NCDOT expands the number of dynamic left turn intersections in North Carolina
A Dynamic Left Turn intersection (DLTi) approach treatment allows some intersections with two left turn lanes to operate more efficiently during off-peak periods, by closing one of the left turn lanes and enabling the display of flashing yellow arrows during those times.
A DLTi approach combines familiar intersection elements in a new way to reduce delay, through the use of overhead lane signals to close one left turn lane during lower-volume periods. This time-of-day turn lane closure enables the display of a flashing yellow arrow during off-peak periods for the single remaining open left turn lane. Since off-peak periods are the majority of the day, this can significantly reduce delays for both left turns and opposing through traffic. However, both left turn lanes are available during peak periods as need be to maximize approach capacity.
America’s first dynamic left turn intersection opened on February 14, 2020 at the Tryon Road/Cary Parkway intersection in Cary. RTA proposed and helped develop the DLTi concept with NCDOT and the Town of Cary. The intersection approach has been operated dynamically for more than five years; here is a media story about the site one year after installation. NCDOT and Cary have periodically modified the proportion of the day that the intersection operates under various left turn lane phasing and available lane combinations. A prior analysis showed that the intersection had already paid for itself due to the computed value of delay savings versus the cost of the DLTi approach treatment.
Earlier this month, NCDOT and contractors installed a new dynamic left turn intersection approach in Apex, for northbound Kelly Road traffic at the US 64 eastbound ramps.
The Apex location is the second DLTi in the US, and the first at an interchange location. The leftmost left turn lane is always open and available for left turning traffic; it has two overhead illuminated left turn arrows, one in advance of the intersection and one at the intersection.
During off-peak, lower volume periods — which is the majority of the day — less lane capacity is needed for left turns. During those times, the rightmost left turn lane is closed, via the display of two overhead, illuminated red “X” indications, one in advance of the intersection and one at the intersection, to prohibit use of the right-most left turn lane. Pavement markings guide travelers to the left-most, primary, always open left turn lane. A flashing yellow arrow can be displayed during off-peak periods, enabling left turning vehicles to turn left after yielding to oncoming traffic.
This approach significantly reduces delay for left turning travelers from northbound Kelly Road to US 64 eastbound, since they no longer have to wait for the display of a green arrow to execute a left turn. This treatment also reduces delays for southbound Kelly Road travelers, since they will no longer have to wait at a red signal for one or more opposing left turning vehicles to complete their turn with a green arrow during off-peak periods.
Here is a graphic from NCDOT with additional background and details on the DLTi approach treatment.
The regional business community applauds NCDOT for exploring and activating various intersection innovations to simplify travel.
Let’s get moving,
Joe
this post can also be referenced directly via: letsgetmoving.org/DLTiApex
