Goodness of service is measured in ridership per route mile. There are no other metrics.
If the service is good, people will ride it. The better the service, the more people will choose to ride it. Many things contribute to goodness of service--Speed, Frequency, and Reliability are all important.
Why per route mile? If half the route length is junk, the numbers
should reflect that. If cutting half the route destroys the whole route,
it is not a very good route.
There is no help for a route that does not connect to places people want to go. If it goes someplace useful, it will be ridden.
Ridership per route mile does not measure development effects. Development does not affect goodness of service. Development is caused by goodness of service.
For planned routes, for which not ridership can be given, accessibility to jobs and employment (as measured by density) is a reasonable proxy.
Transit use is mediated by the attractiveness of alternate forms of transportation. The difference in time and cost between transit and alternatives is critical. ie: Parking availability, congestion, gas prices.