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Spencer Durham column: How are traffic light cycles determined?

Oct 04, 2023

Spencer Durham

Traffic lights in Kokomo are pictured.

I turned to my coworker James Bennett one night at the office and said, “I thought of a really stupid idea for a column.”

“What’s that?” he asked.

“What if I went and sat at the busiest intersections in Kokomo and timed the light cycles?”

“If you do that, it’ll be one of the most read stories of the month,” James replied.

We’re putting that to the test this month.

I’m sure you’ve found yourself traveling on Indiana 931 — perhaps you’re running late to a meeting because you’re a perpetual five-minutes-behind kind of person — and you hit a red light.

“Great, how long am I going to sit here?” you ask impatiently, side eyeing the clock.

Or maybe you’ve sat at a red left turn arrow at 1 a.m. for what feels like forever and decide to just make the turn because no one else is around.

Been there, done that.

So how do they determine the cycles at an intersection?

I emailed the Indiana Department of Transportation and got linked up with possibly the best guy to answer my questions, Ed Cox.

Cox is the engineering director for INDOT’s Traffic Management Group. He’s oversaw the traffic patterns along the 931 corridor for years.

There are 15 signals along 931. These are the intersections. All are timed, and the cycles vary throughout the day and on the weekend.

There are four different patterns on weekdays.

The morning pattern is in effect from 6 a.m. to 9 a.m.

Midday is from 9 a.m. to 3 p.m. Evening is 3 p.m. to 8 p.m., and overnight goes from 8 p.m. to 6 a.m.

The weekends are simpler. There’s a daytime pattern — 8 a.m. to 7 p.m. — and an evening pattern — 7 p.m. to 8 a.m.

The length of a light cycle is about 120 seconds for most of the 931 lights. This means the time it takes for a pattern to cycle through all options (green, red, turn arrow) is about two minutes.

Cox told me to think of it as a pie chart. Part of the pie is green, another part is red and a smaller sliver is for the turn arrow.

And Cox has news for all of you who think the red lights last forever.

“It’s probably not as long as you think,” he said.

The pie chart analogy didn’t immediately click with me when I went out and timed some of the signals.

As I sat in my car by Starbucks on Alto Road, I timed how long the light stayed red as cars traveled west across 931. Then I timed how long the light stayed green.

The numbers I got didn’t make any sense. I was lost.

That was until I used the stopwatch feature on my phone, specifically the “lap” button.

I started my timer the moment the light turned red. When it turned green, I hit “lap” and repeated this for about 7 minutes.

Combined, the time the light was red and green would add up to about 2 minutes every time.

The east-headed-west light on Alto Road at 931 stays green anywhere between 41 and 49 seconds. It stays red for about 1 minutes and 15 seconds.

So, if you are driving on Alto Road and hit the red light at 931 at around 7 p.m. on a weekday (evening pattern), expect to sit there for about 1:15 before the light turns green.

I then moved over to observe the lights for southbound traffic on 931 at the same intersection. The amount of time differed slightly but still added up to about 120 seconds.

The southbound light on 931 at Alto Road stays green anywhere between 24 and 32 seconds. It’s red for about a 1:30 during the evening pattern.

Traffic patterns on 931 are reconfigured every four years. You might have seen crews working on them last fall.

“We try our best to move the most traffic down 931,” Cox said. “My goal is to move you through as many signals as efficiently as possible.”

Patterns are based on a number of variables including the amount of traffic, the time of day and when traffic is the busiest. Cox said INDOT buys data from Purdue University that shows how long it takes vehicles to travel down 931 and how many stops they make.

Back in the old days, INDOT folks would come out and count cars.

Industry can also factor into a pattern. Cox said traffic light patterns near the Subaru plant in Lafayette differ based on shift changes.

Traffic lights near a school might be designed with morning drop off and afternoon pick up in mind. Cox said these patterns can be turned off in the summer.

Cox is probably one of the few people who welcomes complaints. He wants people to tell his office what they think about traffic patterns. It helps INDOT fine tune signals.

“It’s really nice when you can get a lot of local eyes and ears on the system,” he said.

I also timed the signals at Markland Avenue and 931. The times here are different and the cycle appears to be more than 120 seconds.

The northbound lights stay green for about 49 seconds and red for about 1 minute and 57 seconds. Again, I timed these lights during the evening pattern.

For southbound traffic, the light stays green for about a minute, sometimes a little more, and red for one minute 45 seconds, sometimes close to 2 minutes.

If you’re headed west on Markland, expect to sit at a red light for almost 2 minutes during the evening pattern. The green light lasts 48-52 seconds.

Underground sensors play a part in the traffic patterns. If you see a circular, six-sided or eight-sided cut in the road at an intersection, that’s where a sensor is.

These detect the metal in your car (I always thought it was weight). If you’ve ever noticed a metal box embedded in a road, that’s the connection piece for the sensors.

A sensor can extend time. For example, if a sensor picks up on a lot of vehicles in a turn lane, the green arrow will stay green longer. Cox said each cycle has a minimum and maximum time for green.

Sensors are also important for times when there is no traffic in a lane. If a side street lane does not have any waiting cars, the green light on 931 will stay that way.

I had hoped to talk to the city about how its light patterns are configured, however, after four emails, all I got was an outside firm does them.

That’s the nature of journalism sometimes. You don’t always get all the information you want.

This column is part of a series where I answer random questions. Have one for me? Drop me a line.

Have You Ever Wondered is a monthly column by Tribune education reporter Spencer Durham. He can be reached at 765-454-8598, by email at [email protected] or on Twitter at @Durham_KT.

Spencer Durham

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