By Kristina M. Currans
In August 2014, the Institute of Transportation Engineers released the 3rd edition of the Trip Generation Handbook, a 352-page text that has traditionally, until only recently, provided guidance on estimating vehicle trips generated from new development. Among other updates, this new edition includes new chapters summarizing the most recent research and methods developed allowing users to account for people (not just vehicles) in trip generation estimation practices. This industry’s transition to estimating and understanding the “people” traveling to development has been in high demand from communities looking to accommodate multimodal travel, but there still remain a number of limitations in the guidelines presented.
One such limitation is that we currently have few data that allow us to directly estimate person trip rates. Instead we are often required to: (1) estimate vehicle trips using suburban vehicle trip rate data, defined as a “base rate”; (2) convert the vehicle trips into an estimated person trips using an assumed mode share and vehicle occupancy rate from the ITE’s suburban vehicle-oriented data, and; (3) reallocate the estimated person trips into different modes (bike, walk, drive, transit, etc.) based on the urban context of the development. This “direct mode share adjustment” forces the user, first, to assume that person trip rates do not vary across urban contexts and, second, to assume...Read more