Highway bridges are located on all kinds of roads: the following terms are used to describe them, along with the functional classifications associated with each of the three major categories:
Arterials provide the highest level of mobility, at the highest speed, for long and uninterrupted travel. Arterials typically have higher design standards than other roads. They often include multiple lanes and have some degree of access control. The urban arterial system includes the functional classes interstate highways, other freeways and expressways, other principal arterials, and minor arterials. The rural network includes the functional classes interstate highways, other principal arterials, and minor arterials. In this report interstate highways are shown separately, and the other arterial functional classes are combined into the rural and urban other arterial categories.
Collectors provide a lower degree of mobility than arterials. They are designed for travel at lower speeds and for shorter distances. Generally, collectors are two-lane roads that collect travel from local roads and distribute it to the arterial system. The rural collector system includes the functional classes major collectors and minor collectors. In this report they have been combined into a single category. The urban collector system consists of a single functional class, urban collectors.
Local roads represent the largest element in the American public road network in terms of mileage. For rural and urban areas, all public road mileage below the collector system is considered local. Local roads provide basic access between residential and commercial properties, connecting with higher order highways. Both the urban and rural local road networks each consist of the lowest functional class, designated as the local functional class.
SOURCE: adapted from the U.S. Department of Transportation, Federal Highway Administration and Federal Transit Administration, 2006 Status of the Nation’s Highways, Bridges, and Transit: Conditions and Performance report to Congress, p.2-7