Roads that look like freeways cause drivers to behave as if they are on freeways—and to ignore speed limit signs. In fact, drivers can argue that posted speed limits are artificially low.
That is, speed limit signs do not define speed limits; traffic speed defines speed limits.
Police radar speed trailers ("Speed limit 25 mph. You are going 43 mph") placed on Guerrero at Duncan routinely show drivers driving at more than 50 mph--more than twice the posted limit.
According to the San Francisco Police Department, these radar trailers slow traffic, but only until people get used to their location. Police officers therefore constantly move the trailers around the city.
Tightly Packed Traffic Confuses Radar Guns
Worse, tightly packed traffic confuses police radar guns so it's difficult to get accurate radar readings on San Jose/Guerrero. Traffic judges often throw out traffic tickets in such conditions, so the police are reluctant to enforce even the 85th percentile speed limits.
Traffic Speed Determines Speed Limit—not Signs
If a street is designed to encourage drivers to travel at 45 mph instead of the posted 35 mph, police are often at a loss.
First of all, police can’t be present at all times.
Secondly, traffic enforcement agencies that try to address speeding on arterial and residential streets are often hamstrung by state laws which require that speed limits not be set any lower than the actual speed of 85% of the vehicles on a given street.
This “85th percentile” law—adopted to prevent municipalities from setting up so-called “speed traps”—helps insure that high-speed streets are a self-fulfilling prophecy.-- Local Government Commission
Speed Limit Law