Fight Your Ticket & Win in California contains errors regarding 85th-percentile speeds in relation to the determination of speed limits. In the 16th edition, pages 219 through 221 and the legal brief from page 222 to 229 are affected.
The problematic assertions are:
You should resolve any other inconsistencies between the information in those pages and the information provided here in favor of the following. (Readers should also rely on the most recent version of the California Manual on Uniform Traffic Control Devices ("MUTCD").)
Note: Existing engineering and traffic surveys performed before July 1, 2009 don't have to comply with the new criteria noted below until they're due for reevaluation. (MUTCD Section 2B.13, 12f.)
Also note: In addition to what follows, factors that may be considered when establishing or reevaluating speed limits are:
(MUTCD Section 2B.13, 16.)
The 85th-percentile speed is the speed at or below which 85% of vehicles travel. (MUTCD Section 1A.13, 214.)
A posted speed limit must be set at the nearest 5-mph increment of the 85th-percentile speed of free-flowing traffic, with two exceptions discussed below. (MUTCD Section 2B.13, 12a.) So if the 85th-percentile speed were 37 mph, the posted speed limit would normally have to be 35 mph. Or if the 85th percentile speed were 38 mph, the posted speed limit would normally have to be 40 mph.
Here are the two exceptions to this rule.
The following exception for establishing posted speed limits applies to engineering and traffic surveys performed on or after July 1, 2009. If the following 5-mph reduction is applied, an engineering and traffic survey must document the conditions and justification for the lower speed limit and be approved by a registered Civil or Traffic Engineer. (MUTCD Section 2B.13, 12a and 12b.)
The posted speed limit may be reduced by 5 mph from the nearest 5-mph increment of the 85th-percentile speed in accordance with California Vehicle Code Sections 627 and 22358.5. (MUTCD Section 2B.13, 12a.) This provision means that if the 85th-percentile speed were 37 mph, the speed limit could be reduced from 35 mph to 30 mph. Or if the 85th-percentile speed were 33 mph, the speed limited could be reduced from 35 mph to 30 mph.
Section 22358.5 essentially says merely that the Legislature intends that road conditions that are readily apparent to a driver, standing alone, would not require special downward speed zoning. But Section 627 provides that an engineering and traffic survey must include, among other requirements deemed necessary, consideration of:
Section 627 further says that, when conducting an engineering and traffic survey, local authorities may also consider:
(See Section 627 for more detail.)
The following exception may be used to establish speed limits after January 1, 2012.
Where the nearest 5-mph increment of the 85th-percentile speed would require rounding up, the speed limit may be rounded down to the nearest 5 mph increment. (MUTCD Section 2B.13, 12a.) So, if the 85th-percentile speed in a speed survey were 33 mph, instead of rounding up to 35 mph, the speed limit can be rounded down to 30 mph.