A “new” way to turn left

As a regional business leadership group focused on transportation, we are constantly scanning the landscape for opportunities to make things operate better and faster — for our member companies, their employees, and all of the members of our community.

While we certainly focus on major freeway, transit, and airport projects, we also look at less expensive opportunities for improvements including intersections. And there may be no more challenging aspect of intersections than left turns.

We came up with a new concept to help with traffic flow at intersections after thinking about what typically happens when a second left turn lane is added to help deal with rush hour congestion or backups.

Specifically, when you add a second left turn lane to serve peak period travel, the usual situation is that left-turning drivers no longer receive a flashing yellow arrow at any point during the day. This means that left turns have to wait for a green arrow to come around before they can turn, cycle after cycle. And it means that everyone else has to wait while even one left turning driver receives a dedicated green arrow during the cycle.

In summary, while a second left turn lane can improve peak period operations, its presence will often make an intersection function less efficiently the rest of the day. The reason is that the extra peak period capacity is not needed during lower volume times, but the intersection footprint and traffic signal typically cannot adapt or adjust, and extra delay is the inevitable, unavoidable result.

RTA, NCDOT, and the Town of Cary have developed a new, low-cost concept that deals with this peak capacity / off-peak delay tradeoff at intersections in a unique way.

The new concept, called a “dynamic left turn intersection,” or DLTi for short, combines familiar intersection elements in a new way to reduce delay. A DLTi reduces the number of available left-turn lanes from two to one during off-peak periods — by closing the rightmost left-turn lane using an overhead lane control signal — which enables the display of a flashing yellow arrow during those lower volume times.

The DLTi thus preserves the capacity benefits of having two left turn lanes during rush hour, while allowing the intersection to function more efficiently the rest of the day, and with far less delay.

NCDOT chose the “dynamic left turn intersection” name for this treatment, and I think it fits quite well – because a DLTi varies the number of left turn lanes, and also the signal phasing, to maximize rush-hour capacity while optimizing off-peak delay and efficiency.

It turns out that nothing at a dynamic left turn intersection is “new” — everyone has seen dual left turn lanes, an overhead “X“ lane control signal like they have by PNC Arena, a flashing yellow arrow, etc. What is new is this particular combination of intersection elements, that will allow the intersection to operate more efficiently throughout the day.

NCDOT announced this week that it has chosen two pilot locations for DLTi — the first to be installed in the Town of Cary this month, and another in the Town of Clayton this spring.

RTA was pleased to propose and help develop the DLTi concept with NCDOT and the Town of Cary. In addition, RTA is funding the before-and-after analysis to help evaluate the effectiveness of the new approach. It is our organization’s hope that the pilots will be successful, that we can learn from both installations, and that NCDOT and area communities can expand this technique to many other intersections in the Triangle and beyond.

Helping people get around the community faster and easier is a primary purpose of the RTA business coalition. We applaud NCDOT and the Town of Cary for their partnership and willingness to explore innovative techniques like this, and we are excited to learn more.

You can learn more about DLTi here, including a list of FAQs.


Joe Milazzo II, PE
Executive Director
Regional Transportation Alliance


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