The San Francisco Municipal Transportation Agency (MTA) has instituted 34 road diets since the city's Transit First policy was adopted in 1973, a number that is expected to grow once the bicycle injunction is lifted and new bike lanes are striped.
MTA traffic engineer Mike Salaberry said the city had a number of roads where lane width was excessive and designed only for cars. Road diets, he said, were primarily "to improve how the road works for a wider range of users."
"Older roadway designs were not efficient for using space," said Salaberry, who pointed to numerous streets where wide travel lanes led to long crossing distances for pedestrians and no dedicated space for cyclists. "Given that space is at a premium, we had to find creative ways to allocating the space we do have."
In some cases, making space for pedestrians and cyclists is as simple as painting lines on the street and reducing the opportunity for drivers to reach high speeds. Walk San Francisco's Manish Champsee hailed the MTA's track record on road diets and said they were doubly beneficial for pedestrians. Slower speeds mean less collisions and less-severe collisions when they do happen, which dramatically increases a pedestrian's chance of survival. Another benefit comes from visibility: On a four-lane road reduced to two lanes, pedestrians don't have to worry about the danger of the car in the lane further from the curb not stopping for pedestrians at un-signalized crosswalks.
Champsee pointed to the Valencia Street road diet as a success story that has become a national model for traffic engineers. Valencia was a four lane road until March 1999, when the former Department of Parking and Traffic (the DPT is now part of the MTA) re-striped the street to its current configuration, with two travel lanes, a center median with left-hand turn bays, and bike lanes.
Despite DPT concerns at the time that bicycle collisions would increase, in the year after the lanes were striped, total bicycle collisions and pedestrian collisions declined, and there was a 144 percent increase in bicycle use. Though some vehicles diverted to other streets, vehicle volume was only slightly lower, and it didn't all move to Guerrero Street, where traffic engineers expected it to go. Fears of negative economic impact were also unfounded. As an SF State study noted  subsequently, only 6 percent of merchants surveyed after the bike lanes were installed had negative feedback.
"There is really nothing bad you can say about what happened on Valencia Street," said Champsee, who added that he was at a recent Federal Highway Administration forum on street safety and Valencia was an example the federal agency used to highlight a successful road diet.
Despite the proven positive impacts of road diets in San Francisco, the hurdles to further changes are significant. As the protracted battle over the Bike Plan illustrates, road diets have to undergo significant environmental review and if they impact on traffic flow, they either get scrapped or have to receive an exemption from the city.
"As always, we're up against some of the perverse unintended byproducts of the California Environmental Quality Act (CEQA)," said the San Francisco Bicycle Coalition's Andy Thornley. Despite San Francisco's long-standing Transit First policy and the unquestionable benefit of slowing vehicle speeds, projects can't be expedited faster than they currently are.
"Over and over again, these projects illustrate what's counter-intuitive to some folks," explained Thornley. "If you take four lanes and make them three, it seems that you're losing something. In fact, by providing a left turn lane, the throughput of the street may remain the same or improve, while the safety improves greatly."
The next big road diet project is the redesign  of Cesar Chavez Street from Guerrero Street to the Highway 101 "Hairball" interchange . The city will remove one lane of traffic in each direction, widen the median and install left-turn lanes, and add bicycle lanes. The project will not only represent a new level  of city agency vision and cooperation , but if successful, could set a new standard for livable streets in San Francisco.
Thornley said Cesar Chavez and other similar projects like 25th Avenue in the Richmond would further establish the benefit of traffic calming and multi-modal planning. "In spite of what drivers may think, it will make their trip safer and it might make it faster."
- Arguello from Pacific to Fulton
- Valencia from Market to Tiffany
- Polk from Turk to Vallejo
- Mansell from San Bruno to University
- Harrison from 11th to 22nd
- San Jose Avenue, southbound, from Randall to Arlington
- San Jose Avenue/Guerrero from Randall to Cesar Chavez
- Dewey Boulevard from Laguna Honda to Taraval
- Post Street from Presidio to Steiner
- Turk Street from Arguello to Masonic
- Golden Gate Avenue from Masonic to Broderick
- Fulton Street from Baker to Webster
- Fell Street westbound from Scott to Baker (Laguna to Scott PM tow-away later removed also)
- JFK Drive westbound from Shrader to Conservatory Drive East
- Duboce from Buchanan to Church (conversion from roadway to bikes only)
- Fourteenth Street from Dolores to Guerrero
- Howard Street from Fremont to 11th
- Seventh Street from Townsend to 16th
- Oakdale from Phelps to Industrial, Bayshore to Industrial
- Battery Street from The Embarcadero to Broadway
- San Bruno from Mansell to Campbell
- Folsom Street from 14th to 11th
- Market from 8th to Van Ness
- Potrero from 17th to 25th
- Lake St from Arguello to 3rd Ave
- 14th Street from Market to Dolores
- Alemany from San Jose to Rouseau
- Clipper St from Douglass to Diamond Heights
- 25th Avenue from Fulton to Lake
- 7th Avenue from Lawton to Judah
- Oak Street from Divisadero to Laguna (removal of AM tow-away lane)
- Scott Street from Oak to Fell St
- Gough St northbound from McCoppin to Market