The problem with regulatory signs directing bicyclists to dismount and walk at specific locations, especially width-constrained ones, is that once dismounted the bicycle/pedestrian combination is now nearly twice the width of a non-bike-walking pedestrian or riding cyclist - which reduces the effective usable space even more than if people were to continue to ride.
An agency could argue "if everyone complies with the sign, then it won't matter, because at pedestrian speeds, crash risk is minimal." This could open them up to a response of "Did you monitor the site for compliance, and upon assessing compliance rates, take any action?"
Unwritten Rule #2 of Traffic Engineering: The presence or installation of a traffic control device does NOT necessarily mean the device will be effective or improve safety.