What is a Traffic and Engineering Survey and what does the information mean?

A traffic and engineering survey is used to determine the prevailing speeds and to establish a speed limit. Traffic and Engineering surveys need to have been completed within five years. As defined by the California Vehicle Code:

• 627. (a) "Engineering and traffic survey," as used in this code, means a survey of highway and traffic conditions in accordance with methods determined by the Department of Transportation for use by state and local authorities.
• (b) An engineering and traffic survey shall include, among other requirements deemed necessary by the department, consideration of all of the following:
• (1) Prevailing speeds as determined by traffic engineering measurements.
• (2) Accident records.
• (3) Highway, traffic, and roadside conditions not readily apparent to the driver.
• (c) When conducting an engineering and traffic survey, local authorities, in addition to the factors set forth in paragraphs (1) to (3), inclusive, of subdivision (b) may consider all of the following:
• (1) Residential density, if any of the following conditions exist on the particular portion of highway and the property contiguous thereto, other than a business district:
• (A) Upon one side of the highway, within a distance of a quarter of a mile, the contiguous property fronting thereon is occupied by 13 or more separate dwelling houses or business structures.
• (B) Upon both sides of the highway, collectively, within a distance of a quarter of a mile, the contiguous property fronting thereon is occupied by 16 or more separate dwelling houses or business structures.
• (C) The portion of highway is longer than one-quarter of a mile but has the ratio of separate dwelling houses or business structures to the length of the highway described in either subparagraph (A) or (B).
• (2) Pedestrian and bicyclist safety.

85th Percentile Speed

Q: What is the 85th percentile speed?
ANSWER: If the speeds of all motorists are ranked from slowest to fastest, the "85th percentile speed" separates the slower 85% from the fastest 15%. (Similarly, the average speed separates the fastest 50 nd the slowest 50% of motorists in a normal speed distribution. In other words, the average speed is typically the "50th percentile speed").

Q: Why the 85th percentile?

Why not the 99th, or 75th or 30th percentile?
ANSWER: Two reasons.
1. Most motorists travel at about the same speeds, so setting the speed limit at the 85th percentile legalizes the vast majority of motorists. About 70% of motorists travel in a 10-mph grouping (called the "pace"), which generally covers all but the fastest 15 nd slowest 15%. If the speed limit were set at the average speed, only 50% of motorists would be legalized. Setting the speed limit about 5-mph higher (at the 85th percentile) legalizes the vast majority of motorists. Raising the speed limit another 5-mph wouldn't legalize that many more drivers, because the fastest 15 nd the slowest 15% tend to more widely dispersed in traffic speeds.

2. Research suggests that motorists far outside the normal traffic flow have higher accident rates. A speed limit at about the 85th percentile ("about" because speed limits are posted in 5-mph increments) legalizes consensus of most motorists, and after a reasonable enforcement tolerance, focuses law enforcement on motorists far outside the normal flow.

A Police Officer uses lidar to determine the speed of passing vehicles.

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