MINNEAPOLIS (WCCO) – The signs are often changing: Is it worth 75 cents to jump in the carpool lane? Would you pay $3? $4?

With 23,000 active MnPASS accounts, about 10,000 people each day pay to get into the HOV lanes. But how does the Department of Transportation (MnDOT) decide what the toll will be?

“It’s all automatically controlled,” said Brad Larsen, the MnPASS program manager for the MnDOT.

Sadly, there’s no one sitting in the regional traffic control center yelling, “Raise the price a quarter!”

“There are sensors in the roadway that measure traffic volumes and speed. As the lanes fill up with traffic, the price automatically adjusts upwards,” Larsen said.

The loop detectors are the central nervous system for all of our traffic information. The data from the detectors feeds the traffic maps you see on WCCO in the morning, the travel signs that show the time it takes to get from point A to B, and it also drives the algorhythm that controls pricing for the MnPASS.

“It’s pretty complex,” Larsen said.

The formula takes into account vehicle speed, and lane congestion. And it’s constantly measuring.

“Every three minutes it recalibrates and updates the price,” he said.

The goal is to stay below full lane capacity, which MnDOT defines as 2000 cars per hour.

As the traffic count gets closer to 100 cars in three minutes, the price starts to rise to keep traffic moving smoothly.

The lowest price is 25 cents. The top price is $8.

“It doesn’t stay there for very long, the $8 price discourages people from using it, so it drops down pretty quickly,” Larsen said.

For example, on Tuesday, Feb. 12, there were 5,593 MnPASS trips on Interstate 394 and 3,792 on Interstate 35W. The average toll on I-394 was $1.67, it maxed out at $8. The average on 35W was $1.80, and maxed out at $7.

The total revenue was $16,178 — $9,341 from I-394 and $6,836 from 35W.