One problem I have encountered is with the Vehicles table.
There are some tables that are related to this table, for example Accidents, Departments, Shifts, VehicleMaintainanceRecords.

The primary key to Vehicles table is the Unit#. Now say for example one of these vehicles get totalled or is replaced with a brand new vehicle and we use one of the old unit numbers to a vehicle that has not existed in quite some time. When we lookup accident history on a unit number or look at maintainance records on a unit number, we will not see the correct history.
For example say Unit# 50 had been used on a 1998 Ford F250 many years ago and that vehicle was totalled and say many years down the road we get a new vehicle in and use Unit #50 on a 2009 Chevy 2500. We will no longer have information on the Ford only the Chevy.

I hope I didn't confuse anyone with what I am trying to say.

Thanks,
jc