For Sally Chicos, paying a small fee to escape rush hour gridlock is the best bargain going.

Twice a day when the price is right, the St. Louis Park resident commutes to and from her job in Roseville by using the MnPass lane on Interstate 394. Most days, she spends anywhere from a quarter to a couple dollars per trip.

"I love it. I get to slide right by everybody and see them waiting in line," said Chicos, who has been using the high-occupancy toll (HOT) lane for the past two years. "Going home, it seems so much easier than trying to merge heading out of downtown, and it's usually so cheap. I'd spend 50 cents on gas sitting in traffic. It's a small price to pay to get home fast."

The Minnesota Department of Transportation charges motorists driving alone anywhere from 25 cents to $8 during peak periods to use the special lanes that are otherwise reserved for carpools of two or more people, buses and motorcycles. But just how much a solo driver is charged is determined by an old complex algorithm operated by an outside vendor. It assesses real-time traffic conditions in the MnPass and the free lanes every 3 minutes and sets the price accordingly. Sometimes that leads to wild price spikes.

That's about to change. MnDOT has been working with the University of Minnesota Traffic Observatory to develop a new algorithm that will be run in-house and should better set tolls and control how fast prices rise or fall. It's expected to be in operation by the end of December, said MnDOT freeway engineer Brian Kary.

MnPass lanes are designed to get drivers to and from their destinations faster by keeping traffic flowing at least at 50 miles per hour. When operating optimally, HOT lanes provide for consistent speeds and trip times, while reducing congestion in the free lanes.

Many drivers use the prices posted on overhead signs to judge congestion levels downstream and decide whether to enter the HOT lanes. A sudden burst in traffic can send the MnPass lane price soaring and discourage drivers from entering the HOT lanes, which then become under used. Conversely, MnPass lanes become overcrowded when the price drops too low, thus decreasing the lanes' efficiency.

How much is too much?

Chicos said $3 is about the point when she would think twice about entering the toll lanes. For others, the cutoff seems to be about $5. Anything above that and drivers are less likely to use the lanes, according to MnDOT research.

Dense rush-hour traffic, a lane-blocking crash or a snowstorm that paralyzes traffic has the ability to instantly send tolls skyrocketing or drop just as fast.

HOT lane pricing has been about as bumpy as the pavement on I-394 was before MnDOT smoothed it out this summer. With the new algorithm, which Kary describes as a tweak to the old one, drivers should less rapid fluctuation as conditions change. The goal is to get more people in the HOT lanes, maximize their use and improve overall traffic flow.

"The old algorithm had a lot of price spikes that weren't justified, probably because of its complexity," Kary said. "The continuous pricing algorithm should smooth price transitions."

That should be good for the 27,000 MnPass account holders, a number likely to rise with the Nov. 30 opening of the new I-35E MnPass lane in Maplewood and St. Paul.