Road Scholar: Speed Limits: How They Are Set & Enforced

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In California as in many other states there are three categories of speed limits. The first three are Regulatory (black and white color). The third is Advisory (yellow-colored sign).

1. The Maximum Speed Law/Limit 65mph or slightly higher on some interstates and other highways. This law is plain and simple: you may not exceed the speed limit. The only exception would be a medical emergency which would be "guilty with an explanation".
2. The Basic Speed Law-you may not drive faster than is safe for conditions. If conditions are poor, you may be driving too fast even if within the posted limit. Driving over the limit is only presumed unsafe. If one is slightly over the limit and it is a clear day, you probably won't be cited. If you are, you can still contest the violation by showing that it was a reasonable speed for the conditions: weather, visibility, traffc flow, road condition.
3. Prima Facie Speed Limits - exist whether or not there is a posted limit; unless posted otherwise, a business, residential or school zone (during school hours) speed limit is 25mph; Alley, blind intersection or uncontrolled railroad crossing is 15mph.
4. Advisory Speed Limits when a sign is yellow the posted speed is merely a suggested speed and is not enforced as a regulatory speed limit (black and white sign).

These factors are considered: 

• road type and surface (curve, hill, etc.)
• location and type of access points (intersections, entrances, etc.)
• sufficient length of roadway (1/4 mile minimum)
• existing traffic control devices (signs, signals, etc.)
• accident history
• traffic volume
• sight distances
• test drive results
• speed study

The most important part of the traffic investigation is the speed study. When choosing a speed, drivers take many roadway environment factors into consideration. Therefore, the speed that the majority of people consider prudent is an important value. Data is collected by performing radar checks at selected locations on the roadway under ideal driving conditions. A technical analysis is done on the results to determine the 85th percentile. This is the value indicating the speed at which most (85%) drivers are traveling under. Experience has shown that a posted speed limit near this value is the maximum safe and reasonable speed. Studies have shown that traveling much faster or slower than this value can increase your chance of being in a crash.

As in most states, California has a number of law enforcement agencies which enforce speed laws; The California Highway Patrol (CHP) is our equivalent of what are called state troopers elsewhere. There are also local (city) police departments and county sheriff's offices. Regardless of the citing agency, there are three major means of monitoring and enforcing speed limits; Pacing, radar and patrol via aircraft. Pacing involves the officer following a vehicle for a period of time and comparing his/her speed against the speed of the vehicle to be cited. There is little that a speeding ticket recipient can do in contesting this type of violation other than to question his/her judgment or observation skills as evidenced by information written on the citation....time of day, make model year and color of vehicle and so on. Radar enforced speed zones will have signs posted alerting drivers to the fact that speeds are monitored by radar. As mentioned above, for such limits to be enforceable, a traffic and engineering survey (speed study) must be performed and documented at least once every five years or citations can be dismissed in court if there is a failure to provide such documentation at the request of the defendant.Citations issued by aircraft patrol who monitor speeds and signal the ground officer to make the traffic stop are the third variety. If one chooses to contest this method of speed monitoring, both the aircraft patrollman and ground officer must appear in court or the citation will be dismissed. Note that radar detectors are legal in California.

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