Frontage road

A frontage road (also known as an access road, outer road, service road, feeder road, or parallel road) is a local road running parallel to a higher-speed, limited-access road. Where parallel high-speed roads are provided as part of a major highway, these are also known as local lanes. Sometimes a similar arrangement is used for city roads; for example, the collector portion of Commonwealth Avenue in Boston, U.S., is known as a carriage road.

A frontage road is often used to provide access to private driveways, shops, houses, industries or farms that would otherwise be cut off by a limited-access road. This can prevent the commercial disruption of an urban area that the freeway traverses or allow commercial development of bordering property.

There are several advantages to using frontage roads. One advantage is to separate local traffic from through traffic. When frontage roads are lacking in an urban area, the highway is used as a local road, reducing speeds and increasing congestion. Another advantage is to provide an alternative when the highway is closed or obstructed.

The existence of a frontage road can also be a catalyst for development, and property values along the highway often increase after the construction of frontage roads.

There are also some disadvantages to using frontage roads. When frontage roads are used without controlling the access to the primary road, at every intersection where an intersecting road runs across the primary, the number of conflict points increases one fold for each frontage road, since each frontage road is itself another intersection. A highway with frontage roads can be difficult for pedestrians to cross, for a variety of reasons including, but not limited to when neither the primary road nor the crossing is elevated, or gaps in traffic are few and the intervals between those gaps is long. Such examples include:

A complex example is US 77/Commerce (34°10′49″N 97°08′35″W / 34.1802°N 97.1430°W / 34.1802; -97.1430) in Ardmore, Oklahoma, particularly at the Grand Avenue intersection. Right turns from the central carriageways are not allowed; a slip ramp must be taken to the two-way frontage road, where the turning traffic must yield to the through traffic. Only then can a vehicle make a right turn from the signal on the frontage road.

Furthermore, frontage roads can increase urban sprawl. Land along highways is made open for development, allowing shopping centers and other buildings to sprawl.

Cost can also be a disadvantage, as building a highway with frontage roads can be more expensive than building a highway alone.