Why are so many flat CA state highways signed at 55 MPH?

[heynow415]

Doesn't California have a rule that says that speed limits have to be periodically justified based on the 85th percentile' principal?   This is to help neutralize 'Deputy Dawg' speed traps.

Mike

Yes, it does, for the most part. The default speed limit on two-lane highways is 55, unless posted otherwise. Speeds can be increased to 65 or 70 based on certain criteria and typically only on divided highways or freeways, and some two lane highways designed to expressway standards.  At the other end of the spectrum, the default speed limit in business districts and on residential streets is 25; in school zones and alleys it can be lowered to 15. 
For everything else, posting an enforceable speed limit must be based on an engineering and traffic study (E&TS) to determine what the 85th percentile is and rounding to the nearest 5mph.  Based on that same study and the particular situation of the roadway, the certifying traffic engineer can reduce the 85th speed by another 5mph, such as roads with lots of driveways, blind curves or intersections and the like. E&TS certifications have an initial life of 5-7 years but can be extended up to 10 years if there have not been any significant changes to the roadway or its surroundings (configuration, traffic patterns, new development, etc.).  At that point a new study needs to be done to validate the posted speed.