NCHRP Report 687: Guidelines for Ramp and Interchange Spacing

• Published by:  NCHRP
• Authored by:  Brian Ray
• Co-authored by:  J. Schoen , P. Jenior , J. Knudsen , R. Porter , J. Leisch , J. Mason , R. Roe

These guidelines do not establish ramp and interchange spacing standards. Rather they provide a process and criteria for assessing spacing in a given context to assist planners and designers in considering the feasibility of new or rebuilt interchanges and ramps. Interchange spacing is defined as the distance between the centerlines of successive crossroads with interchanges on a freeway. However, unlike AASHTO, which has established spacing guidelines as shown in Figure 7, the guidelines “de-emphasize the measurement between adjacent crossstreet centerlines and focus on the design, operational, safety, and signing considerations of individual ramps.” AASHTO guidelines are called “reasonable starting points,” but the report notes that “…the design and placement of ramps will ultimately play a greater role in determining the adequacy of spacing. 

Ramp spacing values should ultimately dictate ramp and interchange spacing design decisions.” P. 34 The Guidelines were developed primarily for fully controlled access freeways but could also be applied on controlled access highways. This information will be very useful for the new interchange chapter and possibly other chapters as well. Figure 7. AASHTO interchange spacing guidelines.

A few highlights are noted here.

Design:

• Decision sight distance should be provided at lane drops and exit ramps. Providing decision sight distance can influence ramp spacing values.
• When considering the safety performance of ramp alternatives, the guidelines call for a substantive approach to safety, that views safety on a continuum (rather than as safe or unsafe). This is consistent with the approach used in the Highway Safety Manual. “It is difficult to recommend absolute minimum dimensions from the expected safety outcomes themselves, but it is possible to conduct more meaningful trade-off analysis that considers a variety of important transportation, environmental, societal, and cost factors.” P. 54

Safety is defined as “the number of crashes, or crash consequences, by type and severity, expected to occur on an entity during a specified time period.” P. 55

• The form of an interchange may also influence the spacing assessment results. p. 72 Policy:
• Minneapolis-St. Paul, Minnesota (MPO):
  1. “New or expanded interchanges are not to be provided as a convenience for short trips, to compensate for the lack of an adequate minor arterial and collector system, or to compensate for deficient minor arterials and collectors.”
  2. “Traffic backups resulting from interchange ramp designs must occur on cross streets and frontage roads rather than on the mainline.”
  3. “Interchange ramp configuration and design should provide for preferential treatment of transit and rideshare vehicles.”
• Florida Department of Transportation: When a new or modified interchange application is submitted, review criteria include – “Has an Arterial Access Management Plan been developed and agreed to (where required)?” The report includes a helpful framework to guide the user, as shown in Figure 8.

Figure 8. Interchange planning and design framework.

Ray, B., J. Schoen, P. Jenior, J. Knudsen, R. Porter, J. Leisch, J. Mason and R. Roess, NCHRP Report 687: Guidelines for Ramp and Interchange Spacing, Transportation Research Board of the National Academies, Washington, D.C. (2011)