Signal Timing: The Science of Moving Traffic Safely

Last week we talked about the unseen infrastructure required for simply changing a red light to green. As important as this infrastructure is, signal timing must be planned and deliberate.

This week looks at the calculations and organization used by the Transportation Management Center staff to keep traffic flowing every day.

Planning

Signal timing is not a shot in the dark, according to Eric Claussen, professional engineer with City of Springfield Public Works Transportation Management Division. The first step for a new, or updated, signal project is gathering data.

“By data, I mean turning movement counts,” Claussen said. “We are basically setting detectors in each one of the approach lanes and getting the count on our true volume at each individual intersection.”

Transportation Management Center (TMC) staff will also physically drive the routes affected by the signal project. The data from detectors and driving the routes is used in conjunction with federal requirements designated in the Manual for Uniform Traffic Control Devices to begin signal timing calculations.

TMC staff also consider if the intersection is, or should be, part of the organization’s coordinated plan. Signals on a major arterial are typically part of the coordinated plan. Conversely, outlining intersections run in free operation.

The objective of a coordinated plan is to move vehicles in groups, called platoons, through multiple signals before having to stop at a red light again. By moving vehicles in platoons, intersections on major arterials are less congested.

“Free operation means it is more responsive to the traffic that’s approaching the intersection,” Claussen said. “We may have a favored roadway, but it’s going to be more responsive to traffic.”

Springfield’s traffic is unique for a larger community because the public uses major arterials, such as Glenstone and Kansas Expressway, for commuting instead of a large highway system. As a result, commuters may have to wait longer than they want on side streets to ensure arterials are able to move large volumes of traffic through the city.

A closer look at the signal control cabinet at the corner of Kimbrough and Walnut.

Timing

After considering all the data, factors and information, staff will begin determining the signal’s timing. As mentioned, federal requirements govern the length of timing for things such as yellow, all red, which will be discussed later, and pedestrian crossing duration. These factors affect the signals cycle timing.

Cycle timing refers to the time it takes an intersection to go from yellow though red and green and back to yellow. According to Jason Haynes, traffic engineer with City of Springfield Public Works Transportation Management Division, cycle length is essentially a balancing act.

“Somebody may call and say, ‘you need to lengthen this left turn time; it’s always cutting off the last vehicle,” Haynes said. “We would love to give a little more green time and not cut that last vehicle, but if we do, we are taking it from somewhere else.”

Jason Haynes

Jason Haynes

Additionally, adding or moving even a few seconds of green light time from one signal to another could throw off the entire coordinated plan. Haynes also explained just because a cycle length may be 120 seconds doesn’t mean vehicles are moving the whole time.

“We keep talking about the cycle length and how important it is, but there is a lot of lost time,” Haynes said. “You may have s 120 second cycle but have 30 seconds of lost time.”

Lost time occurs when nobody is moving. Yellow, all red and the time it takes for a driver to react and start moving all contribute to lost time. Claussen believes that lost time is increasing as a result of digital distractions.

“If it’s … peak rush and you have someone in the turn lane looking down because they are texting and only two vehicles may get through where normally eight would,” Claussen said. “Then all of a sudden that’s compiled on the next cycle and instead of having to push eight cars through you are pushing 14 because only two got through on the time before.”

 

Network

Cycle timing is maintained by a partnership within the TMC. The City of Springfield and the Missouri Department of Transportation (MoDOT) work together when developing, monitoring and maintaining timing plans.

“[T]hat’s why we have this partnership just in this building with MoDOT staff, because the traveling public doesn’t understand where those jurisdictional boundaries are,” Claussen said.

MoDOT maintains the majority of major arterials with the City picking up some major arterials as well as secondary arterials and side streets. Currently, MoDOT has about 129 signals within the Springfield area compared to the City’s 140.

Safety

With nearly 270 signals throughout Springfield safety has to be a priority. Haynes explained that a great deal of safety is actually built into the system.

“At every one of the signals there is an all red,” Haynes said. “The signal will sit in red in all directions for a certain amount of time, usually around two seconds.”

This all red helps ensure that vehicles traveling a certain direction have cleared the intersection prior to allowing opposing or cross traffic to begin moving. Despite this built in safety, Haynes said it’s always a good idea to look for traffic to make sure no one is running a signal before moving into the intersection.

The TMC staff has received legal requests after an accident wanting to know if the system had shown green for vehicles coming from opposing directions. According to Claussen, the system is built to prevent this.

“There is a safety device in each one of those controllers called a conflict monitor card,” Claussen said. “Basically, what it does is not allow the controller to show green in opposing directions.”

This is actually done by clipping designated circuits on the conflict monitor card prior to its installation in a signal control cabinet.

 

conflict monitor card

Haynes shows a conflict monitor card that has been clipped.

Contact

The public is encouraged to call if they notice an issue with any traffic signal or intersection.

“When we do get calls, we appreciate it,” Haynes said. “They are our eyes in the field.”

You can reach the City of Springfield by calling 417-864-1000 or by their website at springfieldmo.gov or by their new app gospringfieldmo available for Apple and Android. MoDOT can also be reached at 417-895-7600 or at modot.org.

 

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