Car Idling

I am responsible for a fleet of several governmental light trucks. Employees say they need a new truck when there present vehicle has 70 or 80 thousand miles on it. Although that amount of mileage is not so hugh they add that the vehicle has a lot of idling time, greatly increaseing the wear and tear on the truck. My question if “Does idling time have a large impact on an engine?” The employees say it doubles the mileage.

Why do they say they need a new truck and what data to they have to support their opinion?  Idle time or not that is very low mileage. 

 Factors that you should consider is the breakdown rate of the trucks.  The maintenance of the trucks (any truck that idles a lot should get maintained on an accelerated basis), the reliability of the trucks (how often do they fail)  How critical would a failure be (EMS would be very critical, trash hauling would be much less critical).  How important is it for the trucks to idle that long?  What is failing on the trucks? A flat tire, a fuel pump or a piston?  How long do they idle vs how long are the driven  Do you get 20 miles a day and 15 hours idle time or do they drive 500 miles and idle 30 minutes?