From OpenStreetMap Wiki
Access permission for Public Service Vehicles (psv).
PSV stands for Public Service Vehicles. Use this tag for roads which are designated to public buses etc.
- vehicles used in a passenger service (no matter how many seating positions they might have)
- taxi=* (taxi)
- vehicles with more than 12 seating positions (whether they're used for hire or reward or not)
- bus=* (a bus acting as a public service vehicle)
- heavy motor vehicles with more than nine seating positions