Twenty could be the new 25.

San Francisco is considering a new report that "builds a case" for lowering speed limits on city streets from 25 mph to 20 mph, according to the San Francisco Examiner.

Drivers in the city average 27 to 33 mph on most city streets, according to a Budget Analyst's Office report. That's above the typical 25 mph speed limit — and far about the posted limit of 10 mph that's in effect on Market Street when a bus or trolley train is present (which is most of the time).

San Francisco is in the middle of an effort to reduce pedestrian and traffic collisions called Vision Zero.

Unlike other Vision Zero cities, however, San Francisco has not yet reduced its speed limits, the newspaper reported.

One reason why is a state law that says there's a minimum speed of 25 mph unless otherwise posted, like in school zones, the newspaper reported.

There were 28 fatal collisions in San Francisco in 2011, 17 of which were between vehicles and pedestrians.