Using age structure to create projections of vehicle miles of travel in Rhode Island

This study investigated the relationship between demographic variables, specifically age and gender, and travel demand in Rhode Island. The data sources were the Census Bureau and the Nationwide Personal Transportation Survey (NPTS). Annual vehicle miles of travel (VMT) per driver was used as a measure of travel demand. The goal of the study was to assess the importance of incorporating age and gender into projections of greenhouse gases resulting from future personal vehicle use. I created projections of future VMT in Rhode Island through 2025, varying assumptions about population growth and change in VMT per driver, and either incorporating age structure or using only aggregate population. Values of VMT/driver in 1995, as well as the direction and magnitude of historical trends of VMT/driver, differed by age and gender. The range of travel demand projected for Rhode Island for 2025 was 8 to 12 billion vehicle miles of travel (compared to 7.9 billion in 2000). In projections where VMT/driver was held constant at 1995 levels, including age structure did not affect the outcome; however, when VMT/driver was assumed to grow according to past trends, age structure affected results substantially. The effect of including age structure was to increase projected VMT for Rhode Island as a whole by as much as 400 million VMT annually in 2025. There was also a marked difference between the projections I created and ones constructed by Tellus for the Rhode Island Greenhouse Gas Process. This study highlighted the importance of making well-founded assumptions in travel demand modeling, particularly about how VMT/driver will change over time. It also indicated that, if VMT/driver follows different trajectories of change for different age groups, including age structure may improve the accuracy of baseline travel demand projections for Rhode Island.