15.2 Warrants for Interchanges and Grade Separations

Interchanges are integral features of freeways and are an adaptable solution to improve intersection conditions by improving safety, operational, and traffic congestion. Grade separations without ramps are also integral features on freeways. They can also provide significant system benefits to other facilities.
Since various factors come into play at each location, specific warrants for justifying an interchange cannot be conclusively stated. There are several conditions that need to be considered when deciding whether to use an interchange as a transportation solution. These include:
  • Design designation
    —if the goal is to develop a highway with full access control between selected locations.
  • Reduction of bottlenecks or spot congestion
    —if insufficient capacity at the intersection of heavily traveled routes results in intolerable congestion on one or all approaches.
  • Reduction of crash frequency and severity
    —if some intersections have a very high rate of serious crashes, and inexpensive methods of reducing crashes are likely to be ineffective or impractical.
  • Site topography
    —if grade-separation designs are the only type of intersection that can be constructed economically.
  • Road-user benefits
    —if the road-user costs from delays at congested intersections are very high. Road-user costs include:
    • Fuel and oil usage;
    • Wear on tires;
    • Delay to motorists; and
    • Crashes that result from speed changes, stops, and waiting.
  • Traffic volume warrant
    —if a traffic volume warrant may be the most tangible interchange warrant. Although a specific traffic volume at an intersection cannot be used as the only measure to determine the need for an interchange, it is an important variable. This is especially the case when combined with the traffic pattern and the effect of traffic behavior.
Not all warrants for grade separations are included in the warrants for interchanges. Additional warrants for grade separations include ones that will:
  • Serve local roads or streets that cannot end outside the right-of-way (ROW) limits of freeways;
  • Provide access to areas not served by frontage roads or other means of access;
  • Eliminate a railroad–highway grade crossing;
  • Serve unusual concentrations of pedestrian traffic (e.g., a city park developed on both sides of a major roadway);
  • Serve bikeways and routine pedestrian crossings;
  • Provide access to mass transit stations within the borders of a major roadway; and
  • Provide free-flow operation of certain ramp designs and serve as part of an interchange.
Refer to Chapter 10 of
AASHTO’s A Policy on Geometric Design of Highways and Streets
for detailed discussion on things to consider for each interchange and grade separation warrant.