Interchange (road)

Note: The descriptions of interchanges apply to countries where vehicles drive on the right side of the road.

The concept of the controlled-access highway developed in the 1920s and 1930s in Italy, Germany, the United States, and Canada.

[21][22][19] A system interchange connects multiple controlled-access highways, involving no at-grade signalised intersections.

[4] A cloverleaf interchange is a four-legged junction where left turns across opposing traffic are handled by non-directional loop ramps.

Although they were commonplace until the 1970s, most highway departments and ministries have sought to rebuild them into more efficient and safer designs.

[23] The cloverleaf interchange was invented by Maryland engineer Arthur Hale, who filed a patent for its design on May 24, 1915.

Nicknamed Slussen, it was referred to as a "traffic carousel" and was considered a revolutionary design at the time of its construction.

Assuming right-handed driving, to cross over incoming traffic and go left, vehicles first exit onto an off-ramp from the rightmost lane.

After demerging from right-turning traffic, they complete their left turn by crossing both highways on a flyover ramp or underpass.

The penultimate step is a merge with the right-turn on-ramp traffic from the opposite quadrant of the interchange.

As there is only one off-ramp and one on-ramp (in that respective order), stacks do not suffer from the problem of weaving, and due to the semi-directional flyover ramps and directional ramps, they are generally safe and efficient at handling high traffic volumes in all directions.

Stacks are significantly more expensive than other four-way interchanges are due to the design of the four levels; additionally, they may suffer from objections of local residents because of their height and high visual impact.

[28][29] If local and express ways serving the same directions and each roadway is connected righthand to the interchange, extra ramps are installed.

Their connecting ramps can spur from either the right or left side of the highway, depending on the direction of travel and the angle.

Some designs have two ramps and the "inside" through road (on the same side as the freeway that ends) crossing each other at a three-level bridge.

The disadvantage of the directional T is that traffic from the terminating road enters and leaves on the passing lane, so the semi-directional T interchange (see below) is preferred.

However, in a semi-directional T, some of the splits and merges are switched to avoid ramps to and from the passing lane, eliminating the major disadvantage of the directional T. Semi-directional T interchanges are generally safe and efficient, though they do require more land and are costlier than trumpet interchanges.

This typically increases the efficiency of the interchange when compared to a diamond, but is only ideal in light traffic conditions.

In the dogbone variation, the roundabouts do not form a complete circle, instead having a teardrop shape, with the points facing towards the center of the interchange.

The various configurations are generally a safer modification of the cloverleaf design, due to a partial or complete reduction in weaving, but may require traffic lights on the lesser-travelled crossroad.

Depending on the number of ramps used, they take up a moderate to large amount of land, and have varying capacity and efficiency.

This allows all highway entrances and exits to avoid crossing the opposite direction of travel and saves one signal phase of traffic lights each.

While the compact design is safer, more efficient, and offers increased capacity—with three light phases as opposed to four in a traditional diamond, and two left turn queues on the arterial road instead of four—the significantly wider overpass or underpass structure makes them more costly than most service interchanges.