An interchange is a grade-separated intersection (one road passes over another) with ramps to connect them. For busy roads this is a necessity to keep traffic moving. Traffic signals are sometimes needed to help traffic move through and between the two facilities. Within these pages, we are going to depict and describe some of the various interchanges used to date. There might be some not used in Ohio. All interchanges are designed for the projected traffic for the region. This will make some designs more beneficial than others with respect to operation, right-way impacts, etc.
A complete interchange has to provide access to and from any direction from each facility. Full freeway to street access with a conventional interchange requires a minimum of four ramps, to get on and off in each direction. In Ohio, full interchanges are required for a future design. Partial interchanges are not allowed unless special needs are displayed (park and ride, by-pass routes, etc.)
An exit ramp, or off ramp, leaves the main roadway for another road; an entrance ramp, or on ramp, enters the roadway. These terms make the most sense when one freeway intersects a surface street; entrance and exit are from the point of view of the freeway.
An interchange is a simple solution to a capacity problem. Safety, cost, environment, development and politics can vary at each site. Many interchanges are slight variations of a few basic types.
The most commonly used types of interchanges are the diamond, cloverleaf and directional.
Folded Diamond Interchange
The diamond interchange is the most common type where a major facility intersects a minor highway. The design allows free-flow operation on the major highway but creates at-grade intersections on the minor highway with the ramps. The capacity is limited by the at-grade intersections on the minor highway.
Variations of the diamond interchange include:
Both SPUIs and TUDIs are more compact than a standard diamond, but are significantly more costly to construct.
Cloverleaf or partial cloverleaf designs may be used in lieu of a diamond when development or other physical conditions prohibit construction in a quadrant, or where heavy left turns are involved. A continuous flow design is required where two major facilities intersect. In this case, a full cloverleaf interchange is the minimum design that can be used. The designer should consider collector-distributor roads in conjunction with cloverleaf interchanges to minimize weaving. However, full cloverleafs have deficiencies which need to be addressed before being chosen as the interchange type. Principle disadvantages are:
When Collector-Distributor roads are not used, a further disadvantage includes weaving on the main line, the double exit on the main line and problems associated with signing for the second exit. The full cloverleaf weaving maneuver is not objectionable when the left-turning movements are relatively light, but when the sum of traffic volumes on two adjoining loops approaches about 1,000 vehicles per hour, interference occurs, which results in a reduction in the speed of the mainline traffic. For these reasons, full cloverleafs are discouraged.
Directional interchanges are the highest type and most expensive. They permit vehicles to move from one major freeway to another major freeway at relatively fast and safe speeds.