In reply to tony:
"There is a growing body of evidence that where well-planned measures to reduce road space for private cars are implemented in congested areas and where no alternative network capacity is available, over the long term the predicted traffic chaos does not occur. This evidence is most notably presented in an important report commissioned by the UK Department for Environment, Transport and the Regions and London Transport.
‘Traffic impact of highway capacity reductions — assessment of the evidence (1998)’ S. Cairns, C. Hass-Klau and P. B. Goodwin
Data taken from nearly 100 locations showed traffic chaos to be limited to a ‘settling in period’.
• Wide range of results, with a 25 % average overall reduction in traffic and a 14 % median reduction in traffic (i.e.‘traffic evaporation’).
• A proportion of traffic which had previously used the affected road(s) could not be found in neighbouring streets.
Traffic evaporation is likely to occur where road space has been reduced for private cars and where, due to general traffic levels or the design and area covered, drivers cannot find:
• an alternative route, or
• an alternative time of day to travel, without experiencing severe congestion (recognising that driver behaviour will also be affected by additional factors such as the availability of alternatives including avoiding the need to travel or making use of public transport).
Contrary to widespread assumptions car drivers adapt to changes in road conditions in highly complex ways which computer models cannot accurately predict.
Short term
• initial cramming of roads was followed by searching for alternative routes and times to travel.
Medium term
• More varied and flexible trip-planning;
• changing mode of transport;
• reviewing the need to travel;
• trip combining.
Longer term
• switching locations of activities or even home or workplace.
Individually or in combination these diverse driver responses to congestion can result in a proportion of traffic ‘evaporating’"
http://ec.europa.eu/environment/pubs/pdf/streets_people.pdf