they are used to carry badly-damaged cars from crashes. Lift flatbed: a boom uses a wheel-lift frame to lift the vehicle vertically and load it on the bed. This type of truck can remove vehicles that are parallel-parked. It is commonly used in Europe. These are the most common arrangements; others also exist, such as flatbed units that offer a wheel-lift, boom trucks that can recover but not tow, and wheel-lift units that offer a combination boom with sling. There are also several sizes and weight categories of tow truck. The lightest models are usually based on light truck and van chassis offering boom and tow weights of around , making them ideal for car towing. Medium-duty tow trucks have a boom capacity of . Heavy-duty tow trucks, based on chassis used by semi-trucks, with multiple axles and the ability to tow fully-loaded semi-truck and trailer combinations, have a boom capacity from . Rotators are the heaviest type of tow truck, ranging from (though lighter models do exist) and often come with many other features per customer specification. Most flatbed-type vehicles are based on medium-duty and heavy-duty trucks to provide the chassis strength necessary to carry entire vehicles. Operations Tow trucks are usually operated by private businesses, except for major highways and toll roads, where the road authority may operate the tow trucks for that stretch of road. Some police departments own tow trucks; in the US, however, it is common to contract police tows to private companies. Businesses who operate a large fleet of vehicles, such as school bus companies or package delivery services, often own one or several tow trucks for the purposes of towing their own vehicles. Government departments with large fleets (such as the police departments, fire departments, transportation authorities and departments of public works of major cities) may similarly own one or more tow trucks. Police department tow trucks may also be used to impound other vehicles. Heavy tow trucks are often called to clear semi-truck accidents and straighten out jackknifed trucks. In rural or unorganized areas, companies which operate tow trucks can sometimes also offer additional services appropriate to highway clearance where government-provided ones are unavailable, such as fire suppression. The military also deploys tow trucks for recovery of stranded vehicles. In the US Army, a variant of the HEMTT truck is used for this purpose, the M984 wrecker. For recovery in combat situations