Motorists will be required to give bicyclists three feet of clearance when passing if Gov. Jerry Brown approves legislation now on his desk.

The proposed law has extensive backing from the Sonoma County bicycling community, which sees it as an opportunity to educate motorists as well as provide some protection to cyclists on the area's windy, narrow roads.

"It gives everybody a more clear understanding of what is the safe distance to pass a bike on the road," said Sandra Lupien, outreach director for the Sonoma County Bicycle Coalition. "While people might know you have to pass safely, having the law adds legitimacy and gives cyclists a way to hold drivers accountable."

The bill was authored by Democratic Sen. Alan Lowenthal of Long Beach and would replace language in the vehicle code that requires passing at an unspecified safe distance.

Eighteen states already have the three-foot rule and four others recommend it in their driver manuals.

"This is a widely accepted standard," said John Casey, a staff member in Lowenthal's office.

While most cyclists have stories of being startled after being "buzzed" by a motorist passing too closely, being hit by a passing vehicle is actually the least common collision.

"Most people are amazed at that statistic," said Gary Helfrich, the coalition's executive director. "There is a public perception the biggest safety issue is being hit from behind. What this law helps do is give people a little more confidence about riding on the road."

The consequences of such a collision are serious, usually resulting in a severe injury or fatality, Helfrich said.

Anoush Zebarjadian of San Francisco suffered a severe head injury and was hospitalized for months when he was struck by the side mirror of a vehicle the CHP believes passed intentionally close during the King Ridge GranFondo last Oct. 9.

In 2009, the last year CHP statistics are available, 107 bicyclists were killed and 12,043 injured statewide, although the types of accidents were not detailed.

The bill also requires a cyclist to ride as close to the right side of the road as practicable.

In situations where three feet is not possible, the motorist is required to pass at a speed no greater than 15 mph more than the bicycle is going.

It also allows vehicles to cross over a double line to pass a bicycle. Most drivers do that now on country roads when passing cyclists even though technically it's illegal.

The bill is opposed by the California State Automobile Association because of the 15-mph passing provision.

"There can be significant issues when requiring a driver to slow down," said CSAA spokeswoman Cynthia Murray. "How do you determine the exact speed of the bike?"

The bill establishes a penalty of $35 for violating the three-foot rule and a $220 fine for a violation that results in an injury to a cyclist.

A spokesman in Brown's office said he has not issued a statement on the bill.