Controlled-access Highway - Design

Design

Freeways, by definition, have no at-grade intersections with other roads, railroads or multi-use trails. Movable bridges, such as the Interstate Bridge on Interstate 5 between Oregon and Washington, do require drivers to stop for ship traffic.

The crossing of freeways by other routes is typically achieved with grade separation either in the form of underpasses or overpasses. In addition to sidewalks (footpaths) attached to roads that cross a freeway, specialized pedestrian footbridges or tunnels may also be provided. These structures enable pedestrians and cyclists to cross the freeway at that point without a detour to the nearest road crossing.

Access to freeways is typically provided only at grade-separated interchanges, though lower-standard right-in/right-out access can be used for direct connections to side roads. In many cases, sophisticated interchanges allow for smooth, uninterrupted transitions between intersecting freeways and busy arterial roads. However, sometimes it is necessary to exit onto a surface road to transfer from one freeway to another. One example in the United States (notorious for the resulting congestion) is the connection from Interstate 70 to the Pennsylvania Turnpike (Interstate 70 and Interstate 76) through the town of Breezewood, Pennsylvania.

Speed limits are generally higher on freeways and are occasionally nonexistent (as on much of Germany's Autobahn network). Because higher 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. Guide signs are often mounted on overpasses or overhead gantries so that drivers can see where each lane goes. Exit numbers are commonly derived from the exit's distance in miles or kilometers from the start of the freeway. In some areas, there are public rest areas or service areas on freeways, as well as emergency phones on the shoulder at regular intervals.

In the United States, mileposts start at the southern or westernmost point on the freeway (either its terminus or the state line). California, Ohio, and Nevada use milepost systems in which the markers indicate mileage through the state's individual counties. However, in Nevada and Ohio, and freeways that pass through Kern County, California, also use the standard milepost system concurrently with their respective postmile systems.