SACRAMENTO – The less California motorists drive, the less they might pay for auto insurance under regulations proposed Wednesday.

State Insurance Commissioner Steve Poizner said the plan will give drivers the option of paying for their automobile insurance based in part on how many miles they drive.

The idea is to provide an incentive for motorists to drive less – saving fuel and cutting greenhouse gas emissions. In doing so, they also would lower their insurance premiums.

Such pay-as-you-drive auto insurance policies already are an option in 34 states.

The Brookings Institution estimates that nearly two-thirds of California families would save money under Poizner's plan. The Washington, D.C.-based research organization says the typical family would save $276 annually per vehicle.

Insurance premiums currently are based on estimates of how many miles motorists drive each year. Those estimates are usually inaccurate and don't inspire motorists to drive less, Poizner said.

The regulations will be voluntary and are expected to take effect by fall 2009.

They will allow insurance companies to track mileage through odometer readings or wireless devices placed in vehicles. Some insurance companies contract with General Motors Corp., which uses its OnStar program to track miles driven, said John Boesel, president of CALSTART, a nonprofit organization that promotes transportation technology.

Poizner said his rules will prohibit insurance companies from tracking motorists' locations or how they drive.

“That would be a blatant privacy violation,” he said.

Assemblyman Jared Huffman, D-San Rafael, was pushing a bill earlier this year that would have allowed insurance companies to do just that.

It drew criticism from Consumer Watchdog, a Santa Monica-based nonprofit, which said the practice would constitute an invasion of privacy.

Huffman's bill was approved by the state Assembly, but he withdrew it from the Senate in favor of Poizner's voluntary regulation. The consumer group prefers Poizner's approach, said Carmen Balber, an advocate for the organization.

Samuel Sorich, president of the Association of California Insurance Companies, said insurers had asked Poizner to consider letting them track when and how individual motorists drive.

For example, insurers would like to know if their policy holders are more likely to drive at night or routinely exhibit poor driving habits, like speeding away after a stop or slamming on the brakes.

But insurers will not push Poizner to allow that in the new regulations, Sorich said. His organization, which represents insurers who write policies for about half the 21 million insured vehicles in California, generally supports pay-as-you-drive insurance.

If 30 percent of policy holders participate, it would lead to a reduction in greenhouse gas emissions equivalent to taking 10 million cars off the road by 2020, said Lauren Navarro, a lawyer for the Environmental Defense Fund.