Fares on BART are comparable to those of commuter rail systems and are higher than those of most subways, especially for long trips. The fare is based on a formula that takes into account both the length of the trip and the counties passed through. A surcharge is added for trips traveling through the Transbay Tube, to Oakland International Airport, to San Francisco International Airport, and/or through San Mateo County, a county that is not a member of the San Francisco Bay Area Rapid Transit District.