U of MNUniversity of Minnesota
Center for Transportation Studies

Programs & Labs

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Spring 2001

Under the virtual microscope

Ed. note: Ramp metering, the practice of regulating the rate at which cars enter an urban freeway by means of traffic signals at entrance ramps, is an increasingly common traffic management technique in metropolitan areas with extensive freeway networks. But despite the broad support that ramp metering in general enjoys among traffic managers and researchers, the benefits and drawbacks of particular metering algorithms continue to be widely debated by academics, politicians, and the press. In the Minneapolis-St. Paul metro area, three research teams studied different aspects of ramp metering during and after an experimental area-wide meter shutoff.

From subatomic collisions to traffic jams, digital simulation using powerful computers is a critical tool for researchers studying complex systems. However, traffic systems modeling has been limited in the past by the inability of available software to model the variable movements of thousands of independent vehicles of different types. This kind of "microscopic" simulation is capable of achieving much greater accuracy than simulations that treat traffic as a continuous fluid flow—but the increased detail comes at the price of greater demand on the computer and its human users.

Photo of Panos Michalopoulos Panos Michalopoulos Photo of John Hourdakis John Hourdakis

In the Department of Civil Engineering, Professor Panos Michalopoulos and research fellow John Hourdakis used sophisticated modeling software to create models of several Twin Cities freeway sections based on individual simulated vehicles, and used their simulations to test the effectiveness of ramp metering on the metropolitan freeway system. The meter shutoff period provided a unique opportunity to compare the results of their computer simulation to observed effects on the real traffic system.

The team studied ramp metering through an application in which Mn/DOT's real-time ramp control strategy was compared with the no control alternative on two test sites. The first was a 20-km section of Trunk Highway 169 northbound from the I-494 interchange; this section traverses the metro area without going into the city center. The second site was I-94 eastbound between I-394 and the Ninth Street exit ramp. This section is part of the connection between the two downtown areas, and is often severly congested during peak travel times. In both cases, the simulation encompassed the freeway proper and the ramps up to their intersections with the surface street. Data to create and calibrate the simulations were gathered from pavement-embedded vehicle detectors and by manual counts for three consecutive days at each test site.

Analysis of the simulation results revealed that the use of ramp metering reduced total system travel time in both test areas by 6 to 16 percent. The average freeway mainline speed with ramp control increased by 13 to 26 percent. On both freeways, ramp metering sharply reduced the number of times vehicles stopped in traffic. This smoothing of traffic flow yielded measurable reductions in fuel consumption and pollutant emissions from vehicles traveling on the freeways.

In both the simulations and the real-world measurements, the data confirmed that Mn/DOT's ramp metering strategy has a beneficial effect on travel times for the freeway system as a whole; however, the researchers noted adverse effects on the ramps themselves, such as queues and traveler delays. Similarities between simulated and real-world delays support the hypothesis that certain delays are probably inherent in the current metering algorithm--but Michalopoulos and Hourdakis believe that these inefficiencies could be corrected.

The simulation research also supported the idea that the effects of ramp metering are likely to vary greatly according to traffic volume, which can change unexpectedly. This suggests that an optimal ramp metering algorithm should take into account these variations in daily demand patterns.

As the process of developing better metering strategies gets underway, the simulation techniques used by Michalopoulos offer researchers and planners a new tool for testing new strategies prior to deployment and for fine-tuning the operation of metering strategies that have already been implemented. The ITS Institute's simulation lab will be used to evaluate several alternative metering strategies that may be employed by Mn/DOT on Twin Cities freeways.

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