KENT, Wash. -- Traffic moved smoothly in the car pool lane of Highway 167 between Auburn and Renton on Monday where solo drivers can buy in by paying a variable toll.

But in the first weekday operation of the new HOT lane, many people were confused about a rule requiring drivers to not cross in to or out of the lane except in designated access areas.

A new solid double-white line separates most of the HOT lane from normal lanes, and it is illegal to move in our out of the lane by crossing the double line.

Troopers were pulling over cars throughout the morning for violating the rule, and the fine can cost up to $124. The purpose of the restriction is to make sure drivers are entering the lane where an overhead sensor can read their cars' toll transponder.

Still, state troopers said the day was more or less a smooth ride.

"For the first time out, we're pleased," said Trooper Curt Boyle.

The designated HOT lane entry and exit points are marked with a traditional single dashed white line (scroll down for a map), and signs are placed a half mile ahead of each entrance point.

The cost of the toll varies from moment-to-moment depending on how heavy traffic is, but commuters will pay the price set at the time when they enter the HOT lane, regardless of whether it increases or decreases while they drive.

Transportation Department manager Patty Rubstello said the HOT lane price in the Monday morning commute ranged between 50 cents and $2.00.

Drivers have to watch overhead displays for the cost, which is recalculated every three minutes by a computer.

Department administrator Craig Stone says it's one of only three "dynamic systems" in the nation. A transponder deducts the toll from drivers' pre-paid accounts.

Transportation officials estimated about 8 percent of the cars that used the HOT lane on Monday paid for the lane usage. The lane continues to remain free to cars with two or more passengers.