NEW MATH

UDOT uses ramp meters to keep freeway traffic on an even keel.

New software to calculates meter rates to help smooth out traffic flow on I-15.

UDOT has been using ramp meters to even out freeway traffic flow and improve safety for several years. Metering breaks up bottlenecks, smooths out surges and keeps traffic on an even keel. Without metering, freeway speeds would likely be much slower, and travel times would be less reliable. Metering also improves safety because stop-and-go driving behavior is reduced and platoons are broken up, and fewer side-swipe and rear-end collisions occur.

The present way of metering takes average traffic patterns into account. But, “traffic is never average,” explains John Haigwood, Traffic Engineer at the UDOT Traffic Operations Center. “We can’t account for all the variations.” The graph below  shows variations in traffic during the evening commute, with the black line showing the average.

Sometimes metering can be too restrictive or too free. Restrictive metering allows too few cars to access the freeway, and metering that is too free can fill freeway capacity quickly and cause flow to be sluggish.

Because of variations, sometimes metering can be too restrictive or too free. Restrictive metering allows too few cars to access the freeway, and vehicles may stack on the ramps and available freeway capacity may go unused. Metering that is too free can fill freeway capacity quickly and cause mainline freeway traffic flow to be sluggish.

A new, more responsive way to control metering rates is being tried on I-15 in Salt Lake County.  UDOT is using new sophisticated software  that “automatically adjusts to actual conditions,” says Haigwood. The system is being provided by Wasatch Front Regional Council Congestion Management and Air Quality funds.

Traffic sensors measure backing on the ramp, and traffic speeds at the merge point and downstream, and the software calculates meter rates. “The system uses data collected from traffic monitoring stations that are UDOT fiber network,” says Scott Stevenson, a Traffic Engineer who works with UDOT on the new system. “These existing stations provide the raw data that is used for the travel time signs, traffic flow maps and now, corridor responsive metering.”

Engineers at UDOT’s Traffic Operations Center “are seeing a lot of success,” says Robert Clayton, Director of the TOC. Clayton anticipates having real data soon to back up the observations. In the meantime, engineers are keeping close watch on the test sections to make sure correct parameters are being set so the software can work correctly.

It’s a balancing act, explains Stevenson. “We’re trying to maintain a better flow on the freeway by adjusting the ramps, and visa verse.” By balancing the metering rate of the ramps with the flow of traffic on the mainline freeway, road users can take full advantage of available capacity, backing on ramps can be minimized and the overall operation freeway system can be optimized.

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