An interchange is defined as a system of interconnecting roadways in conjunction with one or more grade separations that provides for the movement of traffic between two or more roadways or highways on different levels. Interchanges are utilized on freeways and expressways, where access control is important. They are used on other types of facilities only where crossing and turning traffic cannot be accommodated by a normal at-grade intersection.
Interchange Spacing. Interchanges should be located close enough together to properly discharge and receive traffic from other highways or streets, and far enough apart to permit the free flow and safety of traffic on the main facility. In general, more frequent interchange spacing is permitted in urbanized areas. Minimum spacing is determined by weaving requirements, ability to sign, lengths of speed change lanes, and capacity of the main facility. Interchanges within urban areas should be spaced not closer than an average of 2 mi (3.2 km), in suburban sections an average of not closer than 4 mi (6.4 km), and in rural sections an average of not closer than 8 mi (12.9 km). In consideration of the varying nature of the highway, street, or road systems with which the freeway or expressway must connect, the spacings between individual adjacent interchanges may vary considerably. In urban areas, the minimum distance between adjacent interchanges should not be less than 1 mi (1.6 km), and in rural areas not less than 2 mi (3.2 km).
Interchange Type. The most commonly used types of interchanges where two routes cross each other are the diamond, cloverleaf, and directional interchanges. When one route ends at an interchange with another route, a trumpet or three-leg directional interchange can be used. Figure 2.40 shows schematic examples of the various types of interchanges. The trumpet interchange (a) has one loop ramp in its design, which is a lower-speed ramp. The three-leg directional interchange (b) incorporates all highspeed ramps in its design. The “one quadrant” interchange (c) has a two-way ramp with at-grade intersections, all in one quadrant of the interchange. This is used primarily in urban areas where the routes are both two-way roadways. Typically, this is utilized as a first stage in a developing area. Right of way in one or more other quadrants is purchased to allow for future expansion. The diamond interchange (d) is the most common type where a major facility intersects a minor facility. The capacity is limited by the at-grade intersections at the minor crossroad. The single-point urban interchange (SPUI) shown in (e) can be used when the minor road traffic volume increases and the diamond operation begins to bog down or fail. It allows the use of a single intersection and usually operates on a three-phase traffic signal. Opposing left turns from either the ramps or the side road do not cross paths and therefore can run in the same phase. The third phase is the through traffic on the side road. The partial clover — leaf interchange (f) can be designed to allow some free-flow right turns from the minor road or at least eliminate the need for left turns from the minor road. Signals are usually required to allow access for left-turning vehicles from the ramps onto the minor road. The full cloverleaf interchange (g) eliminates the need for at-grade signalized intersections by providing continuous-flow movements for all traffic. This is used when two major freeways or freeway-style roadways intersect. Under high volumes of traffic, the short weaving distance between the interior ramp terminals creates congestion problems. The use of collector-distributor roads can alleviate some of this problem by separating the through traffic from the entering/exiting traffic. The all-directional four-leg interchange (h) is the most efficient in terms of handling traffic, but is also usually the most expensive. It requires the most right of way and the incorporation of additional bridge structures to accommodate four levels of traffic.
