The United States is covered with a convenient system of U.S. and Interstate highways. Interstates are always freeways (limited access; no grade crossings), while U.S. Highways may be freeways on some sections and not on others. These roads network between major (and minor) population centers, and can make it easy to cover long distances – or get to the other side of a large city – quickly. Primary Interstates have one- or two-digit numbers, with odd ones running north-south (e.g. I-5) and even ones running east-west (e.g. I-80). Three-digit interstate numbers designate shorter, secondary freeways. An odd first digit signifies a "spur" into or away from a city; an even first digit signifies a "loop" around a large city. The second two digits remain the same as the primary Interstate that travels nearby. The U.S. Highways are generally older routes that lead through town centers. In many cases, Interstates were constructed roughly parallel to U.S. Highways to expedite traffic that wishes to bypass the city.
Speed limits on the interstate highways can vary from state to state, as setting the speed limit is up to each individual state. For example, Interstate 70 from east to west is 65 miles per hour in Pennsylvania and Ohio, 70 miles per hour in Indiana, 65 miles per hour in Illinois, 70 miles per hour in Missouri and Kansas, and up to 75 miles per hour in Colorado and Utah.
The vast majority of interstates do not charge tolls, but those that do are also known as turnpikes. Tolls are also frequently levied for crossing notably large bridges or tunnels.
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