Freeway

All freeways share a number of common features. A freeway has at least four lanes, two lanes in each direction. Many freeways, however, feature more than four, and as many as ten lanes, especially as they near the cities. Lanes are required to be from 11-12 ft (3.35-3.66 m) wide. Shoulder lanes provided on each side of the driving lane for each direction of the freeway allow vehicles to safely leave the traffic stream in the event of an emergency. Shoulder lanes are generally 8-10 ft (2.4-3 m) wide. A median, or center strip, separates the opposing An aerial shot of a freeway system under construction in southern California. Photograph by Tom Carroll. Phototake NYC. Reproduced by permission.
directions of traffic. Medians may vary from 16-60 ft (4.9-18.3 m) wide. The median improves safety by preventing head-on collisions of automobiles traveling toward each other.

Freeways are called controlled access highways. This means that 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 roads, from 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. Generally, one road, usually the road minor to the freeway, is raised on a grade, or slope, so that it is higher than the freeway, and allowed to cross it over a bridge. Ramps are constructed to lead the crossing road to the grade separation. Additional access ramps, often called on-ramps and offramps, connect the freeway to the intersecting road. They allow vehicles entering the freeway to accelerate to the proper speed before merging with the freeway traffic; the ramps allow vehicles leaving the freeway to decelerate to the slower speeds of the crossing road.

As part of the FIHS, freeways are designated by red, white, and blue signs in the shape of shields. Freeways are also numbered, with the numbering system used to indicate the direction of the road. Freeways traveling in an east-west direction are given even numbers; those traveling north-south are given odd numbers.