Matt Hogan, Project Engineer in our Site Engineering [...]
Recently, one of our transportation engineers, Scott Cothron, P.E., spoke at the Alabama Section of the Institute of Transportation Engineers (ALSITE) Fall 2014 Meeting. His topic was “Innovative Intersections & Interchanges – Alabama’s Future?”, a topic he is familiar with since serving on the Federal Highway Administration (FHWA) Every Day Counts – Alabama working group to look at the most promising initiatives for the future of transportation in the state.
Scott’s work has focused on intersection and interchange geometrics, which have been identified as promising tools to improve traffic flow and safety, and tools that we expect to see used much more in Alabama in coming years.
Since intersections and interchanges are planned points of conflict that inherently create conditions that could result in a crash, identifying and implementing new tools is crucial for safety. The FHWA reports that more than 20% of the 33,808 roadway fatalities in 2009 were intersection or intersection-related. They also reported that the relationship of total fatalities to intersection or intersection-related ones has not changed greatly in the last 25 years.
Alternative geometric intersection and interchange designs are specifically designed to reduce or alter conflict points, allowing for safer travel for motorists, pedestrians and bicyclists.
Here’s a look at three of the most promising designs that Alabama drivers should expect to see more of in the future:
1. Roundabouts — A roundabout is a form of circular intersection where traffic travels counterclockwise around a central island. Entering traffic must yield to circulating traffic. Roundabouts change the nature of intersection conflicts by eliminating perpendicular crossings and opposing direction turns in favor of low-speed merging and diverging maneuvers.
2. 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. Traffic is moved 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.
3. 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. Several U-turn based designs involve motorists making a U-turn maneuver at a median break about 400 to 1,000 feet away from the main intersection in lieu of a traditional direct left-turn.
Since most Alabama drivers are probably not very familiar with these designs, they may appear complex initially. But evaluation and observation has shown that users actually find them easy to navigate. Some of the above configurations are already being used with great success in Alabama.