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Good Question: Why do certain traffic signals take so long to turn green?

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You’re waiting at a red light for what seems like an eternity. You cuss and you yell but it’s not having any effect.

Finally, the light turns green. You’re hoping that traffic moves quickly so that drivers like you, who are farther back in the line of traffic, can make it before the light turns red again.

You make it to the light just as its turning yellow.

Time to step on it, you think to yourself.

You make it through the light just in time. The light is now red, but there are three cars right on your tail behind you that don’t want to wait.

We’ve all been the driver stuck in traffic during a red light. It’s never convenient and it’s always annoying.

This week’s Good Question is prompted, in part, by an email we received from an user regarding the flow of traffic along the Interstate 15 interchange at Exit 119 in Idaho Falls, which connects I-15 and U.S. Highway 20. Here is a portion of an email we received from Andrea.

“Each night around 6 p.m., (I come) in to (Idaho Falls) via Bellin and then Grandview. There are times that we have to wait six light changes to get past the I-15 interchange.”

“I clocked it last night. The (green) light for eastbound traffic lasted 19.6 seconds. Then our red light lasted one minute and three seconds. As things stand right now, it is a no win situation. If the eastbound light is adjusted, it makes traffic on I-15 back up which is a safety issue. But traffic on Grandview backs up almost to the Skyline light.”

“Maybe it is time we revamp this interchange and (improve) the flow of traffic.”

Andrea is not the only person who feels this way. Anyone who has driven through this intersection knows exactly what she is talking about.

I reached out to the Idaho Transportation Department for a response to this email.

“If there’s a delay on Grandview, that’s a lot safer than having traffic backed up on the highway,” ITD Engineer Matt Davison told me. “The (on-ramp for exit 119) has priority. It it doesn’t, traffic will back up on the highway and you’ll have people driving 70 mph into the back end of traffic.”

Davison said safety is the number one concern at this interchange. The state is trying to find a solution to the traffic bottleneck issue and rework the interchange network so that it is safer for everyone.

Courtesy ITD

ITD’s website indicates a study is currently underway to identify and analyze corridor improvements at six different interchanges in Bonneville County.

Karen Hiatt, ITD’s Engineering Manager says they will be holding a public meeting at Skyline High School in early September to discuss the results of the study and hear proposed solutions. The exact date and time are still being determined. will post more information when it is available.

How do traffic signals work?

I also spoke with the city to learn how traffic signals, in general, work.

Kent Fugal, the city engineer for Idaho Falls, says every intersection with a traffic signal has a set of programmed timing plans. The timing for each signal at each intersection varies based on traffic demand at different times of day.

“We don’t just say ‘the left turn signal will be on for 12 seconds.’ There’s a minimum time and a maximum time for each signal,” Fugal says.

The amount of time it takes a light to change from green to yellow to red and back to green is called a cycle length.

“At an intersection like Hitt and 17th, for example, (a cycle length) could be as much as three minutes. Most of our intersections are less than that, around 90 seconds, on average,” says Bruce Scholes, the city’s Generation and Signalization Foreman. “If you let people sit very long, they get antsy. We try to keep (traffic) moving as much as possible.”

“If there are vehicles that are waiting to make a movement, our detection system picks up those vehicles,” Fugal says.

How does the traffic detection system work?

A standard detection system, according to Fugal, is a Loop Detection system underneath the pavement.

“There are several turns of wire wrapped around a loop. As a vehicle crosses those loops, it induces an electro-magnetic current flow in the loop. That current flow tells the controller there is a vehicle waiting at the light.”

The more mass a vehicle has, the easier it is for the system to detect it. Some motorcycles are not easily detected, Scholes says. The higher the speed limit, the greater the spacing for the detectors.

Stock photo

Another type of detection system used in town is video cameras. You may have seen cameras attached to a mast arm near the traffic light. Fugal says those cameras are used for traffic detection, not surveillance as some have speculated.

A third detection system used is a radar detection system.

Fugal says each type of detection system has limitations. Power outages and weather factors can impact the effectiveness of each system.

It’s worth noting that Idaho Falls traffic signals are connected to Idaho Falls Power. Since the city has its own power grid, they are able to use city employees to maintain traffic operations.

To report traffic-related issues in Idaho Falls, call Idaho Falls Power at (208) 612-8430.