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.