Freeway maps include the superhighways, motorways, expressways, and often toll roads (where fees are paid for road usage). All freeways are roads specifically designed to allow for the free flow of traffic and typically feature four or more traffic lanes in each direction, medians to divide the opposing directions, full access control, a system of ramps to prevent merging and diverging traffic from interrupting the traffic flow, and grading to separate intersecting traffic on other roads. As of the 2000s, about one-third of the total number of miles driven on roads utilize a freeway system. Many freeways now contain up to sixteen or eighteen lanes, especially as they near the cities. Lanes are required to be a particular width. Shoulder lanes, also of a specified width are provided on each side of the driving lane for each direction of the freeway to allow vehicles to safely leave the traffic stream in the event of an emergency. A median, or center strip, separates the opposing directions of traffic. Medians may vary in width but the median improves safety by preventing head-on collisions of automobiles traveling toward each other.
Freeway traffic is limited in where it may come onto or leave the freeway. These entrance and exit points are referred to as interchanges. Minor roads and driveways are diverted away from the freeway so that their traffic does not interfere with the freeway traffic flow. Many small local roads and streets to other highways in the freeway system, intersect with a freeway. Grade separation prevents the intersection of two roads traveling crossways to each other from interrupting each others’ traffic flow.