You can cruise through traffic in the new express lanes that opened Friday morning on U.S. 101, and it’s free for carpools of three or more people, but it’s a $500 fine if you’re in that lane without a FasTrak.

Folks who drive U.S. 101 on the Peninsula have had a SoCal-style express toll lane available from the San Mateo/Santa Clara County line down to Sunnyvale since earl 2022. A new 20-mile stretch of express lane in San Mateo County soft-launched for “toll equipment testing” in November, but hits its full stride effective today.  KTVU reports the new 101 express lanes started today, and as seen below, this presents the possibility of much faster travel northbound and southbound between San Bruno and Redwood City. And commuters can now ride in express-lane comfort — which is free for carpools of three or more — all the way from San Bruno to Sunnyvale.

The NBC Bay Area video above is a nice explainer of a somewhat complicated FasTrak tolling system for the new express  lanes, which are meant to encourage carpooling, but solo drivers can still drive in that lane. “People driving in carpools with three or more people, as well as buses, will be able to travel free of charge in the express lanes,” as NBC Bay Area reports. “Customers driving clean air vehicles such as cars that run on fuel cells, battery, or plug-in hybrids that have a valid clean-air vehicle decal from the Department of Motor Vehicles will qualify for a 50% toll discount when the lanes first open.”

The express lane system is now in effect Monday-Friday, 5 a.m.-8 p.m.

The above map shows where the new express lanes are in effect, represented in the blue and white line (the green line is a South Bay express lane that was already in effect).

The cost is based on dynamic pricing that changes constantly, and road signs will flash the current cost. The method of determining this cost is as clear as mud. But per FasTrak, “Two-person carpools using a properly set FasTrak Flex or FasTrak CAV toll tag receive a 50% discount. Carpools of 3+ people and motorcycles may use the lane for FREE with a FasTrak Flex or FasTrak CAV toll tag. Eligible clean air vehicles (CAVs) traveling as a single-occupant receive a 50% discount when using a FasTrak CAV toll tag.”

Do you want to ride but feel you can’t afford it? Big government will hook you up! “The Community Transportation benefits program helps cover transportation costs for qualifying individuals in San Mateo County by leveraging income from the new express lanes to help low-income individuals,” Tasha Bartholomew of the San Mateo County 101 Express Lane Joint Powers Authority tells NBC Bay Area. “We can provide participants with either a $100 transit credit on their Clipper card or a $100 toll credit on a FasTrak flex transponder, depending on if they want to take public transportation or if they’re in their car.”

But you need a FasTrak toll tag to ride in that express lane, no matter how many people are in the vehicle. Riding in that lane without a FasTrak toll tag carries a $491 fine.

Traffic authorities around the Bay Area have been discussing pay-to-play express lanes for over a decade now, and there are now 155 miles of express lanes, primarily in the East Bay, on I-680, I-880, part of I-580, and on Highways 237 and 85. The I-680 express lane project is continuing to expand, but the express lanes on 101 are the first to require 3 or more passengers to qualify as a carpool.

Related: Bay Area Toll Authority to Crack Down on Unpaid Bridge Tolls With Penalties and Liens on Registrations [SFist]

Image: @101ExpressLanes via Twitter