Intersection Design

New, innovative designs offer roads and highways owners a wide array of options that deliver more benefits to drivers, cyclists, and pedestrians at a much lower cost. Non-traditional intersections can offer double the lane capacity compared to traditional designs. Non-traditional designs (with the exception of roundabouts), rely on reducing the signal phases of signalized intersections down to two phases. This allows all traffic on the main roadway to move half of the time, while all traffic on the crossroad to move the other half of the time. This is in sharp contrast to the percentage of time that all traffic is permitted to move under the operation of traditional, eight phase signalized intersections.

 Examples of non-traditional intersections include:

• Roundabouts
• Single point urban interchanges
• Quadrant intersections
• Continuous flow intersections
• Para-flow intersections
• Superstreets
• Median U-turn intersections
• Diverging diamond interchanges
• Bowties
• Displace left diamond interchanges
• Para-flow interchanges

Better Traffic Flow Needs Less Land

Allowing a greater percentage of traffic to move a greater percentage of the time means fewer lanes are needed to move the same amount of traffic with non-traditional intersections than with traditional designs. By reducing the need for additional lanes, less right-of- way is needed to construct the interchange. As a result, the construction costs of non-traditional designs is substantially less than their traditional counterparts.

Better Design Means Safer Roads

In addition to increasing capacity and reducing costs, all non-traditional intersections are thought to be substantially safer than traditional designs, due to the reduction of the number of conflict points. Conflict points have always served as a surrogate for safety (with the least number of conflict points being the safest). Traditional intersection forms typically have 32 conflict points, while most non-traditional forms have 16. Likewise, a traditional diamond interchange has 30 conflict points, while most non-traditional interchange forms have only 16.

Fewer Stops Cause Less Pollution

Many areas of the country are in non-attainment for air quality. As a result of having two signal phases, the efficiency of non-traditional designs permits more vehicles to be moving within a specified time period than the traditional multi-phased designs which operate with more delay and idling while waiting on their signal phase to turn green. Based on VisSim models, air pollution can be reduced by 25%-30%, depending on the pollutant, with non-traditional designs when compared to the same traffic volume with traditional designs.

Typically, non-traditional designs provide a win/win/win/win over their traditional counterparts by providing:

• Higher capacities lane for lane
• Safer designs with less conflict points
• Reductions in air pollutants
• Less expensive designs

WSP in Action

WSP is proud to deliver these benefits to our clients. In North Carolina (US), implementation of superstreets meant travel time improvements by 20%, reduction in collisions by 56%, and fewer injuries and fatalities by 63%. In the case of the A19/A1058 Coast Road Junction Improvement (UK), our primary objective was to relieve congestion by realigning the A19 to pass beneath the existing grade separated junction. A significant reduction of 25% in capital works cost has been achieved through a rigorous approach to value engineering and focused risk management.