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:
(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:
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.
UPDATE, 14 AUGUST 2019: a Freedom of Information request shows that although traffic over neighbouring bridges has increased, around 9,500 car journeys a day have evaporated. See www.hammersmithbridge.org.uk/n/n42/changes-in-traffic-flow.
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