases of research, resulting in the examination of over 70 case studies of roadspace reallocation from eleven countries, and the collation of opinions from over 200 transport professionals worldwide.

The findings suggest that predictions of traffic problems are often unnecessarily alarmist, and that, given appropriate local circumstances, significant reductions in overall traffic levels can occur, with people making a far wider range of behavioural responses than has traditionally been assumed. Follow-up work has also highlighted the importance of managing how schemes are perceived by the public and reported in the media, with various lessons for avoiding problems.

Finally, the findings highlight that well-designed schemes to reallocate roadspace can often contribute to a multiplicity of different policy aims and objectives.

Download "Disappearing traffic? The story so far" (PDF)

(Note that one of the examples in the study is the closure of Hammersmith Bridge for one month in 1997, when traffic levels were seen to drop significantly.)

The possible reasons for a reduction in traffic given in the study are:

  1. Capacity increases on other routes, or changes in traffic management, or changes in driving style, which pack more vehicles into the same space.
  2. The reduction in capacity on the specific road or area (in this case, the bridge),  may be offset by adequate spare capacity on alternative routes or at other times of the day. Consequently, people may change their route or journey time, but the overall number of trips and vehicle mileage is likely to remain relatively unchanged.
  3. People changing their mode of travel, choosing to visit alternative destinations, changing the frequency of their journey, consolidating trips for different purposes, altering the allocation of tasks within a household to enable more efficient trip-making, car-sharing, or no longer making journeys (e.g. by working from home occasionally). Longer-term responses included changes in job location, changes in household location and changes in developers’ choice of location for new development.

The study highlighted that many of these changes are being made all the time anyway, for other reasons. Underlying aggregate traffic patterns, there is a complex ‘churn’ of individual turnover. Hence, for example, surveys of number-plates have shown that as many as 50% of cars on a major commuter route on two subsequent days can be different, even though overall traffic levels remain similar. This means that when road conditions change, a range of travellers are affected. Some are people who are used to making a particular journey in the same way every day, who are likely to be relatively resistant to changing their behaviour. However, many others will in any event be making a mixture of minor and major changes to their journey patterns. Hence, a roadspace reallocation may simply tip the balance in a decision that is being made for other reasons.

One important point in the report is that the problems with traffic levels after a change in roadspace will be minimised if the change is well planned, and advertised well in advance. Clearly, this hasn't been the case in the closure of Hammersmith Bridge to motor vehicles; and the situation has been further complicated by the subsequent changes in bus routes and side roads.

But the study shows that in almost every case, the total amount of traffic falls after a decrease in road capacity, and we have reason to hope that this will eventually happen here as well.

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