Roadways: Interchange/Intersection

Interchange/Intersection

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. Safety improvements should be addressed by reducing or eliminating problematic conflict points. Capacity needs should be based on traffic analysis in the design year, not to exceed the 20 year forecast. Prior to proposing new signals, interchanges, or major geometric changes, designers should evaluate operational and minor geometric improvements that could address identified needs. Innovative intersection treatments that may provide needed improvements while minimizing impacts and costs should be considered. Intersection and interchange spacing guidelines balance access and mobility for a given roadway functional classification and context. Signal warrants should be met prior to installing new traffic signals. At-grade intersection treatments, including innovative designs, should be fully considered before selecting grade separation on an uncontrolled or partially access controlled roadway.

Discussion 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. If needs call for implementing a traffic signal, perform a signal warrant analysis. In some cases, even if a signal is warranted, other treatments, such as roundabouts, can be considered to help control traffic movements. 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. 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 Practical Design Implementation Guidance