OLYMPIA — Carpoolers will face new rules if they want to travel at no cost in the express toll lanes opening next year on I-405 between Lynnwood and Bellevue.

They will need to install a transponder in their vehicle to avoid getting charged and carry at least three people to qualify as a carpool during the busiest times of the day, under plans drawn up by state transportation officials.

Right now on Highway 167, which has the state’s only existing non-bridge toll lanes, carpools of two or more occupants can use the lanes at any time, do not pay a fee and do not need a transponder. Single drivers who want to drive in the lanes must have a transponder, known as a Good To Go pass, and pay a toll.

“It’s going to be a big change,” said Patty Michaud of the communications and marketing division of the state Department of Transportation.

WSDOT is looking to open the 17 miles of express lanes on I-405 next fall. Before then, officials know, they must conduct an extensive public education campaign and work with lawmakers concerned by the significance of the changes for carpoolers.

First, though, the state Transportation Commission must act on the department’s recommendations concerning the operating rules and the toll rates. That won’t happen until early next year.

As envisioned, WSDOT wants to define a carpool as at least three people during peak hours — essentially the morning and afternoon commutes — and two people the rest of the time.

And every carpool vehicle will be required to have a switchable device to use the express lanes for free. The units will have separate modes for HOV, which stands for high occupancy vehicle, and toll. Carpoolers must switch their unit to HOV to avoid paying a toll, otherwise they’ll get a bill in the mail.

The new devices, when available, will also work on the Highway 520 and Tacoma Narrows bridges and the Highway 167 toll lanes.

Those who never plan to carpool in the new I-405 express lanes can still use the lanes if they pay the toll. Drivers would use their standard Good To Go account to pay a toll or, if they don’t have an account, they would receive a bill in the mail at a higher toll rate. Cameras will be used to take photos of license plates to send bills to those without a transponder.

Toll rates are the other piece to be put in place by commissioners.

Prices could range from a minimum of 75 cents to a maximum of $15, with drivers paying between 75 cents and $4 about 90 percent of the time, according to a report delivered to the state Senate Transportation Committee last month. Travel in Highway 167 high-occupancy-toll (HOT) lanes costs a minimum of 50 cents to a maximum of $9.

Commissioners were to act in November but delayed decisions until early next year to spend more time working on the details and consulting with concerned lawmakers.

“It is too important to not get it right,” said Reema Griffith, executive director for the commission.

Four Republican state senators want the panel to wait until the end of the 2015 legislative session.

They don’t like the notion of charging carpoolers who use the express lanes without a transponder unit, even if they meet the requirements for a high occupancy vehicle. They also oppose adding a $2 processing fee to each bill collected by mail.

“This represents a policy change, which should be thoroughly evaluated by the Legislature prior to action by the Commission,” reads the Nov. 13 letter they sent to the panel. It is signed by Sens. Andy Hill of Redmond, Steve Litzow of Mercer Island, Joe Fain of Auburn and Curtis King of Yakima, who is chairman of the Senate Transportation Committee.

Fain said they want to be sure the department and commission fully explore the effects on drivers.

“I want to keep an open mind to these proposals, but we need to slow down to get a sense of the broader impact and the difficulty of implementing a new system that is a significant change for drivers,” said Fain, who serves on the Senate Transportation Committee.

Commissioners took the letter “very seriously” and want work with lawmakers to ensure they don’t produce something that will be blocked, Griffith said.

“The idea is not to get people riled up and put out something half-baked,” she said. “We want to be sure what’s put out there is our best effort.”

Jerry Cornfield: 360-352-8623; jcornfield@heraldnet.com.