by Michael Brown, PE, AICP
* Traffic Engineer, New Urbanist
* Founder of Urban Innovators
What is a U-Turn Intersection?
Have you ever tried to turn left from a parking lot onto a busy arterial, and found it impossible to get a gap in both directions? It is often faster and safer to just turn right, then make a U-turn wherever possible. U-Turn intersections formalize that action. This diagram shows how left turns can be denied at the main intersection and rerouted as either "Thru-U-Right" or "Right-U-Thru" through the "bulb-out" ellipses. The result is greater efficiency, which can then support additional development. Reduced delay means you can reduce speed limits and add traffic calming curves and streetscape without making drive trips take longer. Pedestrian refuge in former left-turn lanes and reduced conflicts make it safer as well.
The diagrams below depict U-Turn variants. Most applications have been auto-oriented, but these focus on how they can also be used for converting Stroads into Walkable Boulevards. Variants are often called by various names including "Bowties, Median-U’s, Superstreets, Thru-Turns, Michigan-Lefts, and Reduced Conflict U-Turns (RCUT)." They utilize U-turn strategies such as "Loons, Teardrops, Ellipses, and Roundabouts."
North Carolina Concepts
The North Carolina Department of Transportation hired Urban Innovators and NC State University to demonstrate how U-turn designs could be applied to walkable settings, and evaluate some of the pros and cons of doing so. Below are diagrams from that effort.
This is a combined U-Turn / Quadrant, where lefts can be rerouted to either. Today the intersection maxes out at 3,800 vehicles per hour, but the concept design can support 6,000 (1.6x, or 60% more). The new vehicle capacity, combined with improvements to biking, walking, transit, and general mixed-use efficiency, mean the overall area can likely support 3x more development than at present. If the market takes full advantage of that potential, it will offset about 700 acres of sprawl, and eliminate about 20-miles of roadway and utilities. Expensive? Yes! But a LOT cheaper than the alternative!
America has thousands of locations with open medians where it is possible to make a left turn from unsignalized driveways onto a Stroad. But it is very hard to get a gap in both directions, and very dangerous when you "go for it."
For safety, engineers install "raised medians" which force "right-in, right-out." Businesses hate this because it makes it harder for customers to get in and out of their site. The diagram below shows that if engineers also will install frequent U-turns as part of the raised median project, both safety and accessibility will improve tremendously. U-turns just before the intersection help reduce the volume entering the intersection, which also reduces delay.
Big trucks exist. If you deny lefts at the main intersection, you may need to install a "bulb-out teardrop" wide enough for trucks. The diagram shows how you can do this, and still have a Livable environment.
New Urbanist Solution?
For greenfield environments, there are better concepts that fit better with New Urbanism. But since existing suburbs are not ideal, this may be the best compromise for dealing with a difficult reality. In our opinion, it's a tool that every New Urbanist should add to their toolbox.
There are many more design styles within this family. Contact Urban Innovators for more info.
Advantages of U-Turns
-
Impressive vehicle-capacity gains
-
Shorter signal cycles
-
Safer for both autos and pedestrians
-
Enhances and motivates TOD
-
Often very low cost
-
Politically easier to obtain lanes for transit
-
Traffic Calming / Gateway Feel
-
Planted medians, access control, circulation.
-
Compatible with traditional signals
-
Former lefts reclaimed for center-running transit, pedestrian refuge, etc.
Disadvantages
-
Initial confusion for drivers
-
Out of direction travel for lefts
-
Space required for bulb-outs / ellipses
-
Educational effort to win over skeptics
-
Still high volume – doesn’t make cars go away