Transportation Performance Metrics: The Value of Accessibility

Back in 2013, I authored an article on the value of accessibility, aka access to destinations, as a transportation performance metric. Accessibility basically measures the ability to access jobs, goods, or services in a community. While there are many formulations, a simple accessibility metric would be the number of jobs reachable by a 30 minute commute. Over and entire region, one might calculate the average number of jobs a resident could reach. One can analyze specific modes, such as personal automobile or transit, and one can compute a “modal accessibility gap” based on the difference in the results. More complex metrics can look at jobs, health care, shopping, and other destinations.

Accessibility is an important metric because it directly measures the desired outcome of travel. One rarely travels solely for the purpose of traveling. Travel is a derived demand, arising as a mechanism to fill other needs. Other metrics such as delay, average speed, and travel time measure the means, but not the outcome. This also means that it is independent of the approach taken to improve the outcome. It will capture the outcome of urban planning to place more jobs closer to residents, as well as adding or deleting transit service, increased congestion, or improved travel speeds from adding lanes to an existing freeway.

Although know in academic circles, until recently it was not widely used, as it requires large data sets and large computing capacity to calculate. However both of these obstacles have been overcome, making even block by block calculations in a metropolitan area feasible. One of the leading practitioners is the Accessibility Observatory at the University of Minnesota. For more information on accessibility, you can read the article.