Arizona trials use of ramp-meter lights as part of its I-17 wrong-way vehicle system


The Arizona Department of Transportation (ADOT) is trialling the use of freeway entrance ramp meter lights on Interstate 17 as part of the agency’s pilot wrong-way vehicle detection system in the Phoenix Metropolitan area.

During normal weekday rush hour traffic conditions, the alternating green and red ramp meter traffic lights control the steady entry of vehicles onto I-17. Now, and as part of the pilot project, when a wrong-way vehicle is detected by the thermal camera-based technology that ADOT has deployed along 15 miles (24km) of I-17 in Phoenix, the system also turns on the red lights in nearby entrance ramp meters to try to hold ‘right-way’ traffic from entering the freeway.

Via the I-17 pilot project’s decision support system, the red lights on entrance ramp meters are programmed to turn on within three miles (5km) of a detected wrong-way vehicle.

If the vehicle continues along the freeway, additional ramp meter red lights will be activated. The red lights are programmed to return to their normal mode for that particular time of day or night, including turning off, after the system detects that the wrong-way vehicle is no longer in the area.

The first-in-the-nation I-17 system features 90 FLIR thermal detection cameras positioned above exit ramps as well as the mainline of the freeway between the I-10 ‘Stack’ interchange near downtown, and the Loop 101 interchange in north Phoenix. When a wrong-way vehicle is detected entering an off-ramp, the system is designed to trigger a background-illuminated wrong-way sign to try to get the driver’s attention.

Alerts also are immediately sent to ADOT’s Traffic Operations Center (TOC) and the Arizona Department of Public Safety (AZDPS) so traffic operators can quickly activate freeway message boards to warn other drivers, while state troopers can respond faster than relying on 911 calls.

Since it went operational in January this year, the system has detected more than 30 wrong-way vehicles entering I-17 off-ramps and frontage roads. One vehicle was detected on I-17 in north Phoenix on July 5 and the driver was stopped by AZDPS. The majority of drivers in vehicles detected by the I-17 system have turned around on exit ramps without entering the freeway.

“While most drivers aren’t used to seeing a solid red light displayed by a ramp meter for more than a few seconds, we want I-17 drivers in Phoenix to know there is a reason for that to happen – a wrong-way vehicle may be ahead on the freeway,” explained Susan Anderson, ADOT’s systems technology group manager.

“As we test the overall wrong-way vehicle alert system and research its performance over the next several months, it’s important to note that the ramp-meter red lights are one of several countermeasures. The ramp meter lights are traffic signals, so I-17 drivers on entrance ramps should be prepared to stop if the light is a solid red, no matter what time of day.”

Share this story:

About Author


Tom has edited Traffic Technology International (TTi) magazine and its Traffic Technology Today website since May 2014. During his time at the title, he has interviewed some of the top transportation chiefs at public agencies around the world as well as CEOs of leading multinationals and ground-breaking start-ups. Tom's earlier career saw him working on some the UK's leading consumer magazine titles. He has a law degree from the London School of Economics (LSE).