SAN FRANCISCO - An ambitious plan to add bicycle lanes to some of The City's busiest thoroughfares would slow traffic and Muni service, but transit officials say the congestion trade-offs will result in cleaner, calmer streets.

The plan would add 34 miles to San Francisco's existing 45 miles of bicycle lanes during the next several years, along with parking spots and traffic signals for cyclists and colored lanes on some streets.

The improvements are welcomed by cyclists and environmentalists, who say increasing bike lanes will persuade drivers to hop on two-wheelers, ease congestion and reduce vehicle emissions.

However, an extensive environmental review of the plan facing certification by the Planning Commission this week - an important step toward an official groundbreaking - warns the changes would bring congestion spikes, slow Muni vehicles and shrink street-parking options in key areas.

Twenty-seven intersections, some heavily trafficked, were identified in the environmental review as those in which bike improvements would create an "unavoidable significant impact" to street congestion.

Some projects would eliminate vehicular lanes on busy roads, thus increasing the chance for traffic jams, the report said. Air quality would also decrease, since more cars would be idling in traffic, according to the report.

Four intersections along Masonic Avenue between Fell Street and Geary Boulevard would see greater congestion if bike lanes were added, particularly during peak commute hours. The popular 43-Masonic bus line, which operates along the busy stretch, would be slowed by the increased congestion and by a lane dedicated to cyclists. Similar problems would occur along Second Street between Market and Townsend streets, along with an intersection on Church Street at Market and 14th streets, the report said.

The proposal to add bike lanes to Second Street has raised eyebrows among residents in the area. Second Street connects drivers to the Bay Bridge, but under the plan cars would lose a lane between Market and Harrison streets northbound and Harrison and Townsend streets southbound. There would also be restrictions on left turns and fewer parking spaces.

Jamie Whitaker, a Rincon Hill resident, expressed concern that the Second Street plans would clog roads and endanger pedestrians because ambulances and fire trucks would not be able to reach residences.

Whitaker, who is vice president of the Rincon Hill Neighborhood Association, said he supports a wider bicycling network in The City, but he charged the Municipal Transportation Agency with railroading his neighborhood with bike projects without adequately addressing community concerns.

"The [transit agency's] attitude is you're either with us, or against us," Whitaker said.

The transit agency denies the claim, saying it has done adequate public outreach addressing bike plan concerns.

"There are some trade-offs, but there are also quite a few design solutions we've found as well," Muni spokesman Judson True said.

But Leah Shahum, executive director of the San Francisco Bicycle Coalition, argues the network would calm speedy streets and would have the added benefit of attracting commerce.

"I've received 150 letters from businesses around The City supporting the plans," Shahum said, adding that Valencia Street in the Mission district has enjoyed a commercial boom since two of its four lanes were converted to dedicated bike lanes in 1999.

Vehicles vs. bicycles

A report says some intersections in The City would see "unavoidable significant impact" to congestion if the bike plan moves forward.

SoMa

• Five on Second between Howard and Townsend
• Three on Fifth, including at Bryant, Howard and Brannan
• Seventh and Townsend
• 10th/Brannan/Potrero/Division
• 11th/Bryant/Division
• Fremont and Howard

Potrero Hill

• Potrero and 16th

Duboce Triangle

• Church/Market/14th

Lone Mountain/Panhandle

• Four on Masonic between Fell and Geary

Lower Bernal Heights

• Bayshore/Jerrold/Highway 101

Mission

• Five at Bryant, Guerrero, Evans, South Van Ness

Diamond Heights

• Clipper/Portola

Miraloma

• Two on Fowler and Portola and Woodside/ O'Shaughnessy/Portola

Muni lines that would be slowed by new bike lanes, amenities:

• 9, 10, 12, 27, 30, 43, 45, 48, 52

Source: San Francisco Planning Commission