Reducing Congestion on a Budget: Turn Lanes

With today’s fiscally constrained environment, adding turn lanes to congested roads can significantly reduce congestion at a fraction of the cost of building new travel lanes. Turn lanes also improve road safety. Turn lanes are one of several cost-effective techniques that state and local authorities can use to reduce congestion.

Adding turn lanes is substantially cheaper than widening the road. Widening an urban arterial typically costs $6 million or more per mile. Constructing a continuous turn lane on an urban arterial costs less than half, typically about $2.8 million per mile. Constructing turn lanes, at the busiest intersections along the arterial, costs between $1-2 million per mile. Adding a turn lane can cost only 20% of the cost of widening the highway.

There are four different types of turn lane improvements. Each has sizable benefits. The four are: Left Turn Lanes, Indirect Turns, Right Turn Lanes, and Roundabouts. The Federal Highway Administration Office of Operations has more details about the congestion relief and safety benefits of turn lanes. Indented paragraphs contain information and statistics from the Federal Highway Administration Access Management brochure.

Exclusive turning lanes for vehicles remove stopped vehicles from through traffic. Left-turn lanes at intersections substantially reduce rear-end crashes. Exclusive turn lanes reduce crashes between 18 to 77 percent (50 percent average) and reduce rear-end collisions between 60 and 88 percent.

Left-turn lanes also substantially increase the capacity of many roadways. A shared left[-]turn and through lane has about 40 to 60 percent the capacity of a standard through lane. [There is] a 25 percent increase in capacity, on average, for roadways that added a left-turn lane.

Right turning lanes can also substantially reduce congestion and improve safety. Since motorists can turn right on red, there is less total delay. Still dedicated lanes can make a substantial difference at major intersections.

Though there are fewer studies of these impacts, there is a clear relationship between the number of vehicles attempting a right turn in a through traffic lane and its delay to through traffic. This relationship is exponential — each additional car that must wait for a right turn will increase the delay more than the previous car. At intersections with substantial right-turn movements, a dedicated right-turn lane segregates these cars from through traffic and increases the capacity of the road.

At some intersections due to right-of-way or safety issues, adding a traditional turn lane may not be realistic. In these situations Indirect Turns can be a good alternative.

Some of the biggest issues with managing access [occur] at intersections where vehicles must cross traffic. Some states and cities have adopted indirect turns to reduce these conflicts. In New Jersey, the jug-handle left turn requires a right turn onto a feeder street, followed by a left onto a cross street. Detroit has extensively used an indirect U-turn that requires a U-turn past an intersection, followed by a right turn instead of a regular left turn.

Like dedicated left-turn lanes, indirect turns reduce crashes, improve congestion, and add capacity. Crashes decline by 20 percent on average, and 35 percent if the indirect turn intersection is signalized. Capacity typically shows a 15 to 20 percent gain.

Finally, roundabouts are an increasingly popular transportation solution. Roundabouts are not appropriate in all situations. Roundabouts allow traffic to move continuously, decreasing wait times. For drivers making turns, roundabouts offer quicker and safer movements.

Roundabouts represent a potential solution for intersections with many conflict points. Though not appropriate for all situations, roundabouts reduce vehicle movements across traffic. Roundabouts can reduce congestion by keeping traffic moving continuously. Only a few studies have examined the safety benefits of roundabouts. One study of four intersections that were replaced with roundabouts in Maryland found a drop in crashes between 18 and 29 percent and a reduction in injury crashes between 63 and 88 percent. The cost of crashes at these locations (one measure of severity) was also reduced by 68 percent. Overall crashes on roundabouts were more minor than those at left turn locations.

While not every road needs turn-lanes, busy roads with turn-lanes are substantially less congested than busy roads without turn lanes. At a fraction of the cost of widening the highway, turn lanes are a cost-effective way to reduce congestion.

Last month I wrote here why synchronized traffic lights are a cost-effective way to reduce congestion on arterials. Earlier this month I detailed here why using shoulders as travel lanes is a cost-effective method of reducing congestion on highways.