Media Contacts:
Dr. Joseph E. Hummer, 919/515-7733 or hummer@eos.ncsu.edu
Tim Lucas, News Services, 919/515-3470 or tim_lucas@ncsu.edu

August 1, 1999

Unconventional Left-Turn Lanes Reduce Traffic Accidents, Congestion

FOR IMMEDIATE RELEASE

Nearly one out of six collisions on America’s city streets involves vehicles making left turns. In North Carolina alone, about 30,000 left-turn crashes or fender benders are reported each year. The traffic bottlenecks caused by these crashes, and by the often long lines of cars waiting to make left turns at busy intersections, add significantly to rush-hour traffic congestion and delays.

A North Carolina State University transportation engineer says there’s a better way.

Dr. Joseph E. Hummer, associate professor of civil engineering, has identified five unconventional designs for busy intersections that could reduce delays and decrease the risk of left- turn-related collisions.

The five designs he’s promoting – median U-turn, bow tie, superstreet, jug handle and continuous flow intersection – can help ease congestion and improve safety by reducing the number of vehicular "conflict points" in an intersection. They also can reduce delays to through traffic.

Hummer presented overviews and feasibility studies of the five designs, three of which already are used on a limited scale, to about 180 highway engineers and city planners at the Urban Street Symposium in Dallas last month. He and co-presenter Jonathan D. Reid, a transportation engineer at Parsons Brinckerhoff in Morrisville, are the first researchers to review all five designs in such comprehensive fashion.

"Like Glenwood Avenue in Raleigh or Independence Boulevard in Charlotte, many urban and suburban arterials are hopelessly congested," Hummer says. "Engineers have done as much as they can with conventional improvements like dual left-turn lanes and actuated signals. And other conventional improvements, such as widening roads or building flyovers and overpasses, can be prohibitively costly to construct and to secure right- of- way along the affected roads."

The five alternative designs evaluated by Hummer and Reid are:

Median U-turn – Already used in Michigan, this design entails lower construction and right-of-way costs than many conventional options and improves safety by eliminating left turns at the intersection. To turn left, vehicles must drive several hundred feet past the intersection and cross the median in a signal-regulated U-turn lane, from which they can safely re-enter traffic and make a right turn onto the side street.

In tests conducted along a two-and-a-half mile corridor in Detroit, the median U-turn proved to be the most effective design for moving traffic quickly through intersections at nearly all times of day, Hummer says. Though it entails the use of three traffic signals, the signals can be coordinated so most vehicles have to stop no more than once.

Bow tie – Like the median U-turn, this design – which was created at NC State in 1993 by Hummer and former student Jonathan Boone – entails relatively low right-of-way costs and eliminates left turns at intersections. Drivers who want to turn left at an intersection must instead turn right into a one-way roundabout located several hundred feet down a side street. The roundabout safely turns them so they go back through the intersection headed in the desired direction, eliminating the need to cross over lanes of oncoming traffic.

Though the bow tie design is not yet in use, computer simulations based on actual traffic data show it can move traffic through intersections as quickly as median U-turns.

Superstreet – In this design, vehicles on the main road use conventional lanes while through traffic and left-turning traffic from side streets use median U-turns. The design’s main benefit, Hummer says, is that it makes it easy for drivers on the main road to continue getting green lights at intersections, thus reducing stops and delays. Superstreets are not yet in use.

Jug handle – At jug handle intersections, which have long been used in New Jersey, right- hand turns are the only turns you can make. Drivers who want to turn left must follow handle- shaped ramps or lanes onto a side street and then back across the intersection. The advantages of the design are that it reduces delays for through traffic and can be built in a fairly narrow right-of-way.

Continuous flow intersection – The main feature of this design, already in use at about a dozen sites in Mexico and the United States, is that vehicles turning left off the main road turn onto a ramp before the intersection and take the ramp directly onto the side street. This allows through traffic and left-turning traffic to move through the intersection on the same green light, reducing delays and back-ups. This design was patented in 1987 by Francisco Mier of El Cajon, Calif.

"No one design is right for all intersections," Hummer says. "By evaluating the strengths and weaknesses of these five relatively unknown or underutilized designs, our goal is to help highway designers identify cost-effective new ways to address traffic problems."

--lucas--

Note to editors: For a copy of Hummer’s presentation, contact him at (919) 515-7733 or hummer@eos.ncsu.edu, or Tim Lucas, News Services, at (919) 515-3470 or tim_lucas@ncsu.edu.

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