Avoid a Trap! Understand How Speed Limits are Set in California

cop checking speed of cars

What makes a speed limit reasonable, appropriate, and enforceable? An accurate and thoughtful response requires an understanding of both the California Vehicle Code (CVC) and the California Manual on Uniform Traffic Control Devices (CAMUTCD).

speed limit 50The CAMUTCD states, “The setting of speed limits can be controversial and requires a rational and defensible determination to maintain public confidence.  Speed limits are normally set near the 85th percentile speed that statistically represents one standard deviation above the average speed and establishes the upper limit of what is considered reasonable and prudent.  As with most laws, speed limits need to depend on the voluntary compliance of the greater majority of motorists.  Speed limits cannot be set arbitrarily low, as this would create violators of the majority of drivers and would not command the respect of the public.”

In light of these concepts, CVC §40802 defines a Speed Trap and sets forth limitations to the enforcement of speed limits that are not established based on sound engineering principles. Radar enforcement of such unsound speed limits constitutes a legally defined speed trap. To avoid establishing a speed trap, the CVC requires justification of speed limits by an Engineering and Traffic Survey (E&TS). Parameters for an E&TS are provided in CVC §627 and are further explained in the CAMUTCD.

cop checking speed of cars

Two key elements are documented in an E&TS – 1) the 85th percentile speed and 2) conditions not readily apparent to drivers. The 85th percentile speed on a roadway is determined by measuring the speed of vehicles traveling on that roadway, typically with a radar gun. The 85th percentile speed is the speed at which 85 percent of observed vehicles travel at or below.  The 85th percentile speed provides the foundational basis for the posted speed limit, and once measured, is typically rounded to the nearest 5 mph increment to become the posted speed limit. Conditions not readily apparent to the driver provide the primary justification for setting a speed limit below this value, subject to various options provided for in the CAMUTCD.

Option 1 – The posted speed may be reduced by 5 mph from the nearest 5 mph increment of the 85th-percentile speed based solely on engineering judgment. An E&TS that documents the conditions and justification for the lower speed limit must be approved by a registered Civil or Traffic Engineer.

Option 2 – For cases in which the nearest 5 mph increment of the 85th-percentile speed would require a rounding up, then the speed limit may be rounded down to the nearest 5 mph increment below the 85th percentile speed, if no further reduction is used.

For example, if the 85th percentile speed in a speed survey was 37 mph, the speed limit would be posted at 35 mph, or in accordance with Option 1, could be dropped to 30 mph with justification and approval from a registered Civil or Traffic Engineer.

If the 85th percentile speed in a speed survey was 38 mph, the speed limit would be posted at 40 mph, or in accordance with Option 1, could be dropped to 35 mph with justification and approval from a registered Civil or Traffic Engineer. Option 2 allows for a round down from 38 mph to 35 mph without specific justification as long as no further reductions are applied.

The CAMUTCD cautions, “The establishment of a speed limit of more than 5 mph below the 85th percentile speed should be done with great care as studies have shown that establishing a speed limit at less than the 85th percentile generally results in an increase in collision rates; in addition, this may make violators of a disproportionate number of the reasonable majority of drivers.”

There are a number of factors and steps involved in establishing a speed limit and MEA Forensic Engineers & Scientists would be pleased to help guide you through this process as well as any other speed zoning questions related to your case.

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