I'm a little bit late to this issue, but on June 16, 2015, the SFMTA approved changes in a plan called the "Safer Market St Project" to limit the movement that private cars can make on Market St in San Francisco. SFGate has some good coverage of the issue here and here. I want to share my thoughts on the matter.
The origin of this legislation is something called Vision Zero SF, which is a group that is trying to reduce traffic deaths in SF; their declared goal is to achieve zero traffic deaths in SF by 2024. This is an ambitious goal. San Francisco is a city with nearly a million permanent residents, and during week days when people from around the Bay Area commute into the city the population regularly exceeds that number.
Whether or not you believe it's possible to achieve the Vision Zero goal, what is interesting is to look at statistics on which intersections in the city have the most deaths. This is particularly the case for deaths caused by cars hitting pedestrians or cyclists, since anyone who walks or bikes regularly in the city knows that there are certain areas where road layout, lights, turn restrictions, etc. encourage cars to engage in behavior that is dangerous to pedestrians and cyclists. These are the areas where we can most effectively make changes or traffic restrictions that will save lives. What Vision Zero has found is that a hugely disproportionate percentage of fatalities take place on a few blocks of Market St. According to the document I just linked to, a portion of SOMA that accounts for 12% of San Francisco's streets accounts for 70% of total crashes. This is consistent with similar (and older) data from the San Francisco Bicycle Coalition on the matter regarding accidents regarding cyclists. The SFMTA did analysis on the matter and gave this public hearing presentation which outlines their research on the matter and the proposed turn restrictions (which ended up actually being implemented in June).
Opponents of the legislation pointed out that restrictions of any kind on traffic on Market St will likely lead to higher congestion on nearby streets and generally higher congestion in SOMA in general. It's hard to deny this theory: these restrictions will have a serious impact on drivers and their ability to navigate in the area around Market St. And yet, that's kind of the whole point of the legislation. The legislation is being passed to save the lives of pedestrians and cyclists, not to decrease congestion.
I was disappointed in how a lot of the legislation I read portrayed the matter as one between cars and cyclists. As you can see in the Vision Zero literature, there are many more pedestrian related deaths than cycling deaths. The report I linked to earlier showed that in 2014 there were 17 pedestrians killed in traffic related deaths, nine motorists killed in traffic related deaths, and only three cyclists killed. They do point out that San Francisco is one of the few cities in the country where cycling related deaths are increasing, but I think the fact that there are more than five times as many pedestrian fatalities speaks for itself.
As someone who rides a bicycle up and down Market St every week as part of my work commute, I strongly support the legislation that the SFMTA passed. I hope that they are able to move ahead with similar legislation in the future. Market St is incredibly dangerous to bike down, and I have frequent close encounters with cars on Market St. My personal take on the matter is that of the encouters I have with motorized vehicles, very seldom do I have problems with MUNI buses or taxis. Most of Market St in SOMA already has designated lanes for buses and taxis that puts them in the center lane away from cyclists. Market St is especially dangeorus due to the divided lanes for bus stops. This lane division was not originally anticipated when Market St was built, and consequently the right-most lane is very narrow in many places and does not have sufficient space for cyclists. Additionally, many pedestrians who are getting on or off MUNI will cross the right-most lane to get to the divided bus stop area, and in doing so cross into the way of traffic. The restrictions that the SFMTA passed apply primarily to private vehicles that are in the rightmost lane or turning into the rightmost lane, right where they're the most likely to hit pedestrians and cyclists.