How does MnDOT set signal light times?

Signals at the intersection of Highway 371 and Barclay Avenue in Pine River looking south.

When does a yellow light change too fast?

That's a question people who frequent Pine River may ask themselves occasionally when they have the misfortune of hitting a yellow light in town at the wrong moment. Asked if they think the yellow light in Pine River is too short, 13 Facebook followers answered in the affirmative.

“It definitely is way too short of a time length from yellow to red. Most times it feels like quite the abrupt stop and in the winter it's not safe to stop that fast without sliding. And if you don't stop you're running a red light,” Lori Roller said.

“I drive Northern MN for work and I swear it's the shortest light I come across. With most stop lights, if you enter past the white turn lines doing the speed limit you will make it through the light. Not in Pine River!” Travis Kelley said.

“I've told Shawn there is absolutely no way you can get through that light if it turns yellow as you hit the intersection.” Lori Lentz Bartholomay said.


“Sometimes I think that there is a chance I will be rear ended. I know that I can not take even a slight chance on the yellow so I hit the brakes. The person (unless they are local) behind me can't possibly know why I would be stopping so suddenly on a yellow,” wrote Tina Hanneken.

Local authorities chimed in and agreed that the lights seem to change too fast. Pine River Police Chief Paul Sand said he has noticed that motorists often have to make a difficult choice when this signal is yellow.

“When you see it go from green to yellow and red it seems to be really fast,” Sand said. “I sit and watch from different places and if I see them within a half a block from the old Family Dollar by the alleyway and the light turns yellow at 30 miles per hour you either dynamite the brakes or go through. At a normal light you just go through because you figure you have time. By the time they are through the intersection it's turning red already.”

Public Works Director Mike Hansen has received complaints from people in the past regarding the speed of the red light.

“We have in the past had questions about why it is so short,” Hansen said. “It's all MnDOT controlled and I've quizzed MnDOT before. They said they set it for the speed that goes through Pine River. I have heard from numerous people about it being fairly short. There aren't many people that go through that intersection more than me in a day's time and you can bet there are a lot of people going through red lights because they think they have longer. It is pretty short. If you're doing 30 and that thing turns yellow you better be putting your brakes on immediately or you are going to sail through a red light.”

So the question is, does the light really change too fast?

The Minnesota Department of Transportation has a calculation they use to set the yellow light intervals. They use the following equation:

Y=t + 1.467 v


2(a + 32.2g)

In this equation Y is the yellow interval in seconds, t is the perception-reaction time (MnDOT assumes this at 1 second), v is the posted speed (30), a is the deceleration rate (MnDOT uses 10 feet per second squared) and g is the grade of the approach (0 at this intersection). Using this equation, the yellow traffic signal for vehicles driving through the intersection on Highway 371 should be 3.2 seconds approximately. How does this compare to the actual timing on that light?

It turns out the yellow signal in all four directions were the same on Jan. 8. All of these lights are three seconds, which, barring any misreadings, is slightly shorter than the MnDOT equation recommends. How does this compare to other cities with traffic signals?

Pequot Lakes also has a traffic signal, however, the speed limit in Pequot Lakes is higher. Adjusting for a 35 mile per hour speed limit, the MnDOT equation suggests a yellow signal interval of 3.56 seconds approximately. The northbound yellow signal in Pequot Lakes on Jan. 8 was 4 seconds. In Walker, where the speed limit is also 30 miles per hour, both yellow signals in town were timed at just over 3.2 seconds on Jan. 8. If Pine River's timer really is set at 3.2 seconds then motorists should be experiencing the same issue in Walker as they do in Pine River, unless there are other factors at play.

Interesting enough, this is a discussion that much larger cities have. In Chicago in 2014 and 2015 the city's three second yellow lights attracted media attention. Some not only claimed the yellow lights changed too fast, but that the city was intentionally setting them low in order to rack up fines from traffic violations. Pine River is not Chicago, but it may surprise some that the two cities have something in common.

In Minnesota, the yellow light intervals are set according to that equation based on what county engineers call “decision distance”. That equation is meant to protect against what highway engineers call the “dilemma zone”, which is that conflict drivers experience when choosing between running the red or stomping their brakes.

In theory, this equation should help give drivers the optimal amount of time to choose whether to stop safely at a traffic signal, or continue to drive through in a safe manner. Given that applying the brakes at full force is not a safe way to stop at an intersection, the above sources seem to be suggesting the equation is not working in Pine River, but why not? Delage said it could be a failure in sensors on the traffic signal. In addition to having a specifically timed sequence, the traffic signal in Pine River is also designed to sense if a vehicle is approaching from approximately the same amount of time it takes for the yellow light to transition to red.

If sensors are working correctly then a traffic signal should remain on green (called vehicle extension), again, in order to help drivers avoid being stuck in a situation where they have to choose between slamming on the brakes or running the stop light. If the system is working properly they should have enough time to decide this safely.


“The vehicle extension timer comes into play each time a vehicle crosses a detector while the approach phase is green,” Delage said. “As it name implies, it extends the green indication by the time programmed as the vehicle extension parameter and begins to countdown. If another vehicle crosses the detector during this countdown, then the timer is reset and it starts again.”

Though the light in Pine River was timed at 3 seconds consistently from all sides, DeLage says MnDOT records indicate the signal is supposed to be set for 3.5 seconds for traffic approaching on Highway 371 and 3.2 for traffic approaching from Highway 84. The MnDOT equation, again, suggests the timer should be at 3.2. That being said, it's possible that the timers in Pine River are off or that our attempts at measuring the yellow light increments were faulty for some reason. After a phone interview with DeLage, he said MnDOT would check both the timers and the sensors to see if everything was working correctly.

“We will check it out in the field and let you know if there is anything amiss,” DeLage said.

In the meantime, though it has not been absolutely determined that the yellow lights in Pine River change too fast, even local authorities suggest that might be the case.

The evening of Jan. 9 a technician with MnDOT did find an issue with the light. In an email update, DeLage explained what they found.

“My tech was at the signal this afternoon,” DeLage wrote. “The controller had an excessive amount of time programmed for the vehicle extension. He lowered it from 5 to 3.5 seconds and watched operation.”

This means that the sensors that extend the green light based on oncoming traffic were set incorrectly. This may have been a contributing factor contributing to the feeling that the yellow light wasn't lasting long enough. DeLange did say that the timer on the light was set for the proper time identified by MnDOT, so the actual duration of the yellow light was not changed.

Whether or not this will make a noticeable improvement for motorists in Pine River has yet to be determined.

It is also possible the width of the intersection affects motorists perceptions of the yellow light. If the light does last 3.2 seconds, a vehicle traveling 30 miles per hour will cover 140.8 feet in the same amount of time. With four traffic lanes and two shoulders wide enough to park on plus the width of the crosswalk and approach, that means the intersection is 110 feet wide. If a motorist is only 30 feet from the intersection when the light turns yellow, it will be red before they have completely crossed. This may cause some drivers to feel anxious.

Part of the reason you want to make smooth decisions on whether to brake or continue through an intersection, of course, is safety. While there is no guarantee that the current timing makes a significant negative impact on safety there, Sand has seen several accidents there where the lights may have been a contributing factor, then again, maybe not.

“We've had a couple accidents there,” Sand said. “Some were not due to that.”

Hansen may have experienced one such accident first-hand.

“In fact, I've been hit before,” Hansen said. “I've been t-boned at that intersection.”

So the question remains, is the yellow light at the Pine River traffic signal too fast? That remains in the eye of the wheel-holder.

Signals at the intersection of Highway 371 and Barclay Avenue in Pine River looking north.

Travis Grimler began work at the Echo Journal Jan. 2 of 2013 while the publication was still split in two as the Pine River Journal and Lake Country Echo. He is a full time reporter/photographer/videographer for the paper and operates primarily out of the northern stretch of the coverage area (Hackensack to Jenkins).
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