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In a surprise finding, University of Minnesota researchers say that motorists who pay to drive in the carpool lanes on Interstates 35W and 394 are more likely to use them when tolls are high and to travel in the general traffic lanes when prices are low.

They also found that drivers who use the HOT lanes (High Occupancy Toll) are paying $1 to $2 per minute to save time. That’s more than $60 an hour.

Authors David Levinson and Michael Janson concluded in their ongoing study that drivers use the advertised toll price at entry points as an indication of the severity of congestion in the general purpose lanes and are more likely to use express lanes when prices are high to avoid sitting in traffic.

MnPass uses dynamic pricing, meaning the cost to drive in the HOT lane changes based on traffic conditions. Prices range from 25 cents to $8 and are adjusted every three minutes based on conditions. The average price is $1.50 to $2, said Brian Kary, MnPass freeway operations engineer.

Kary said he agrees with the study’s premise that drivers use toll information to judge how bad congestion will be downstream, but that there is a point when people will stop paying. MnDOT research indicates HOT lane use starts to drop off when the price hits $5. The U study did not indicate at what price HOT lane usage increased or decreased.

“There is a point when people will stop paying,” Kary said. “There is a limit to that. If we put it at $9, people won’t pay. When the HOT lanes are moving at the same speed as the regular lanes, people are less likely to jump in.”

Levinson and Janson calculated time savings and found that drivers in the I-394 HOT lane saved about 1.7 minutes in the mornings and about one minute in the afternoons. Time savings on 35W were about three minutes in the mornings and 1.2 in the afternoons.

Motorists use HOT lanes on average two to three times a week, Kary said. And even though time savings might not be that significant, there are other tangible benefits.

HOT lanes provide an absence of stop-and-go traffic and predictable travel times. It also provides for a lower-stress commute.

“You don’t have to worry about having to leave 10 minutes early to be to work by 8,” Kary said.

Levinson and Janson used two years of tolling and traffic data and modified pricing models on I-394 for three weeks in December 2012 and January 2013.

Tim Harlow • 612-673-7768