Curious Iowa: How are traffic lights timed?

Cedar Rapids resident Jim Fox spends a lot of time behind the wheel in downtown Cedar Rapids. Last year, he started a part time “retirement gig” driving people with disabilities to their medical appointments. During his time behind the wheel, he’s begun wondering about the stoplights downtown.

“I suppose the best description is they seem to be consistently inconsistent,” Fox told The Gazette in an email. Fox has noticed that depending on the time of day, the lights operate differently.

“Some red lights seem to last 15 to 20 seconds, while others seem to go on for an eternity,” Fox said. “It’s the inconsistency that makes me wonder what is the rationale (if any) behind the lights.”

Fox turned to Curious Iowa, a Gazette series that answers readers’ questions about our state, its people and the culture, for an explanation. Buckle up, we’ll take you for a ride through the factors and calculations that go into traffic light timing.

Why does the timing of traffic lights vary?

Cedar Rapids city traffic engineer Cari Pauli said a number of considerations go into the length of traffic lights. These include time of day, the volume of vehicles on the road, type of intersection and whether pedestrians are present.

First, understand that there are coordinated and uncoordinated phases. Coordination allows multiple intersections to sync up. Think about those times when you hit green lights back to back to back. According to the National Cooperative Highway Research Program’s Signal Timing Manual, coordinated phases are given a fixed minimum amount of time each cycle under a timing plan. Uncoordinated phases operate freely and not in coordination with other signals.

Now let’s add in actuation. Actuation refers to whether a vehicle is detected at the intersection. For example, if you are turning left on a mainline road, like First Avenue, the green arrow is actuated. This means it will only turn green if a vehicle is detected in the left turn lane. If there’s no vehicle present, the left turn phase will be skipped.

So, it’s important to pull your vehicle up to the stop bar and stay put.

“If a vehicle is stopped too far back from the stop bar or too far beyond the stop bar, they may risk not being detected.” Pauli said.

At upgraded signals, video cameras can detect a vehicle, but there are still loop detectors in use around the city. Loop detectors use a wire in the pavement to detect the presence of a vehicle.

“It resonates at a particular frequency and when a vehicle traverses the loop, it increases the frequency and sends a signal to the controller that says a vehicle is present,” Pauli explained.

Loop detectors can fail, and as signals are upgraded, that technology is being replaced. If you don’t see a camera at an intersection, you can assume it is using loop detection.

The presence of a pedestrian also can impact how long you sit at a red light. Pauli explained that the city is adding pedestrian actuation, meaning a pedestrian pushes a button to get the signal to cross. Pauli said some of the city’s signals do not have this feature, so pedestrian crossing comes up every cycle regardless of whether someone is there or not. Upgrading those signals will improve efficiency, she said.

Unsurprisingly, timing plans also change based on the time of day. Coordination keeps traffic flowing at peak hours but at off-peak hours, it is more efficient to switch to a different plan.

“You might run a cycle length of 90 seconds in the morning, but then when it drops off at 9 o’clock, we might run a 70-second cycle length because there isn’t as much traffic and we want it to be really efficient,” Pauli said. “And so then we don’t have a longer cycling to make people sit and wait on the side streets longer.”

How often are timings updated?

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The City of Cedar Rapids has 175 traffic signals. This total does not include the pedestrian signals, warning flashers or signals of that nature. Traffic signals on major roadways are retimed once every three years on a rotating schedule. This means about 40 intersections per year have their timing upgraded. Minor arterial and collector roadways — the roads that “collect” traffic from local roads and funnel it to arterials — are retimed once every five years, which is about 10 intersections annually.

Cedar Rapids has 23 coordinated groups that make up approximately 90 percent of the city’s signalized intersections. For example, Pauli said, 27 signals in the Westdale Corridor, which includes Williams Boulevard, Edgewood Road and Wiley Boulevard, are currently being reviewed.

Retiming is a big undertaking. It takes time to pull the data and to implement it, and the plans are associated with peak and off peak hours during the morning, midday and evening.

“It’s not just, ‘Hey, we put out a timing plan for this’,” Pauli said. “It’s multiple [plans] throughout the day for all those signals on that corridor.”

Are there industry standards?

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Pauli said there are industry standards but there is some variation. For example, Cedar Rapids uses all red timing, which is when all lights at an intersection show a red light briefly before moving on.

“So we implement that, but some cities do not … so there is a little bit of gray area and it’s kind of up to the discretion of the cities,” Pauli said. “but most … are standard practices that are used.”

A lot of calculations go into determining how long cycles should be. If you want to dive into the math, take a peek at the National Cooperative Highway Research Program’s Signal Timing Manual. That manual provides guidance and industry standards for traffic engineers across the country.

Pauli explained that the “85th percentile speed” is a key factor in determining the timing plans, like red and yellow clearance intervals. The 85th percentile speed is the speed at which 85 percent of vehicles are traveling at or below.

“And that’s an industry standard where you use that,” Pauli said. “that’s the typical percentile speed that you’re using to design a roadway or to post the speed limit.”

If there is a traffic signal that you believe isn’t functioning the way it should be, contact the traffic engineering team at the department of public works. You can do this by calling 319-286-5176, submitting your concern in the My CR app or emailing traffic@cedar-rapids.org.

The traffic engineering team is able to look into complaints using Pan Tilt Zoom cameras.

“If a person says ‘Hey, I was at the intersection of First Avenue and Sixth Street and I sat there for four cycles and I was there at 4:55,’ we can go back and look,” Pauli said. Based on what is found, adjustments can be made.

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Comments: bailey.cichon@thegazette.com