Flow improvements and vehicle emissions: effects of trip generation and emission control technology

This paper examines whether road schemes that increase the availability of road space or which smooth the flow of traffic result in increased vehicle pollution. Economic theory indicates that increases in road space and the consequent decreases in travel time will tend to increase total vehicular travel, an effect known as induced travel. The net impacts on vehicle pollution have largely been a matter of conjecture with some arguing that policies to reduce congestion (by adding more road space) will reduce pollution by smoothing the flow of traffic and reducing stop and go traffic, while others argue that induced traffic will overwhelm this effect. This paper uses a micro-simulation model, integrated with a modal emissions model, to evaluate the overall strategic policy question of how changes in available road capacity affects vehicle emissions. The analysis examines alternative vehicle fleets, ranging from a fleet with no emission control technology to relatively clean Tier 1 vehicles. Results show emission break-even points for carbon monoxide, hydrocarbons, nitrogen oxides, fuel consumption and carbon dioxide. Increased traffic is found to quickly diminish any initial emission reduction benefits.