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Stop complaining about the traffic light schedule? Go to this site.


I must be wise and crazy.

After all, one way or another, small gripes that cost me seconds always end up turning into massive research projects. My February article on the slow-moving catwalks at Salt Lake City International Airport is a classic example that I’m still laughed at for – a minute wasted on an airport ride turned into a survey trip of Several weeks.

This week’s data column is in that vein. I recently found myself frustrated with having to wait about three seconds longer than usual at one particular intersection, that of Interstate 80 and 700 East. You see, before, the intersection was religiously flowing northbound: one green light led to another, which meant I didn’t have to stop at the second light.

My expectations were bursts end of July. All of a sudden the green light at the next signal didn’t come on, forcing me to stop… before the green light came on a few seconds later.

Like I said, that’s a minor complaint.

Since then, I’ve talked to Utah Department of Transportation signal engineers, researched the system they use, and learned how it all works. To be honest, I’m pretty impressed. I also learned the best way to report problems to the UDOT – so that if you have a jam at the traffic lights, you too can take action.

Transparent Technology

As you probably know, most intersections are associated with a multitude of technologies.

There’s the traffic light controller itself, usually found in a metal box at one corner. Most intersections have sensors just before the intersection lines, which detect whether cars are waiting at a light or crossing. Many also have cameras placed atop traffic lights to allow UDOT workers to monitor these intersections with their eyes.

That’s not all. Many intersections are also equipped with radar detectors or other sensors, located about 300 to 400 feet before an intersection, to count traffic approaching a light, as well as to know the speed at which cars are traveling . Sensors typically track information 10 times per second, which means sensor data alone is approximately 19 megabytes per day for each intersection. Some traffic light controllers upload this data to the UDOT servers in real time; in others, they download it every 10 or 15 minutes.

That’s a lot of data when you multiply it by the roughly 2,000 UDOT lane intersections. That ends up being about a terabyte per month statewide. But it’s useful information, which can be used to optimize traffic light timing throughout Utah.

How does UDOT keep track of all this? In 2011, the department developed the Automated Traffic Signal Performance Measurement System, with help from Purdue University and the Indiana Department of Transportation. ATSPM is open source software that allows signal engineers to monitor individual stop lights and the entire system to see where things can be improved. Federal transportation managers have since encouraged adoption of the system by various transportation departments across the country.

And because they were after my heart, they put an awesome amount of transparency into the system. In particular, the entire data set is available online for everyone to see – not just UDOT traffic engineers, not just consultants, not just academics who have requested access to UDOT, not only journalists. Someone. It’s all at UDOTtraffic.utah.gov/ATSPM.

“Our goal of maximizing mobility, improving safety, preserving infrastructure and strengthening the economy,” says the ATSPM’s frequently asked questions page, “means that everyone who wants to use data must have access to the data without restrictions”.

I like it so much.

Use this traffic data

So what can you do with this data? Well, you can measure the efficiency of the operation of traffic lights. There are a number of different goals that signal engineers can examine – and optimize – in the ATSPM:

• Arrival on the green: What percentage of vehicles arrive at an intersection where the light going in that direction is already green? To maximize this, engineers frequently place lights along a street on the same cycle time – say, 90 seconds – so that lights in one direction can synchronize.

• Approach time per vehicle: Approaching an intersection – or waiting at an intersection – how long is each car delayed, on average?

• Split failure: How many times does a red light come on before all the traffic waiting at a light can get through?

• Pedestrian phases: Pedestrian traffic differs enormously. Most intersections assume that pedestrians will cross streets at around four feet per second, or 2.7 mph. Some intersections near schools or seniors’ residences require slower walking times. In general, how effectively does an intersection handle pedestrian traffic alongside its vehicular tasks?

All of this can be calculated for any period of time. So, for example, I can look at the intersection of 4500 South and State Street, and see which directions saw green at what times between 5 and 6 p.m. on Thursday.

The first three phases of the Timing and Actuation report for the time of 5 p.m. on August 25. for the intersection located at State Street and 4500 South. (https://udottraffic.utah.gov/ATSPM)

Or take my little gripe intersection, at I-80 and 700 East, where I had to wait a few seconds longer than usual. Arrival on the green percentage for northbound cars took a sudden drop, just like I experienced:

This was also accompanied by a longer average delay at the intersection.

Again: ATSPM is online for everyone, so you can pull charts like these, and many more for the intersections you use regularly. The biggest difference is that, on the back-end of the system, UDOT signal engineers can adjust the plans that the actual lights use to try to minimize traffic.

Report needed fixes

How do signal engineers know that things are not working well at a particular intersection?

First, there is fairly regular recalibration. UDOT visits each of its intersections every few years (more often in places with extreme growth, like the southwestern end of the Salt Lake Valley) to recalculate the average traffic count and reconstruct traffic timing. fires from there.

What about more pressing issues? In some cases, the system may trigger red flags. For example, if a sensor at a particular brake light shows no cars triggering it for an unusual amount of time, it likely means the sensor is broken. The system notifies the engineers, who then instruct the technicians to go to the signal to investigate.

Sometimes it’s a quick fix that can happen within days. Other times parts must be shipped and the road temporarily closed to fix the faulty sensor. In either case, signal engineers will find a way to fix the timing of the light – for example, telling a light to always give one direction a green light every cycle, even if no car is detected in it. this direction due to the broken sensor.

In a large number of cases, problems require human detection. This can come either from the signal engineers monitoring the system or from the general public. UDOT has several means of contact, but the preferred method is through the “Click ‘n Fix” app, available for Apple and Android devices. There you can send reports about things not working properly – traffic lights, yes, but also things like potholes and broken cameras.

In the case of the intersection of I-80 and 700 East, the slight delay in the timing of the lights was actually causing a rush hour traffic problem. Cars waiting for the light to change sometimes blocked turning lanes for traffic exiting the freeway. When this happened, engineers investigated the problem. UDOT suspected that a power surge caused two connected lights to disconnect, resulting in inconsistent timing of the green light.

Coincidentally, the issue was resolved a day before I called UDOT. Green finish percentages are back to normal – as is my ride.

Now, how can I get my seconds back?

Andy Larsen is a data columnist for The Salt Lake Tribune. You can reach him at [email protected].

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