Starting Saturday on the 110 Freeway, anyone wishing to ride in the car-pool-turned-toll lanes — now dubbed ExpressLanes — must have a transponder.

The same will be true to drive in the car-pool lanes on the 10 Freeway, starting in late January or early February.

Even car poolers and van poolers must have a transponder, though they will not be charged tolls.

Once motorists sign up online or in person for a transponder, they will be mailed the unit to attach to the inside windshield.

The transponders are free, but motorists must put at least $40 into an account to begin. Every toll is then automatically subtracted from the motorist's account after a signal from the transponder is read overhead. The fee amount lights up on an overhead sign at each entry point. One fee is the amount until the next exit; the other fee represents the cost until the end of the lane. The toll will vary from $0.25 to $1.40 per mile — the more traffic the higher the toll.

As the motorist's account approaches zero, more funds must be added.

Motorists will set their transponders to one, two, or three people in the car.

On the 110 Freeway, cars with at least two people are considered car pools. On the 10 Freeway, cars with two people are considered car pools during off-peak hours, but must pay the toll during peak hours — 5 a.m. to 9 a.m. and 4 p.m. to 7 p.m. Monday through Friday. Cars with three or more people are considered car pools at all times.

Motorcycles can use the ExpressLanes for free with a transponder. Some electric cars will be allowed to use the lanes for free after March 1, 2014.

Low-income commuters — defined as a family of three with an annual household income of below $37,061 — can receive a 63 percent discount. They start by paying only $15 into the account and Metro will pay $25.

Auto Club of Southern California members can open an account and get a transponder for $32, said Metro's Stephanie Wiggins, executive officer in charge of the program.

Motorists who are in the ExpressLane without a transponder can be caught by Metro photos. They will be billed for the toll by Metro, which doesn't have the authority to issue traffic citations. If they don't pay the toll within five days, a $25 late fee is applied; after 30 days, an additional $30 late fee is applied. The late fees can be waived by setting up an account with Metro.

Overhead readers can signal California Highway Patrol officers if someone is cheating — saying they have two or three people in the car, when there is only one person.

If a motorist is pulled over by the CHP for illegal use of the toll lane, the fine is $341.

Also, all participants in the ExpressLanes program will pay a $3 monthly maintenance fee. However, the fee is waived after four one-way trips in the lanes and for low-income drivers, said Wiggins. ExpressLane users that take the bus or light-rail with a Metro Tap Card can also get credits that will help waive the $3 monthly fee.

"We do expect the majority of customers won't be charged this fee," Wiggins said.

In addition, motorists who pay in cash must pay a $25 security deposit for their transponders. Metro will refund the deposit when the transponder is returned in its original working condition.

steve.scauzillo@gmail.com 626-544-0843