A warning system being tested along 15 miles of Interstate 17 in Phoenix alerted authorities to a vehicle traveling the wrong way.
No crash resulted from the incident early Thursday and the wrong-way driver was arrested on suspicion of DUI.
Arizona Department of Transportation officials say it was the first vehicle detected on I-17 travel lanes in Phoenix since the wrong-way system went into operation in January.
The $4 million system includes 90 thermal detection cameras positioned above exit ramps and the mainline of the freeway between the Interstate 10 "Stack" interchange near downtown Phoenix to the Loop 101 interchange in north Phoenix.
It's designed to alert authorities so traffic operators can warn other drivers via overhead message boards and state troopers can respond faster than relying on 911 calls.