Traffic management schemes that are designed to increase travel by public transport, such as park-and-ride and bus priority, can receive considerable support as a means of reducing the dominance of the car in urban areas, and hence reducing the impact of road traffic on air quality. However, in order to assess the impact of increasing bus usage on urban air quality, detailed knowledge is required of the behaviour of emissions from buses under a range of operating conditions. The applicability of current bus emission data to the assessment of traffic management schemes is uncertain, and this Review is presented as the first step in the identification of the most appropriate information for the task. The Review briefly discusses the factors that influence the fuel consumption and exhaust emissions of buses, and the test cycles on which emission measurements are based - including steady-state and transient engine test cycles, and chassis dynamometer driving cycles for vehicles. The results from published vehicle-based emission tests are summarised. The techniques available for modelling bus emissions are also examined. (A)

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