I remember seeing these signs in Berkeley some years ago. They are in complete violation of the MUTCD (wrong color, symbols, and font), but they do convey a useful message, particularly by citing the relevant city and state codes. I think (an improved version of) these signs would nicely complement Shared Lane Markings that I have suggested as an alternative to door-zone bike lanes in the Jamaica Plain business district. Perhaps a combined sign would do the trick, with an arrow pointing left under the Ride on the Street message and another arrow pointing right for the Not on the Sidewalk message.
Someone is sure to raise the “sign pollution” issue. Well, I can point to dozens of signs that could be removed from the corridor. I counted 10 “SLOW”" signs (or, less grammatically, “DRIVE SLOW”), and perhaps a few more have sprouted since then. These signs violate the basic principle of the MUTCD that signs should convey a clear meaning. How fast is SLOW? Only a little bit above the statutory 30 mph speed limit? Ironically there are no speed limit signs in the whole Centre-South corridor.
An even larger source of sign pollution are pedestrian crossing warning signs. They are haphazardly posted a block or two before a crosswalk and (rarely) near the crosswalk. Usually they are too high to be noticed. Boston has recently used these warning signs in a much more effective way: at the crosswalk, mounted as low as possible, with an arrow pointing to the crosswalk. This helps alert drivers of the need to yield and reinforces the crosswalk pavement markings when they can be difficult to see (at night and after the thermoplastic begins to wear).
The tragic loss of a young bicyclist this week reminds me of another warning sign that we really do need: to alert cyclists about the dangers of trolley tracks. This sign is used in Portland, Oregon, but nowhere else that I know of. It is not in the MUTCD. Fortunately, we no longer need them in the Centre-South corridor (since the tracks were paved over after being an unused hazard for 23 years), but we do need them in the remaining places that trolley tracks are in the road (not in a reservation): along the E branch from Heath St to Brigham Circle, and in Cleveland Circle.