Notes from Martin Catala:

In order for the transit score to be meaningful there is a need to assess three things:

How close are you to transit (distance)

How readily available is it? (frequency)

Can it get to where I need to go? (this is the hard one)

 

What makes this so hard is based on the purpose of the trip.  Often there is a tendency to address only the work trip.  Which only accounts for 15% of all trips and only about 18% of the miles traveled.  (see below from the State of Commute) Most American’s perform what is called trip chaining. 

 

However, I believe the WalkScore addresses this by providing the number of businesses within a given distance or commute shed. 

So what I would say to do is this (feasibility aside). 

The index would be based on:

1.       Availability and Frequency of Transit Service—

a.       Something like: Number of possible buses that are within ½ mile distance of a given address.  For example, there are three bus stops within ½ mile of a given address and the during the course of a day each stop is served by a bus 12 times resulting in 36 bus opportunities. 

                                                              i.      You could consider ¼ mile for bus transit and ½ mile for rail transit.  That is the traditional approach. 

b.      Considering you are looking at GTFS data I assume you could develop a weighted score based on peak commute time opportunities. 

                                                              i.      Haven’t thought this through…but buses available at peak travel times would be assigned higher score or those not in peak a lower score. 

2.       Access to destinations

a.       Number of business to be reached by transit within 25 minutes. (the average commuting time in America) from that location.  I am guessing creating a 25 minute transit commute shed and selecting which fall within that shed. 

Based on data from the National Household Travel Survey 2001