You're frustrated that your customers don't have a place to park, in your private parking lot, because "unauthorized" vehicles are parked there. Before installing those signs saying, "unauthorized vehicles towed at owner's expense," keep in mind these signs should include, at a minimum, the following:
Restrictions on parking
Disposition of vehicles found in violation of the parking restrictions
Maximum cost to the violator, including storage fees and any other charges that could result from the disposition of a vehicle parked in violation of parking restrictions
Telephone number and address where the violator can locate the violator's vehicle
For more information, refer to the Arizona Revised Statutes, 1998 Edition, Title 9-499.05.