As gas prices are rising, more and more people will make the smart decision to use transit. Walk Score has released its first ranking of 25 U.S. cities by how transit-friendly they are, not only in the quantity of transit options, but by how accessible they actually are to citizens. They determine this using some pretty sophisticated algorithms and heat maps (more on that below), making it easier for people who are moving to pick a new home that has good accessibility to transit, and easier for city officials to figure out what the transit weak-links are in their cities.
How the Ranking WorksThey aren't just taking guesses to come up with these scores. It's a pretty sohpisticated system:
Calculating the Raw Transit Score
To calculate a raw Transit Score, we sum the value of all of the nearby routes. The value of a route is defined as the service level (frequency per week) multiplied by the mode weight (heavy/light rail is weighted 2X, ferry/cable car/other are 1.5X, and bus is 1X) multiplied by a distance penalty. The distance penalty calculates the distance to the nearest stop on a route and then uses the same distance decay function as the Walk Score algorithm.
Normalizing Scores from 0 to 100
Since any measure of transit infrastructure (number of stops, number of weekly trips, etc.) will have its own unique range, it is necessary to normalize the raw Transit Score to generate a Transit Score from 0 to 100.
The amount of transit infrastructure can vary by several orders of magnitude. Scales for measuring things that have an extremely large range of normal values (sound volume, earthquake intensity, etc) are typically logarithmic - a bus stop in a small town might see three trips a day, whereas downtown Manhattan might see tens of thousands. If Manhattan had a Transit Score of 100, then on a linear scale a small town's downtown might have a Transit Score of 0.01, whereas a logarithmic score might rate Manhattan as 100 and a small town as 10. The logarithmic score matches a rider's experience better: the added utility of one additional bus in a small town may exceed the addition of 10 new routes in downtown Manhattan.
In order to normalize from 0 to 100, we need to pick a "perfect score" location. To do this, we averaged the Transit Score of the center of a five U.S. cities where we had full transit data (San Francisco, Chicago, Boston, Portland, and Washington, D.C.) to create a canonical 100 Transit Score.
Here's San Francisco's transit heat map.
You can read more about the methodology here.