Interchanges are essential components of freeways for providing reasonable access and mobility. However, interchanges can greatly diminish the traffic operations, safety, and capacity of the through lanes of the freeway. Transportation agencies are tasked with constructing new freeways with interchanges, reconstructing existing freeways and interchanges, and adding interchanges to existing freeways.
The AASHTO Policy on Geometric Design of Highways and Streets contains guidelines on the distance between successive ramp terminals. On urban freeways and other facilities that carry large traffic volumes, two or more ramp terminals are often located in close succession. To provide adequate space for signing, adequate gaps for entering motorists, and sufficient weaving lengths, the AASHTO policy provides minimum ramp terminal spacing dimensions for various ramp pair combinations. Spacing between successive ramp terminals depends on the classification of the interchanges involved, the function of the ramp pair (entrance vs. exit), and the potential for weaving. The guidelines provided in the AASHTO policy are acknowledged to be based on operational experience and recommend basing actual spacing on operations and safety procedures derived from applied research.
Although the location and spacing of interchanges and ramps on freeways has a major effect on the ability of a freeway to carry traffic effectively, this is a topic for which little research or literature has been published. Recent research indicates that a majority of freeway accidents occur at interchanges and in weaving sections between closely spaced entrance and exit ramps. The spacing of interchanges on an urban road network can also result in tradeoffs between providing adequate service and access with both safety and operations. As a result, making sound decisions requires a clear understanding of the impacts of ramp and interchange spacing on safety and operations.
Research is needed that will provide additional information on ramp and interchange spacing for DOT practitioners that can also be used in future editions of relevant AASHTO manuals, including the Policy on Geometric Design, the Highway Capacity Manual, the Manual on Uniform Traffic Control Devices, and the upcoming Highway Safety Manual.
The objective of this research is to develop guidelines for selecting appropriate ramp and interchange spacing based on safety and operational impacts.