Highway Geometric Design3rd Stage

InterchangesInterchanges (grade separated intersection)

An interchange is defined as a system of interconnecting roadways inconjunction with one or more grade separations that provides for themovement of traffic between two or more roadways or highways ondifferent levels.

Interchanges are utilized on freeways and expressways, where accesscontrol is important. They are used on other types of facilities onlywhere crossing and turning traffic cannot be accommodated by anormal at-grade intersection.

InterchangesAn interchange can be a useful an adaptable solution for many

intersection problems either by reducing existing traffic bottlenecks orby improving safety. However, many factors that should be consideredduring interchange design:

1. Topography of the site

2. Traffic characteristics

3. Land availability

4. Impact on environment

5. Economic factors

Interchanges• A grade separation refers to the crossing of two roadways by a physical

separation so that neither roadway interferes with the other. An interchange is

a grade separation with connecting roadways (ramps, loops, or connections)

that move traffic between the intersecting highways.

• Classifications. Interchanges are classified in a general way, according to the

number of approach roadways or intersection legs. The following

subsections include a brief description and some of the advantages and

disadvantages of each of the following types of interchanges:

1. Three leg interchanges

2. Four leg interchanges

InterchangesThree leg interchanges:

• Three-leg interchanges or (T and Y interchanges) can take any of several forms,although all of the forms provide connections for the three intersecting highways.Three-leg interchanges should be used only after careful consideration becauseexpansion to include a fourth leg is usually very difficult.

Trumpet. The most widely used 3-leg interchange is the trumpet type. This type ofinterchange is particularly suitable for the connection of a major facility and afreeway. Preference should be given to the major turning movements so that thedirectional roadway handles higher traffic volume and the loop the lower trafficvolume.

InterchangesDirect. High-type directional three-leg interchanges are those in which all

movements are provided without the use of loops. These interchanges should beused only where all movements are large. They contain more than one structure or,alternatively, a three-level structure.

Interchanges• This design:

accommodates the T-intersection flow of traffic at the junction of two roadways.

allows for traffic on a secondary two-way street to merge onto a multiple laneroadway.

Interchanges

Four Leg Interchanges

• Four-leg interchanges can take a wide variety of forms. The choice of interchangetype is generally established after careful consideration of dominant traffic patternsand volumes and system considerations. The three primary types of four-leginterchanges are as follows:

1. Diamond Interchanges

2. Cloverleaf Interchanges

3. Directional Interchange

Interchanges1- Diamond Interchanges

• The diamond interchange is the most common interchange, especially in urbanareas, since it requires less area than any other type. The diamond interchange isused almost exclusively for major-minor crossings since left-turn movements aremade at-grade across conflicting traffic on the minor road. Separation betweenfrontage road intersections in diamond interchanges in urban or suburbanconditions should be 300 ft [90 m] as a minimum.

Interchanges

Typical Diamond Interchanges.

• The diamond interchange may have several different configurations

InterchangesSingle point diamond.

• A special type of freeway-to-arterialinterchange has received attention duringrecent years.

• In this type of interchange, the freewaymainlines may go either over or under thecrossing arterial and the turn movementsoccur at-grade on the arterial.

• This type of interchange has applicationonly in specialized locations.

Interchanges• This design allows:

• for the interchange of a major roadway with a secondary dual or multiple lane roadway.

Interchanges2-Cloverleaf Interchanges• Cloverleaf interchanges are very common in the world. These types of interchanges

were popular in the early era of freeway construction, but are usually no longerconsidered preferable for freeway to freeway movement, especially wheninterchange volumes are high.

• Cloverleaf should not be used where left-turn volumes are high (exceed 1200vehicle/hr) since loop ramps are limited to one lane of operation and have restrictedoperating speeds.

Interchanges• Full cloverleaf. The four-quadrant, full cloverleaf, eliminates all left-

turn conflicts through construction of a two-level interchange.

InterchangesPrimary disadvantages of the cloverleaf design include the following:

1. large right-of-way requirements

2. capacity restrictions of loops, especially if truck volumes aresignificant

3. short weaving length between loops

4. trucks have difficulty with weaves and acceleration

When used, cloverleaf designs should include collector-distributor roadsto provide more satisfactory operations as further noted in the section onCollector-Distributor Roads.

InterchangesThe cloverleaf design has serious objections:

1. Takes large area

2. Vehicles desire to make a left turn must make a270°turn and travel will be on sharp curves andsteep slope

3. Vehicles leaving the curved loop in one quadrantmust weave through those entering the adjacentloop from the through roadway. Where weavingvolumes are high, the weaving section must belong.

Interchanges• This design

allows for the interchange of two expressways or major roadways.

has shared entrance and exit weave lanes.

InterchangesPartial cloverleaf (semi-cloverleaf) interchanges

• A cloverleaf without ramps in all four quadrants, is sometimes used when sitecontrols (such as railroads or streams running parallel to the crossroad) limit thenumber of loops and/or the traffic pattern is such that the left-turn conflicts causedby the absence of one or more loops are within tolerable limits.

Interchanges3- Directional Interchanges• Interchanges that use direct or semi-direct connections

for one or more left-turn movements are called“directional” interchanges. When all turningmovements travel on direct or semi-direct ramps ordirect connections, the interchange is referred to as“fully directional”.

• These connections are used for important turningmovements instead of loops for the following purposes:

1. to reduce travel distance

2. increase speed and capacity

3. reduce weaving

4. avoid loss of direction in traversing a loop.

• “Fully directional” interchanges are usually justified atthe intersection of two freeways.