Cars line up at a ramp meter.

Major Ramp Metering Upgrade Reduces Freeway Delays

Motorists are experiencing less delay on metro-area highways, thanks to major changes to the Twin Cities’ ramp metering system.

The Minnesota Department of Transportation has reconfigured ramp meters to be more in sync with real conditions. With changes to the turn-on and turn-off criteria, the meters are actually running for a shorter period of time and are only activated when needed.

Ramp meters are traffic lights placed on freeway entrance ramps that control the frequency that vehicles can enter the highway. Sensors embedded in the pavement collect the vehicle traffic data used to time approximately 440 ramp meters.

Staff at the Regional Transportation Management Center, which manages the ramp meters, say the whole system is operating better because of changes that were implemented approximately one year ago (based off a 2012 study).

University of Minnesota-Duluth professor Eil Kwon developed the system’s new software algorithms. In a case study of Highway 100, he found that the delay on the mainline dropped by nearly half.

On northbound Highway 100, the amount of “delayed vehicle hours” — defined as the vehicle hours of traffic flow with speeds less than 45 mph — that motorists experienced dropped 48 percent during the months of October and November in 2012 when compared to the same period in 2011. During the same time period, total volume on that section of northbound Highway 100 increased by 2.7 percent, Kwon said. In spring 2013, the amount of delayed vehicle hours had been reduced by 17 percent.

These results are preliminary, as additional analysis is needed to determine if these results are typical throughout the system on other freeway corridors. However, based on a personal savings of $16.50 per hour, the scenario described above represents a cost savings to motorists of $1,353 to $3,447 per day (depending on the season). That’s as much as $339,150 to $861,640 per year for just a six-mile stretch of highway.

More efficient

Under the old system, each ramp meter would turn on based on current traffic conditions, but the criteria to turn on were easily met, causing the meters to turn on too soon. The old system did not have turn-off criteria, allowing meters to run until a pre-set time of day.

With the new system, improvements were made to make the meters respond more appropriately to current traffic conditions. The turn-on criteria were improved so that meters come on only when needed, and turn-off criteria were added, allowing meters to turn off when traffic conditions improved.

The new metering system is particularly effective at reducing the number of meters operating on light traffic days.

“On days like the ones leading up to Thanksgiving, where traffic may be 10-to-15 percent less than normal, instead of, say, 150 ramp meters being on at a particular time, now maybe only 50 ramp meters will be operating,” explained MnDOT Freeway System Operations Engineer Jesse Larson.

Upgrades to the ramp metering system also allow for a better picture of what traffic is like at a given moment, because it’s now based on corridor density rather than traffic flow.

Traffic flow is the measurement of the number of vehicles passing a given point. Using traffic flow was flawed, in that similar traffic flows can occur at different speeds. The old system couldn’t differentiate between 1,000 cars passing by at 20 miles per hour versus 1,000 cars passing at 60 miles per hour, for example.

Corridor density, on the other hand, is the number of vehicles per lane per mile. By measuring density instead of traffic flow, the system has a more accurate picture of what current conditions are like on the freeway.

Another bonus: ramp meters will no longer release a bunch of cars simultaneously once an entrance ramp fills up. That’s because the system can now detect the ramp filling up and release the extra cars gradually instead.

The amount of hours vehicles wait at entrance ramps fell by nearly 50 percent during the fall months along a section of Highway 100.
Twin Cities ramp meters now turn on and off based on live traffic conditions.

Related Resources

Development of Freeway Operational Strategies with IRIS-in-Loop Simulation study

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