University of Virginia Library

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GRADE SEPARATIONS AND INTERCHANGES

from Chapter X, A Policy on Geometric Design of Highways and Streets,
NCHRP, December 1979

"The greatest efficiency, safety, and capacity are attained when
intersecting through-traffic laws are separated in grades." "The
type of grade separation and interchange along with their design,
is influenced by many factors, the principle factor being design designation....traffic volume, character of composition of traffic,
design speed, and type of control of access."

Other controls: signing, economics, terrain, right-of-way.
Basic type of interchanges can vary extensively in shape and scope.

• Fig. X-1A-Trumpet or jug-handle ramp configuration

• Fig. X-1B-Three-level, directional, three-leg interchange

• Fig. X-1C-Not suitable for freeways but practical for highway-parkway
  connections where trucks are prohibited and
  design speed is low

• Fig. X-1D-Typical diamond which has variations with frontage
  road and collector-distributor ramps

• Fig. X-1E-Partial cloverleaf which favors heavier traffic volumes

• Fig. X-1F-Full cloverleaf generates weaving movements that must
  occur on collector-distributor roads

• Fig. X-1G-Fully directional interchange-example is four-stack
  interchange in Los Angeles

Open-road capacities can flow without interruption when intersecting
roads are separated by a structure. The high initial cost
of grade separations must be justified on the two considerations of
(1) elimination of traffic bottlenecks, and (2) correction of existing
hazards. Six items (or warrants) will justify an interchange:

1.

• Design Designation: whether or not the access will be fully
  controlled between terminals

2.

• Elimination of bottlenecks or spot congestion: inability to
  provide essential capacity of one or both roads

3.

• Elimination of hazard: locations of high accident frequency

4.

• Site Topography: physical properties of site make at-grade
  intersection impossible

5.

• Road-User Benefit: costs due to delays, fuel, time and
  accidents require improvement; relation of road-user benefit
  

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  2

  
  
  
  to cost of improvement; annual benefit divided by annual
  cost of improvement; annual cost is sum of interest plus
  annual amortization.

6.

• Traffic Volume: volumes exceed capacity of at-grade intersection;
  elimination of conflicts greatly improves movement
  of traffic.

Additional Warrants:

1.

• Local streets cannot be terminated outside of right-of-way

2.

• Access to areas not served by frontage roads or other access

3.

• Railroad grade separations

4.

• Unusual concentrations of pedestrians such as parks on both
  sides of roadway

5.

• Bikeways or pedestrian crossings

6.

• Access to mass transit within major arterial

7.

• Free-flow aspects of certain ramps and completing geometry
  of interchange