Highway Interchanges

A highway interchange is a type of road junction that allows highway traffic to pass through the junction without intersecting with other traffic. This is usually accomplished with grade separation, the construction of transportation routes at different heights for the purposes of avoiding mutual disruption. In this sense highway interchanges are a descendent of grade-separated railroad junctions, which originated to prevent road traffic from intersecting with the rails. Interchanges are an alternative to the use of traditional intersections with traffic lights or four-way stop signs, because with the higher speed and more dense traffic of highways, stops are more likely to cause traffic jams, and accidents caused by those who run the stop or fail to brake in time are both more likely and more severe.

A complete interchange includes sufficient exit ramps (which lead traffic onto the intersecting or proximate road) and entrance ramps (which lead traffic onto the highway) to provide access both to and from all directions, which generally means [Page 733]either four ramps (when intersecting with an ordinary road) or eight ramps (when intersecting with another highway). Interchanges can become complicated when multiple roads are involved, when highway exits and on-ramps are proximate, or when multiple grade separations are used within a short distance, all of which can create confusing traffic patterns, increase the amount of traffic that must change lanes, or increase the complexity faced by merging traffic.

There are a number of different highway interchange designs, some of which are meant for specific numbers of roads or other local conditions. One of the earliest developed was the diamond interchange, generally used for a highway crossing a more minor road. In such interchanges, the highway continues over the road via a bridge, while off-ramps diverge at a slight angle to meet the intersecting road and become an on-ramp back onto the highway on the other side.

This is one of the simplest forms of interchange and does not considerably add to the space needed for the roads, especially compared to more complicated interchange systems. It remains common throughout the rural United States and in many suburban areas. Moderate variations on the diamond design include the double roundabout, which combines the basic idea of the diamond interchange with the use of two roundabouts (one for each direction of traffic), which is used on several western interstates as well as in Minnesota; the tight diamond, a diamond interchange used in urban areas and other places with insufficient right-of-way; and the dogbone interchange, which uses modified roundabouts that don'st form a complete circle.

Areas with heavy traffic may require other interchange types, with additional lanes for turning. In urban areas a directional interchange is often more common. A directional interchange, also known as a stack interchange or butterfly junction, uses two pairs of left-turning ramps. The design was first used in Los Angeles at the Bill Keene Memorial Interchange, which connects U.S. Route 101 to State Route 110, and opened in 1953.

A cloverleaf interchange loops through landscaping near Cologne, Germany. The cloverleaf design was first popular in Europe and Latin America; the first U.S. version was built in 1929 in New Jersey. Some have since been converted to updated designs.

...