Foreign keys in the Location file

[levyj]

Our unique ID for vehicles is VIN, not license plate, so we may want to add it to the Location file spec.

As an alternative, should we just have unique IDs for each session (and possibly trip, which already has Shared Trip ID) and require those the the Location file instead of driver and vehicle? It seems as if the conceptual scope for Location is a session, not a vehicle or driver.