a theoretical study has been carried out to estimate the effects of varying the size of bus in the central london bus fleet and the degree of private car restraint on the overall travel costs in the area. the model used was a single-link model in which the central area of london was represented by the average main road of the area. for each size of bus considered the overall travel times for cars, buses, goods vehicles and taxis were calculated and costed. to these were added the operating costs of all vehicles on the network. this procedure was carried out for each size of bus in turn and the whole process repeated for each level of private car restraint considered, ranging from present conditions up to full restraint where no cars are left on the network. considering, initially, time and operating costs only, an optimum bus size for present conditions in central london was found to be a 55-seater bus and for conditions of complete restraint of private car traffic, a 40-seater bus. in these calculations (where time and operating costs only are considered) the greatest community savings occur under full restraint. however, if assumed values of the subjective costs which drivers bear on transferring to bus travel (because of loss of privacy, convenience and comfort etc) are introduced into the calculations, a level of restraint at rather less than half of all private cars appears to give the greatest community savings, and at this level of restraint the optimum bus size is again a 40-seater. the calculations suggested that the mean direct journey speed (origin to destination as the crow flies) of all bus and car travellers would be expected to rise from 6km/h at present to 71/5 km/h when about half of the present cars are restrained and to 8 km/h under complete restraint. complete restraint of private traffic would be expected to reduce total time and operating costs of the central london street network by about 20 m per year (18 per cent). if the subjective costs mentioned above are included, full restraint would be expected to increase total community costs to a level somewhat higher than the present level, and an optimum level of restraint as mentioned above is found at which the overall community saving would be expected to be about 10 million per year.(a)(paper presented at fifth international symposium on traffic flow theory and transportation. university of california, berkeley, june 16-18, 1971)

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