Three major factors influence the emission level of vehicles:
- Vehicle characteristics. The weight of the vehicle, including 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 and engine become less efficient. Technological improvements, like catalytic converters, have significantly reduced CO, CO2, and HC emissions but have increased NOx 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 do not enable optimal travel speeds, particularly in urban areas. Congestion, traffic signals, and freeways discriminate travel speeds and thus raise combustion and emission levels. The majority of urban vehicle-km traveled is at speeds other than the optimal speed for emission levels.
- 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 resulting from combustion.