Roadways: Interchange/Intersection

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Primary Guidance

  • At-Grade Intersections and Grade Separated Interchanges identified for improvement should go through a needs evaluation process to determine specific safety, capacity, and operational deficiencies to be addressed
  • Address safety improvements by reducing or eliminating problematic conflict points
  • Based capacity needs on traffic analysis in the design year, not to exceed the 20 year forecast
  • Prior to proposing new signals, interchanges, or major geometric changes, evaluate operational and minor geometric improvements that could address identified needs as innovative intersection treatments may provide needed improvements while minimizing impacts and costs
  • Intersection and interchange spacing guidelines balance access and mobility for a given roadway functional classification and context
    • Meet signal warrants prior to considering a new traffic signal.
    • At-grade intersection treatments, including innovative designs, should be fully considered before selecting grade separation on an uncontrolled or partially access controlled roadway


At – Grade Intersections

  • Uncontrolled Intersections

Signalization entails significant costs and ongoing maintenance and should be implemented only when necessary. Signalization is discouraged when safety and operational concerns at uncontrolled intersections may be addressed by other treatments such as implementing a yield or stop condition, signing, roadway lighting, or minor geometric changes. Signal warrants should be met before considering a traffic signal as a viable option; however, meeting signal warrants does not mean that a signal should be installed. A traffic study needs to be completed to evaluate each alternative to determine which one would provide the most benefit. In some cases, even if a signal is warranted, other treatments such as roundabouts or no-build may provide the greatest benefit/cost.

  • Signalized Intersections

Existing traffic signals should be evaluated to see if timing can be optimized for specific locations, or if within a corridor, multiple signals may need to be coordinated in order to improve performance. Network solutions or innovative at-grade intersection designs should be considered when appropriate to help reduce the cost of traditional geometric and operational solutions. Some examples of innovative intersection designs may include Continuous Flow Intersections, Michigan U-Turns, J-Turns, Maryland T’s, etc. Impacts to all roadway users who will use the facilities should be considered.

Grade – Separated Interchanges

  • Grade Separating an Existing Intersection

Grade separation should be selected when conflicting high traffic volumes exceed those that can be handled efficiently and safely with an at-grade intersection. Occasionally, it may be appropriate to grade separate an intersection with a disproportionate rate of crashes that is not likely to be reduced through at-grade improvements or an intersection on a corridor transitioning to full access control. Impacts to all roadway users who will use the facilities should be considered.

  • Existing Interchanges

Consider options to maintain elements of the existing interchange and replace only those elements needing improvement before considering full reconstruction options. Innovative solutions such as a Diverging Diamond Interchange or ramp metering can be considered to address needs while minimizing impacts and costs.

  • Interchange Spacing

New interchanges should be appropriately spaced to eliminate conflict with existing interchanges within the corridor.

See Also