Sunday, March 19, 2017

Foundations of Evaluating Public Transit Networks, Introduction

When comparing two complex things of the same type, condensing the complex thing into a single comparative score can simplify decision making. This is a common strategy in a variety of disciplines. In sports, measurements like American football's Passer Rating and baseball's Wins Above Replacement (WAR) form a single score from many components of a player's performance. In the case of WAR, the combined season-long WARs of players on a team closely correlates with that team's number of wins. In this way, baseball players can be compared and ranked, even if they have very different sets of baseball skills. In real estate, Walk Score combines many elements of livability into a single score that homebuyers can use when comparing houses in different neighborhoods. Rotten Tomatoes uses a weighted aggregation of reviews to rank movies.

For transit planning, it would be helpful to have an analogous concept for evaluating how useful a public transit network is. Transit planners could use such a score in a variety of contexts. When considering multiple proposals to restructure transit service, an objective score can be used to select the better one. When a new transit line is planned, the score difference, between the transit network with the new line and without it, can be measured against the line's monetary cost to ensure that it is a good use of resources.

Of course, such a score is unlikely to be the final word in any public transit planning decision. Public transit agencies operate in the context of a government and community. The values of those entities may be extremely difficult to capture in a single score. Nevertheless, having scores available, and having tools that demonstrate how the scores were derived, can help guide conversations within an agency and among the community of riders. They can provide a compelling explanation of why certain decisions were made or why certain alternatives were not considered.

To create a score, a tempting choice may be to consider the number of riders as a score for how useful parts of the transit network are. Using ridership, however, makes it easy to draw erroneous conclusions. Low ridership can be cited as a reason to cancel a transit route, as some may attribute the lack of use to redundancy. However, the route might be fundamentally useful, but not often ridden because it does not run frequently enough for customers to rely on it. Additionally, doing any sort of speculative planning, such as adding new lines, requires projecting ridership, which is imprecise at best. On top of that, ridership is difficult to measure, requiring rider counters to work reliably and in a large enough sample to be representative.

Public Transit Analytics's core score does not use ridership. Instead, the score is motivated by one of the company's tenets: to help build transit networks that are more useful. A useful transportation improvement is one that allows more people to access more of the places that they desire to go. In this series of posts, I will discuss how Public Transit Analytics has derived its own process for measuring exactly that.