Thanh -- November 25th, 2008
Have you driven recently on Port Republic Road, East Market Street, or Main Street, tried to make a left turn at a light and wondered “did that light just skip my turn?” Then found that the green arrow turns on when the other lights turned red again? You may have even noticed the new sign that reads: “Delayed Green Arrow.”
In Spring 2008, the City of Harrisonburg’s Public Works Department began a new Transportation Management Program which began to look at City traffic patterns a little differently than it did before. With the help of modeling software to better visualize how traffic flows respond to signal timings and two full time staff members dedicated to signal timing plans, traffic studies, and coordination between intersections, some changes have been occurring throughout the city’s transportation network.
For example, at select intersections, a “delayed green arrow” pattern has been implemented to improve coordination between intersections along a corridor (major street). By delaying the green arrow and not having it “automatically” turn on and as a result make traffic in the opposite direction wait longer, models show that a larger number of vehicles would experience significantly less delays (less time stopped at a signalized intersection) with this “plan.”
But what about the vehicle wanting to turn left? When there are “breaks” in oncoming traffic the left turning vehicle could make its turn with the “yield on green”, or should it find no breaks in traffic, the light will know that it has been waiting, and at the end of the permissive green (aka, the green ball), it will stop oncoming traffic with a red light and provide a green arrow. Regardless, the vehicle will have an opportunity to safely turn. This form of left turn sequencing is also known as a “lagging left”.
The following intersections are currently using lagging left turns:
- Port Republic Road @ Bluestone Drive
- S. Main Street @ Bluestone Drive
- S. Main Street @ Grace Street
During its first 6 months, the City’s Transportation Management Program has modeled and coordinated several major corridors, including (but not limited to):
- Port Republic Road between Devon Lane and Bluestone Drive
- S. Main Street between Port Republic Road and Cantrell Avenue
- High Street between South Avenue and Gay Street
- East Market Street between Burgess Road and Chestnut Ridge Drive
- University Avenue between Evelyn Byrd Avenue and East Market Street
- Reservoir Street between Neff Avenue and Evelyn Byrd Avenue
With each adjustment in the coordination plans, these corridors are continually monitored and may require additional tweaks in the future as traffic patterns and volumes change. Additional corridors will also be studied and put into coordination as appropriate.
One thing to take into consideration when driving a coordinated corridor is that you may wait a little longer on the side street than you used to, as the coordinated system is programmed to hold on green on the main drag in order to allow traffic to progress smoothly. Just about every light in the City has the ability to detect the presence of your vehicle, which is done using the cameras you see on the signal poles. During periods of heavy volume, coordinated corridors are often programmed to turn off this detection for the main drag (or coordinated corridor) to allow for vehicle progression to occur despite gaps in traffic that would otherwise cause the signal to change to green for the side street.
The City of Harrisonburg’s Traffic Engineering page is currently under development and staff hopes to have a primer on traffic signal coordination made available soon.
Thank you to Brad Reed, Traffic Analyst with Harrisonburg’s Public Works Department, for his assistance with and contribution to this story. Disclaimer: Thanh is also employed by Harrisonburg’s Public Works Department as Public Works Planner.