In the summer of 1969, the Minnesota Department of Transportation (MnDOT) tried a two-week experiment to alleviate congestion on Interstate 35E near downtown St. Paul. The agency installed traffic signals on ramps leading from Maryland Avenue, Roselawn Avenue and Wheelock Parkway to southbound I-35E to see if putting space between vehicles getting onto the freeway would improve traffic flow.
The metering operation worked.
“It found that through traffic will cooperate with a single car merging from the ramp, but has a tendency to be hostile to platoons of cars entering from the ramp,” according to an article published in “Minnesota Highways,” MnDOT’s employee newsletter at the time.
Ramp meters became permanent at Maryland and Wheelock in October 1970, and a half-century later the Twin Cities has 461 meters — one of the largest networks of the quick-cycling lights in the nation.
“We were an early adopter,” said Brian Kary, MnDOT’s director of traffic operations, noting that the first meters in the nation appeared in Chicago in 1963 and Los Angeles in 1967.
Today, the meters are one of MnDOT’s most effective tools in its efforts to keep traffic flowing as congestion worsens, Kary said. The 2018 Metropolitan Freeway Congestion Report found that motorists are caught in congestion 25% of the time during the morning and afternoon commutes, the highest levels since the state began conducting annual highway surveys.
MnDOT defines congestion as traffic moving at 45 mph or less.
In a study carried out in the early 2000s, MnDOT found using ramp meters during rush hours made travel times 22% faster, kept traffic moving 7 mph faster, and resulted in four fewer crashes on highways and freeways each day. MnDOT has not done a comprehensive study since, but Kary said drivers continue to benefit from the meters.
“When we can keep the mainline flowing, you can get 2,000 vehicles per lane past a point per hour,” he said. “When things are stop-and-go, you maybe get 1,500 per lane per hour.”
Ramp meters have come a long way over the past 50 years. In their infancy, the algorithm that controlled meters had to be adjusted by hand. Later versions used timers.
Today, meters are connected to a series of more than 5,000 sensors embedded in the pavement and radar sensors, allowing meters to update to real-time traffic conditions and turn off when they are not needed.
Meters generally are used from 5:30 to 9 a.m. and 2 to 7 p.m., but will flash yellow during those hours when there isn’t enough traffic to warrant their use.
Drivers may have to wait a few extra minutes on the ramp — up to a maximum of four minutes — but that’s the key to keeping freeway traffic moving, Kary said. Breakdowns in traffic flow occur near interchanges when surges of traffic attempt to enter freeway all at once.
“They get frustrated when they have to sit and they ask ‘Why am I waiting?’ ” Kary said. “It might take a little longer to get on the freeway, but traffic is moving because you are waiting. The light is doing its job.”
Even when freeways and ramps are jammed beyond capacity, such as on westbound I-394 at Hwy. 100 during the evening rush hour, the meters still serve a purpose.
“They can help get traffic get back in recovery mode,” Kary said.
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