WHICH COMES FIRST - THE TRAFFIC OR THE ROAD?

The article summarises a paper presented to the 1985 PTRC summer meeting describing a study of highway schemes in London the results of which suggest that new roads provide no significant relief in the immediate area but can create traffic problems elsewhere. Present methods of assessing the benefits of reduced congestion use a fixed trip matrix. However, traffic can be redistributed by a new road where longer journeys at higher speeds become possible and take the place of previous trips. Also if a new road runs parallel to a public transport service the new road could become more attractive to an individual than the public transport service. Similarly, trips by cycle or by foot can be switched if the cost of travelling by car becomes more acceptable. Improved orbital links could increase orbital car journeys at the expense of radial public transport trips. Results are given of several before and after surveys at major schemes in London. Short term studies tend to register mainly reassigned traffic; distribution and modal split effects combine to work through the system for months and even years. One such study showed that only a 4.5 per cent reduction was achieved by a road designed to be relieved by one of the largest schemes. See also IRRD 285682. (TRRL)