Roads with variable speed limits, also known as smart motorways, are common in countries including the U.S., UK and Germany. Normally, rule-based systems monitor the number of vehicles on one of these roads and adjust speeds accordingly. One such road is a 27-kilometer section of the I-24 freeway near Nashville, Tennessee, which was experiencing a problem that besets many busy roads: when there are too many vehicles, phantom traffic jams appear when drivers brake, slowing vehicles to a crawl and risking crashes as fast-moving vehicles come up behind.
To address this, Daniel Work at Vanderbilt University in Nashville and his colleagues trained an AI on historical traffic data to monitor cameras and make decisions on speed limits, deploying it in the I-24 control room in February. Initially, the AI was tested alongside existing software – telling operators what it would have decided — and faced teething problems.
“They look at it and for the first five minutes, everybody’s like thumbs up, and then they start going to thumbs midway and then it’s thumbs down. So hey, it failed in the first 10 minutes,” says Work.
After some tweaks, the team launched a new system in March that has been able to operate unaided ever since (arXiv, doi.org/nbcz). The AI works 98 per cent of the time, but will occasionally call for a change in speed limit that is larger than 10 miles per hour, in contravention of federal law.
“It’s a bad idea if the measured speed is going 80, 80, 80, 80, 20 — we don’t want that,” says Work. “We want it to go 70, 60, 50, 40, 30.” To ensure this, safeguards switch control back to the old system for the remaining 2 per cent of the time.
It isn’t clear how drivers have responded to the system or whether it has improved traffic. The Tennessee Department of Transportation, which manages the I-24, didn’t respond to a request for interview. Work says data on the project won’t be released until later this year, as it is still being analyzed, but he is positive about the results.
“I think that we’re just scratching the surface of a whole new way to operate freeways,” says Work. “Anything that we can do to reduce the number of crashes that happen on that roadway, the number of fatalities that happen on that roadway, is worth doing.”
Oliver Carsten at the University of Leeds, UK, says that without more data it is impossible to gauge whether the AI is a net benefit or detriment. But he says that some sort of variable speed limit system is key to safety on busy roads.
“There's a well-known limit — 2,000 vehicles per lane per hour — at which point you can suddenly go from everything running smoothly at 70 miles an hour to a total breakdown where everything comes almost instantaneously to a total stop,” says Carsten. To maintain the traffic movement, you need to bring the maximum speed down, he says.
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