Shasta County maintains 1193 miles of County roads. Speed limits are set based upon Engineering and Traffic Surveys (E&TS) conducted in conformance with applicable codes. The California Highway Patrol (CHP) enforces those speed limits. The CHP has specifically requested E&TS along twelve roads to permit enforcement with radar. Complete details are included in the attached summary table and individual E&TS.
Speed limits are designed to match the 85th percentile speed based upon an E&TS. The underlying assumption is that six out of every seven motorists are traveling at a speed which is safe and prudent. Most of our existing speed limits were set in the latter half of the twentieth century. Some may have been noncompliant when they were established. Speed limits must be reviewed every seven years if they are to remain enforceable with radar. Vehicle designs have improved. Codes have been modified to limit downward adjustments. The net result has been an upward trend in enforceable speed limits. Lower speed limits may be permitted under certain circumstances (accident rates, school zones, residential and commercial districts). These exemptions have been applied as available and applicable. The CalTrans Traffic Manual provides guidance as follows:
“The setting of speed limits requires a rational and defensible procedure to maintain the confidence of the public and legal systems. By following a uniform procedure, agencies can establish speed limits that are uniform throughout the state and avoid influence from political pressure or emotional perceptions.”
“…Studies of the effects of establishing, raising and lowering speed limits include federal studies FHWA-RD-92-084 and FHWA-RD-98-154… The empirical data in these studies demonstrates that setting the speed limit too high or too low can increase collisions. Speed limits that are set near the 85th percentile speed of free flowing traffic are safer and produce less variance in vehicle speeds. Because of this, the 85th percentile is used to establish the upper limit of operating speeds that are considered reasonable and prudent.”
“Regardless of the posted speed limit, the majority of drivers will continue to drive at speeds at which they feel comfortable. The question then arises, “Why do we even need to post speed limit signs?” In part, posting speed limit signs:
drivers who consider their speed safe and reasonable.
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Allows for fair enforcement for those drivers who are going exceedingly faster or
slower than the speed the majority of drivers consider safe and reasonable.
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Determines many other design criteria such as clear recovery zone distance, yellow
light timing, taper lengths for any type of lane closure or construction, set-back detectors,
size of signs, and other roadway features.”
Highway safety performance is a function of enforcement, engineering, education and emergency responders. The CHP and the County are working to identify corridors where safety may be enhanced through integrated efforts (speed limits, collision analyses, road improvements, enhanced enforcement, etc.). The CHP and the County are conducting studies and have applied for various grants. Related items are anticipated to come before the Board within the next year.