Source: adapted from W.P. Anderson et al (1995) Simulating Automobile Emissions in an Integrated Urban Model, Paper presented at the Conference of the American Association of Geographers, Chicago, Illinois.

Automobile Emission Factors

• Characteristics of vehicles. The weight of the vehicle and its load has a direct impact on combustion, along with its aerodynamics and the friction between its parts (gear, transmission, brakes, wheels, etc.). As a vehicle ages, it tends to consume more fuel because its parts become less efficient. Technological improvements, like catalytic converters, have significantly reduced CO, CO₂, and HC emissions but have increased NO_x emissions.
• Driving characteristics. The optimal speed of a vehicle ranges around 60 km per hour, so a transportation system permitting optimal travel speeds and uninterrupted flows would provide minimal combustion levels and emission factors. Obviously real road conditions, particularly in urban areas, do not enable optimal travel speeds. Congestion, traffic signals and freeways have a high tendency to discriminate travel speeds, and thus raise combustion and emission levels. The majority of urban vehicle-kms traveled are at speeds other than the optimal.
• Atmospheric conditions. They influence combustion in a direct manner, where low temperatures increase combustion, and indirectly by influencing road conditions. For instance, during winter car engines require more energy to warm up (cold start) and keep comfortable conditions for passengers. Snow and rain accumulation, wind and impaired visibility increase combustion. Cooling systems for the engine and the passenger consume more energy as the ambient temperature rises. Further, cold temperatures increase CO and HC emissions.