Intersections and interchanges are planned points of conflict where motorists, pedestrians and bicyclists cross paths or change direction. This inherently creates conditions that could result in a crash. The Federal Highway Administration reports that over 20 percent of the 33,808 roadway fatalities in 2009 were intersection or intersection-related, and that that relationship of total fatalities to intersection or intersection-related ones has not changed greatly in the last 25 years.
As part of the ongoing effort to improve the safety performance of all roads, the Federal Highway Administration (FHWA) encourages State Departments of Transportation (DOTs) to consider alternative geometric intersection and interchange designs, which are specifically designed to reduce or alter conflict points, allowing for safer travel for motorists, pedestrians and bicyclists. Past and ongoing FHWA studies of various alternative intersection and interchange designs implemented within the last few years document the magnitude of both safety and operational improvements.
Roundabouts, diverging diamond interchanges (DDIs) and intersections with displaced left-turns or variations on U-turns are proving to be a few of the effective alternatives to traditional designs.
Roundabouts and Mini-Roundabouts
A roundabout is a form of circular intersection in which traffic travels counterclockwise around a central island and in which entering traffic must yield to circulating traffic. Roundabouts fundamentally change the nature of intersection conflicts, by eliminating perpendicular crossings and opposing direction turns in favor of low-speed merging and diverging maneuvers. Modern roundabouts should not be confused with other forms of circular intersections, such as neighborhood traffic circles or rotaries. Mini-roundabouts are essentially roundabouts that feature central and splitter islands that are fully traversable in order to accommodate large vehicles within constrained right-of-way.
Diverging Diamond Interchange (DDI)
The DDI enhances and simplifies the operation of the intersections at a diamond-style interchange by removing from the signalized intersection the turns on to and off of the ramps. This is accomplished by moving traffic to the left side of the roadway between the ramp terminals. The DDI design reduces the number of perpendicular conflict points as compared to an equivalent conventional diamond layout.
Intersections with Displaced Left-turns or Variations on U-turns
With displaced left-turns, motorists cross opposing lanes at an intersection several hundred feet away from the main intersection. Motorists then travel on a road parallel to the main road until they turn left with the through traffic at the main intersection. Similarly, several u-turn based designs require motorists to make a u-turn maneuver at a one-way median about 400 to 1,000 feet away from the main intersection in lieu of direct left-turns at the main intersection.
The geometric patterns of these alternative forms may appear to be complex designs; however, evaluation and observation show that users do find them easy to navigate. The primary benefit to these designs are enhanced safety performance through fewer or less severe crashes, but operational improvements have also been found, through overall reduced delay and less time spent stopped at red lights.
These alternative geometric designs may not be the right solution for every intersection or interchange project. Nevertheless, DOTs should consider and evaluate them early in the project scoping, planning and decision-making stages, as they may serve as more efficient, economical and safer solutions than traditional designs. For instance, the Missouri Department of Transportation found that it could reduce construction time and project costs by more than half when employing the DDI design over a single point diamond interchange alternative at several locations.
The benefits associated with alternative intersections and interchanges are not limited only to the agencies that construct them. Improved safety and reduced congestion can provide direct and indirect economic benefits to businesses and communities. The economic benefits combined with improved safety, mobility and maintained access to properties near intersections and interchanges will contribute to an enhanced quality of life in communities where the alternative designs are implemented.
By considering alternative intersection and interchange designs through a consistent evaluation process, DOTs will be better able to identify safer, more efficient and economical alternatives. Several DOTs have adopted policies and processes and developed screening tools to facilitate consideration of different forms of intersections and interchanges early in the project development lifecycle. Several DOTs that are leaders in deploying alternative intersection and interchange designs have documented their implementation lessons, and developed initial design guidelines that continue to evolve as new data is collected.