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A frontage road (also access road, service road, outer road, and especially surface road, or feeder) is a non-limited access road running parallel to a higher-speed road, usually a freeway, and feeding it at appropriate points of access (slip ramps). In many cases, the frontage road is a former highway already in existence when the limited access road was built. In other cases they may be built prior to construstion of the highway. Frontage roads are frequently one way roads when then exit on both sides of a highway.
Frontage roads provide access to homes and businesses which would be cut off by a limited access road and connect these locations with roads which have direct access to the main highway. Frontage roads give indirect access to abutting property along a freeway, either preventing the commercial disruption of an urban area that the freeway traverses or allowing commercial development of abutting property. They add to the cost of building an expressway due to costs of land acquisition (but that might be offset on occasion, see below) and the costs of paving and maintenance. However, the benefits of nearby real estate can more than offset the cost of building the frontage roads. Furthermore, a frontage road may be a part of an older highway, so the expense of building a frontage road may be slight. Conversely, the existence of a frontage road can increase traffic on the main road and be a catalyst for development, hence there is sometimes an explicit decision made not to build a frontage road.
A backage road is a similar concept, but lies on the other side of the land parcels that abut the frontage road. It serves mainly to provide access to those parcels without using the frontage road.
Collector-express
The successor to the concept of service/frontage roads in urban freeways is the collector-express system, which is designed to handle closely spaced interchange ramps without disrupting through traffic. Unlike service roads, the collector lanes are typically high-speed full controlled-access lanes, conforming to freeway requirements. The collector lanes may also be known as a collector/distributor road and slip ramps provide access to and from the express/mainline lanes.
Service roads and collector lanes are not needed in suburban freeways which tend to be designed with interchanges spaced further apart and which have property development located a fair distance away (to avoid noise and pollution of the freeway).
Texas
Most Texas freeways have frontage roads on both sides, even in remote rural areas. For example, over 80% of Houston freeways have frontage roads*. Many frontage roads in urban and suburban areas of Texas also have the convenience of Texas U-turns, which allow drivers to avoid being stopped by traffic lights when making a U-turn.
In 2002, the Texas Department of Transportation proposed to discontinue building frontage roads on new freeways, citing studies that suggest frontage roads increase congestion. However, this proposal was widely ridiculed and criticized and was dropped later the same year*.
Texas is the only state that widely constructs frontage / access roads along it's highways. Entering and exiting from access roads can be very confusing to drivers from other states. Signaling is very important, as it's not just for the drivers behind you, but also for oncoming traffic on the access roads.
In the greater Houston area frontage roads are referred to as Feeder Roads
Ontario
The only freeway with a significant remaining network of service roads is the QEW. However, most of the slip ramps in the Mississauga section have been removed during major reconstruction in the 1970s. Service roads are no longer able to directly access the QEW; they have been rerouted to intersections with major roads which have interchanges with the QEW. Nonetheless, the service roads are positioned too close to the QEW which makes the widening of the freeway difficult unless all the private properties along the service road are bought out (which is unlikely).
The only remaining slip ramps connecting to service roads are on the QEW running through St. Catharines. These dangerous low-standard ramps (due to lack of acceleration/de-acceleration lanes) are due to be replaced in a planned extensive reconstruction of the QEW.
Highway 427 had its service roads replaced with a collector-express system in the 1970s. However, it has several RIRO onramps and offramps to serve residential traffic in addition to its standard Parclo interchanges with major arterials.
China
In China, roads running next to expressways, taking outgoing traffic and feeding incoming traffic, are called either service roads or auxiliary roads (fudao locally). Where expressways cross larger urban areas, such frontage roads may run next to the expressway itself. Much of the Beijing portion of the Jingkai Expressway, for example, has, in fact, China National Highway 106 acting as a split-direction frontage road.
See also
"Frontage road"