The Drive: Ramp meters keep metro traffic flowing

Ramp meters have been part of the metro area's traffic network since the 1970s, but many motorists still don't understand their function.

The Minnesota Department of Transportation aims to keep highway and freeway traffic flowing at 50 miles per hour or faster, but during peak periods, congestion sets in when motorists on the mainline are forced to slow down to accommodate large groups of vehicles trying to get on the road. Ramp meters are designed to space out the merging traffic and "maintain that peak flow for as long as possible," said Brian Kary, a freeway operations engineer for MnDOT. "Once congestion sets in, you have fewer cars driving down that roadway because they are basically parked."

Recent upgrades to the system of 440 meters throughout the Twin Cities are helping to keep mainline traffic moving while minimizing the amount of time motorists queue up at red lights on entrance ramps. Sensors embedded in the pavement can now send real-time traffic data to computers, which use algorithms to determine if traffic flow is approaching congestion levels and meters need to be turned on.

In the past, meters would turn on as soon as traffic reached congested levels any time between 5:30 and 9 a.m. and 2 and 6:30 p.m., and would run until the programmed shut-off time. The criteria for turning the lights on was based on traffic flow rather than traffic density. Traffic flow is determined by the number of vehicles that pass a given point, while density is measured by the number of vehicles per lane mile. Simply put, the old system could not tell the difference between 1,000 cars passing a certain point at 20 mph vs. 60 mph. The new system can, meaning ramp meters will now turn on only when needed and revert to their blinking yellow status as soon as traffic conditions improve.

The system also can release cars at a quicker rate — but not en masse — when entrance ramps fill up with too many cars or wait times get too long. The average wait time at a red light is 2 minutes with the maximum not exceeding 4 minutes, MnDOT says.

"It's more responsive," said Jesse Larson, a MnDOT freeway systems operations engineer. "Now maybe they run only 45 minutes during the rush on a lighter traffic day. We have seen that on [observed] federal holidays and weeks leading up to, say, Thanksgiving."

MnDOT put in the new ramp metering system after a 2012 study on Hwy. 100 found that congestion-related delays dropped by 48 percent during October and November when compared with the same months in the previous year, even though traffic volume was up almost 3 percent. In the spring of 2013, delays were down by 17 percent. While results have not been that dramatic on all corridors, Kary said motorists should not be experiencing unnecessary delays due to ramp meters coming on too soon or staying on too late.

New system or old, motorists have long been frustrated when held back when traffic is cruising at 50 mph.

"It is counterintuitive when you see traffic on the mainline flowing and you think, 'I'd be halfway to work if I were out there with them,' " Kary said. "The reason the mainline is doing 50 is because we are metering. As we let more cars on, everything slows down and the roadway is able to deliver less traffic. People have a hard time grasping that concept."