A client of mine is a courier. they want an estimate of the travel time between a client location and a courier (using a mobile phone that regularly uploads gps co-ords to a filemaker table). Courier may be on a bicycle or in a car. The desire is to prioritize dispatching to people who are in a time-based vicinity of the client.
So, you can assume there's a "Trip" table, with client_gps field, and courier table with "current_location_gps" field. and a time_between calculation.