Though much of their construction was funded by the federal government, Interstate Highways are owned by the state in which they were built. All Interstates must meet specific standards such as having controlled access, avoiding at-grade intersections, and complying with federal traffic sign specifications. Interstate Highways use a numbering scheme in which primary Interstates are assigned one- or two-digit numbers, and shorter routes are assigned three-digit numbers where the last two digits match the parent route. The Interstate Highway System is partially financed through the Highway Trust Fund, which itself is funded by a federal fuel tax. Though federal legislation initially banned the collection of tolls, some Interstate routes are toll roads.