A number of studies of the effect of traffic conditions on fuel consumption and emissions are reviewed. A model based on driving vehicles in traffic is described in which the fuel consumption of a vehicle in urban traffic is expressed as a simple function of trip speed. Data from a variety of sources, including additional field data, detailed computer simulation, the same vehicle tested on different fixed urban driving schedules, and small segments of the Federal Test Procedure (FTP) have been all shown to fit the model. A similar model of HC emissions as a simple function of trip speed is derived from analyzing small segments of FTP data. Data from a variety of sources, including published EPA relations, detailed computer simulation, and dynamometer replication of street data have been all shown to fit this model. No simple models were found for CO and NOx. In general, it is found that urban traffic, despite its seemingly disorganized appearance, yields rather consistent relations between fuel consumption and average speed, and between HC and average speed.