The Science of Traffic Signals in Marshfield

Intersection of Veterans/Centeral. Photo Credit: Steven Okonek/ Tri-Media.

City’s Engineering Department Programs & Maintains Traffic Signals

There are 26 sets of traffic signals in the City of Marshfield and each of them is programmed with safety and efficiency as a priority. Accounting for traffic, trains, pedestrians, emergency vehicles, and other factors, traffic signals are a complex system that all drivers encounter on a regular basis.

“There are a lot of things I can program into the overall traffic signals,” said Tom Turchi, City Engineer. “When doing the math, the formulas used are just incredibly fun. They take into account reaction times, slopes of road, vehicle friction, etc… it’s a really involved process.”

How an intersection flows is developed through a signal timing plan, which factors in traffic volumes, approach speeds, lane geometry, pedestrian and other special considerations, and signal timing software.

A light can be “pre-timed” (runs on a fixed cycle regardless of traffic), or “semi actuated control” (has traffic detection on side streets to change light as needed).

Another option, “full actuated control” (which detects all approaches to “call” or service a given movement as long as traffic is present or until and maximum time interval is reached), is only used at one intersection in the City of Marshfield: 29th Street and Central Avenue.

Traffic is detected through inductance loops (which are metal wires buried under the road and signal the controller when there is traffic), video cameras, or a micro-wave.

Intersection of Veterans/Centeral. Photo Credit: Steven Okonek/ Tri-Media.

“There is plenty of science behind all of this. There have been numerous studies to determine what is best. These are directed through the Federal Highway Administration, which keeps traffic signals consistent from state to state,” said Turchi.

Aside from the physical components of a traffic signal, there are also a lot of technical


formulas that are considered when programming an intersection, one being the “cycle length.”

Defined as the time it takes for an intersection to go from green, through yellow and red, and back to green on the same phase controller, the typical cycle length in the City of Marshfield is 90 seconds.

Cycle lengths are composed of “phases,” which designate how long a light stays green, yellow, or red. In Marshfield, for example, the “all red phase” typically lasts 2-seconds, allowing for traffic to clear an intersection during a cycle.


Adding to the complexity, the formulas include factors such as when the lights change and how traffic flows, and these are all programmed into the traffic signal controller. Calculations used to be done by hand, but software developments have made it a much easier and efficient process.

Additionally, cycle lengths can be programmed for different times of the day to accommodate rush hours, resulting a truly complex yet functional system.

Turchi noted that he often hears driver complaints about how long a red light is at any given intersection, and explained that there are several reasons why the lights are programmed the way they are.

“A lot of people don’t understand why and how they are the way they are. They are concerned about their own specific movement, not the system as a whole,” he said. “What we’re trying to do is reduce the amount of fuel that is used and try to program it to basically operate most efficiently with what we have, and safely.”


A smart system, traffic signals are interconnected through a master controller which keeps traffic flowing.

“What we’re trying to do is keep the through street moving at a specific speed and trying to reduce the amount of cars that have to stop at a given light,” said Turchi. “This reduces fuel usage, which helps the environment because it lowers emissions that take place during acceleration.”

“I’ve done the calculations,” he added. “Any time you have one car accelerate from an intersection, it’s basically like dumping a tablespoon of fuel out. If you only have two cars on the side street, it’s only two cars accelerating as opposed to however many on the main road.”

He explained that in major corridors (such as downtown), signals are programmed so that anyone obeying the speed limit will continue to get a green light.

“If you pass through and you aren’t speeding, you should be able to hit the lights without stopping,” he said.

Currently if there is an issue with a traffic signal, or need of reprogramming, Turchi must travel to the site, hook up his laptop, and conduct the work on-site. In the future, Turchi hopes to implement a software program that would allow him to make modifications, as well as monitor signals in real-time, right from his office.

“This would improve safety and efficiency, and allow me to make adjustments as needed,” he said. “For example, if there was an accident and officers had to detour traffic to another location, I could program an adjustment into that controller from my office, based on Police Department input.”


In a typical cabinet (traffic signal controller), there are several fail-safes, including a conflict monitor. That way, if there is a programming error, the intersection will recognize this and go to all-flashing-red.

“It will actually within 4/100s of a second throw the intersection into a fail-safe mode, which is an all-red flash,” said Turchi. “It can fix itself sometimes, but otherwise stays like that until someone manually fixes it.”

Marshfield’s oldest traffic signals (at Fourth Street/Peach Avenue) are currently being replaced and upgraded to new video technology, with the intersection of Doege Street and Central Avenue, as well as St Joseph Avenue and McMillan Avenue, next on the list. The former is the lighted intersection with the most accidents in the City, consisting mostly of rear-endings.

With St Joseph Avenue/McMillan Avenue, there is currently discussion about the best course of action, whether to install traffic lights or a roundabout.

“When you take a look at the difference between a traffic signal or roundabout, accident rates are typically about the same, but in a roundabout they are less severe,” said Turchi. “There are side-scrapes instead of head-ons or t-bones. It’s a safer situations in the long run, but it will depend on the room available at that site.”


Programmed to be both environmentally friendly and with the intent of improving driver safety, Turchi is proud of the current traffic signal system in Marshfield.

“We are reducing accidents at a pretty good rate. About 3% per year typically,” he said.

Residents with questions or concerns are encouraged to contact Turchi’s office at 715-486-2034.

News Desk
Author: News Desk