The Transport and Road Research Laboratory was commissioned by the Secretary of State of Transport to evaluate the effects on parking and traffic behaviour of the use of wheel clamps to enforce parking regulations in central London. The report describes the results of the evaluation which was based on before and after studies. The most important conclusions were that although the number of illegal acts on yellow lines did not change appreciably the durations of parking acts and the parking density on yellow lines declined by 30-40 per cent, and the time taken up by illegal parking on yellow lines fell by 40 per cent. Changes in behaviour at parking meters were small and did not generally differ from changes in control areas. At residents' bays within the clamping zone the percentage of acts which were illegal fell from 35 per cent by about a third. Following the reductions in roadside parking densities on yellow lines, the journey times of test vehicles travelling at average traffic speeds within the road network declined with respect to equivalent measurements in control areas. The annual saving in motorists' time costs over the clamping zone as a whole was estimated to lie between about 9M and 15M and the resource cost saving (fuel, etc) at between about 2M and 3M (1979) prices. (A)

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