Transit availability—the opportunity to use transit service at a particular location—is a key determinant in transit use. If transit service is unavailable near a potential passenger’s origin and destination, it is not a viable travel option. The Florida Department of Transportation’s transit level-of-service (TLOS) indicator is a measure of transit availability that incorporates service coverage, frequency, and duration; the availability and quality of pedestrian routes to transit stops; and population and job density. The TLOS indicator’s basic concept is that at any given minute, a transit vehicle serves a small group of people; that is, those people who could board a vehicle when leaving their job site or residence that minute, walk no more than a specified distance to a transit stop, and wait no more than a specified time for a vehicle to arrive. Geographical information system–based software developed for the TLOS project can be applied to every transit vehicle for each minute in a day. The data can be compiled for time frames ranging from 15 minutes to 1 week to assess the amount of service for each part of a transit system’s service area. The TLOS indicator was tested in Tallahassee, Florida, and produced results compatible with, but more detailed than, the availability measures contained in the Transit Cooperative Research Program’s Transit Capacity and Quality of Service Manual. Potential applications include service evaluation, transportation modeling, and improvement of modal-split calculations.