Don't leave any wiggle room in your policies regarding when employees can use company vehicles for personal use. Make those policies clear and precise, then train employees accordingly.
If you don't provide understandable direction to employees, a court could interpret that as implied consent allowing employees to drive company cars for personal use. As a result, your organization would become liable for crashes that occur during that personal-driving time.
Recent case: Roseville Toyota employee Derrick Lewis drove a car owned by the California dealership to run a personal errand on his lunch break. Lewis rear-ended another car and injured a couple in that car. The couple sued Roseville Honda, and the court awarded them $277,662 in damages.
The court said the company's employee handbook had no specifics regarding personal use of company property. It also cited the company's lack of protocol for employees using company cars, such as a driving log or review of gas records. That indifference, the court said, created an inference of permission to use company vehicles for personal use. (Taylor v. Roseville Toyota, 2006, Cal. App., C050008)