Transit service spatial availability is a key parameter in attracting ridership. The TCRP Transit Capacity and Quality of Service Manual (TCQSM) provides a simple methodology to analyze service coverage level of service (SC-LOS) for fixed-route transit service. SC-LOS is determined from the percentage of transit-supportive area (TSA) that has transit service coverage, that is, that lies within a certain walking distance from a bus stop. The TCQSM method of determining TSA is based on zonal demographic and economic data and may not lead to the most accurate estimation of SC-LOS for an area with a significant influx of transit passengers from external sources, such as from across an international border. When such a ridership pattern exists or is suspected, the onboard passenger origin-destination (O-D) survey method proposed in this research can be used to gather trip data to supplement the TCQSM's TSA estimates. Analysis showed that neglecting the data obtained from a recent onboard O-D survey conducted in the west side of the U.S. border city of El Paso, Texas, would result in an underestimation of the percentage of service area covered by transit and the SC-LOS.