Dispelling misconceptions about speed limits

Have you noticed there are four different speed limits posted along Torrey Pines Road at different junctures, ranging 25-45 mph? (Photo by Daniel K. Lew)


By Todd Lesser

La Jolla Traffic & Transportation Board Chair

• Increasing the speed limit will cause automobiles to drive even faster.

• Increasing the speed limit will increase accidents.

• Reducing the speed limit will slow the speed of traffic.

Although widely perceived to be true, these are all myths not supported by the facts. Most state and local agencies use the 85th percentile speed of free flowing traffic as the basis in establishing speed limits. What this means is that 85 percent of the traffic is traveling at or below this speed.

Depending on the specific traffic question at issue, every 5, 7 or 10 years, the California Vehicle code Section 40802 requires state and local agencies to re-evaluate non-statutory speed limits on segments of their roadways that have undergone a significant change in roadway characteristics or surrounding land use since the last review.

If a speed study shows the 85th percentile is driving faster than the posted speed limit, the speed limit on the road can no longer be enforced using radar.

Over the last few months, there have been many recommendations by the city to increase the speed limit on roads throughout our community. This has led to a lot of uprising by local neighbors who incorrectly assume that increased speed limits will have many negative results.

The statistics show that people regulate their speeds according to road design and drive no faster than they feel is a safe speed.

For instance, if the roads are narrow, they tend to drive slower. If there is an obstacle in front of them, like a traffic circle, they slow down. Drivers “vote with their feet,” states the Institute of Transportation Engineers.

In addition, their evaluation of the studies, including data from the U.S. Department of Transportation, has shown “crash rates are lowest at around the 85th percentile. ”

According to the Southern California Automobile Association, “Before- and after-studies consistently demonstrate that there are no significant changes in traffic speeds following the posting of new or revised speed limits. Furthermore, no published research findings have established any direct relationship between posted speed limits and accident frequency.”

Contrary to popular belief, according to the California Department of Transportation, “Studies have show that establishing a speed limit at less than the 85th percentile (Critical Speed) generally results in an increase in accident rates.” In other words, if a speed limit increase is necessary to correctly reflect the 85th percentile, then the road is safer at that speed than at the previously posted speed limit below the 85th percentile.

If you would like to comment on traffic related issues in La Jolla, I recommend getting on our e-mail list: ljsa.org/lists

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Posted by Staff on Mar 14, 2014. Filed under News, Opinion. You can follow any responses to this entry through the RSS 2.0. You can skip to the end and leave a response. Pinging is currently not allowed.

4 Comments for “Dispelling misconceptions about speed limits”

  1. James

    Once the speed limits accurately reflect the 85th percentile can the speed limit be enforced again? And would the school zones still be 25mph?

  2. This article is exactly correct. It states the principles of setting the safest speed limits which have been known for 70+ years.

    Politicians that fight for artificially low posted limits set below the 85th percentile speeds want one or both of two things. 1) They want more ticket revenue from speed traps. 2) They want to satisfy well-meaning but uninformed citizens who believe the myths noted in the article to make re-election easier.

    People who support true traffic safety should support 85th percentile posted speed limits. See our website for the science.

    James C. Walker, Life Member-National Motorists Association

  3. Leroy

    Oh. I guess that’s why CA sticks to the ridiculous 70 mph speed limit through the deserts and other low population areas.

  4. Of course, proper 85th percentile speed limits can be enforced – and school zones are treated as a special case for the hours children are likely to be present.

    The difference with 85th percentile posted limits is that safe drivers going along with the normal speeds of traffic are no longer targets for “road tax” collections in speed traps. Only the drivers who are well above the normal flow speeds can be ticketed – and they are the only ones that should be, for speed alone.

    James C. Walker, Life member-National Motorists Association

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