The MTC, which has jurisdiction over nine Bay Area counties including Sonoma County, is implementing the commuter benefit program.

The Bay Area Air Quality Management District will enforce the law. Companies that don't offer commuter benefits by Sept. 30 could face monetary penalties, the district said.

The law expires after 2016, at which time the program will be reevaluated, MTC officials said.

San Francisco, Richmond and Berkeley have city ordinances requiring companies to offer commuter benefits. The state law was modeled on those ordinances.

To comply with the law, companies can choose one of four options:

--Companies can allow employees to exclude up to $130 of their transit or vanpooling expenses each month from taxable income. This option is the least financially onerous on employers and could even save businesses money in payroll tax, the MTC said.

--Employers could provide a tax-free subsidy of up to $75 per month to cover transit costs. In this option, companies would likely give employees transit vouchers or something similar such as a pre-paid Clipper travel card. This benefit could help companies recruit and retain employees, the MTC said.

--A business or group of businesses could provide a free or low-cost bus or shuttle for employees. Some tech companies, such as Google, provide buses for employees that live in San Francisco and work in Silicon Valley.

--A company could propose a package of different benefits that encourages alternative commuting. Some options include installing secure bike parking, showers and lockers and offering a subsidy for commuters who bike to work, implementing telecommuting, providing preferred parking for carpoolers and promoting carpooling programs such as Carma.

In Sonoma County, about 600 employers are now required to offer the benefits to 70,000 employees, which observers say could help nudge commuters into changing their behavior.

"I think it's great," said Amy McCrary, project coordinator at the Santa Rosa-based Climate Protection Campaign, which operates the Carma Carpooling program in Sonoma County. "It's that push that helps to get more employees to use alternative commute modes."

Some Sonoma County businesses already offer commuter benefits and may be in compliance with the law.

At Medtronic, which has more than 800 workers in Santa Rosa, the company subsidizes a vansharing service for employees who live in San Francisco and Marin, said spokesman Joe McGrath. The vanpool fleet consists of six 12-passenger vans equipped with 4G Wi-Fi hotspots that provide instant connectivity to the Medtronic network, he said, and the program eliminates more than 31,000 individual commuter trips over the course of a year.

Medtronic also uses WageWorks, a benefit administration firm, to offer employees a pre-tax commuter expense deduction from their paychecks.

Employees at Agilent Technologies' Fountaingrove campus have a menu of alternative commuter options. Spokesman Jeff Weber said the commute package may comply with the law's fourth option, designed for companies that are not easily accessible to public transit.

Agilent offers preferred parking for carpoolers and low-emission vehicles, free electric vehicle charging, a guaranteed ride home program and promotes Carma Carpooling and biking to work, Weber said.

"Our feeling all along has been to do the right thing for the community and the environment," he said.