Expanding the capacity of the overall transportation system through expansion of some key roads and highways, increased cost-efficient and convenient transit, and improved sidewalks and bike paths. The TRIP report also looked at the TTI and FHWA data to determine whether the addition of road capacity has an impact on traffic congestion levels. TRIP studied the data from the 68 cities cited in the TTI report in order to look at each metro areas' increase in congestion levels. Travel delays and road capacity were factored in comparison with population increase, to allow credible comparisons between slow and fast growing areas in terms of their ability to manage traffic congestion. Relative to population growth, areas that were more aggressive in increasing regional road capacity experienced congestion increases that were 40 percent less than traffic congestion increases experienced in areas that were less aggressive in adding lane mileage.
Traffic congestion increases as urban population density increases.
There was no evidence that a high level of road mileage expansion, relative to population increases, stimulates urban sprawl. In fact, urban areas that experienced the largest increase in urbanization between 1982 and 1997 actually experienced lower increases in lane mileage, relative to population growth, than the communities that experienced the smallest increases in urbanization. Metro areas that did the best job of minimizing travel delays caused by congestion expanded their road system at a rate more than double that of the metro areas that had the highest level of traffic delay increases, relative to population growth. The increase in traffic that occurs over time on expanded roads is the result of a variety of factors, such as drivers coming from other more congested roads, drivers changing their travel times, increased population, reductions in household sizes, increased labor force participation and increased prosperity and the preference for suburban lifestyles. A 1998 Federal Highway Administration report found that increased vehicle travel on expanded road capacity is largely the result of this diverted traffic, either from nearby routes or from shifts in travel times.