General Characteristics

High-capacity freeway interchange in Los Angeles, California.

Freeways, by definition, have no cross traffic in the form of other roads and railroads. Elimination of cross traffic is typically achieved with grade separation using underpasses and overpasses. In addition to sidewalks attached to roads that cross a freeway, specialized pedestrian bridges or tunnels may also be provided. These structures enable pedestrians and cyclists to cross the freeway without a long detour to the nearest motor vehicle crossing. Movable bridges are occasionally present on freeways, requiring drivers to yield to river traffic.

Access is typically provided only at interchanges, though lower-standard right-in/right-out access can be used for direct connections to side roads or driveways to adjacent property. In ideal cases, sophisticated interchanges allow for smooth, uninterrupted transitions between intersecting freeways. However, sometimes it is necessary to exit onto a surface road to transfer from one freeway to another. Exits are sometimes numbered to help drivers identify their exit.

Two-lane freeways, often undivided, are sometimes built when traffic volumes are low or right-of-way is limited; they may be designed for easy conversion to one side of a four-lane freeway. Otherwise, freeways typically have at least two lanes in each direction; some busy ones can have as many as 16 lanes or up to 18 for short distances. These wide freeways may use separate collector and express lanes to separate through traffic from local traffic, or special high-occupancy vehicle lanes, either as a special restriction on the innermost lane or a separate roadway, to encourage carpooling. These HOV lanes, or roadways open to all traffic, can be reversible lanes, providing more capacity in the direction of heavy traffic, and reversing direction before traffic switches. Sometimes a collector/distributor road, a shorter version of a local lane, shifts weaving between closely-spaced interchanges to a separate roadway or altogether eliminates it.

Freeways can have frontage roads, normal surface roads parallel to and on either side of the freeway, to provide access to adjacent properties. Frontage roads typically have one-way traffic flow in urban areas and two-way traffic flow in rural areas.

Except on some two-lane freeways (and very rarely on wider freeways), a median separates the opposite directions of traffic. This strip may be as simple as a grassy area, or may include a crash barrier such as a Jersey barrier to prevent head-on collisions. On some freeways, the two carriageways are built on different alignments; this may be done to make use of available corridors in a mountainous area or to provide narrower corridors through dense urban areas.

Speed limits are generally higher than on similar non-freeways, and are sometimes nonexistent. Because the high speeds reduce decision time, freeways are usually equipped with a larger number of guide signs than other roads, and the signs themselves are physically larger. In major cities, guide signs are often mounted on overpasses or overhead gantries so that drivers can see where each lane goes.

In most parts of the world, there are public rest areas or service areas on freeways. Many countries also provide emergency phones alongside freeways at regular intervals.

To reduce the probability that high-speed freeway traffic will have to slow down for slower same-direction traffic, access to freeways is usually limited to classes of motor vehicles that are powerful enough to maintain a certain minimum speed. Some countries partially restrict the use of motorcycles or ban them completely from freeways.