Secure your vehicle ignition with a code providing a definable time laps during which-one the vehicle can be used. This way you can secure your vehicles by an integrated timer defining when your vehicle can be used by whom.
For example you can determine that your vehicles can only be used from 6 am until 10 pm and only on weekdays. Or your vehicle can only be used from Monday until Friday next week.
If the allowed usage period is over, the user should request a new code allowing him to use the vehicle again (mostly after paying a new usage window).
By using a master-code you can reset allowed accesses or use the vehicle whenever you need to.