Comments on: Ask the traffic engineer: How are speed limits set?

By: Eli Damon

Eli Damon — Fri, 23 Jul 2010 17:00:15 +0000

I don’t know why most motorists drive so recklessly fast. But truckdrivers are put under great pressure to deliver their cargo as quickly as possible, which is more quickly then reasonable. They are often heavily penalized for delays, either directly or indirectly. It is a case of unbalanced feedback. They get very little feedback about speeding but a lot of feedback about delay.

By: Al Grayson

Al Grayson — Sat, 10 Apr 2010 21:36:05 +0000

In my experience most drivers go too fast for conditions except on high speed highways where the speed limit is already 70-75 mph. There fear of being caught and the high fines keep them at a more reasonable speed.
Too often I see heavy motor freight trucks going at really excessive speed. On the famous German Autobanhnen trucks and buses are limited to 100 kph (62 mph) which is enforced via a recording meter connected to the drive axle. Cars and motorcycles are provided with a recommended maximum speed of 130 kph (80 mph) but no enforced speed limit. With the German experience (their trucks are of similar size and weight to ours) as a guide I do not understand why American truckers think it is OK to go so fast. I have followed a heavy truck at over 110 mph on I-24! And another passed the both of us!!
Just a couple of days ago I saw a twin trailer truck swerve off the road at over 80 mph in high wind gusts. It then swung back onto the traffic lanes, then whipped back and forth several times before settling down. Another truck was attempting to pass it but waited until the swaying stopped. On the same trip I saw a number of trucks run off the road, kicking up dirt and trash. Not every one was due to wind, either.
Many car drivers are likewise foolish. A car running alongside me suddenly began to swerve back and forth, nearly hitting my car. Its swerving was severe enough to make its tires screech. then it ran off the road onto the median and slid around sideways in the grass. When I glanced at the driver at the first, I saw she was hanging onto her cellphone, still holding it to her ear as she overcontrolled her car with one hand on the wheel.
Goofy stuff like this is a near-everyday occurrence in my driving experience. People cut into my lane, pull out in front of me, you name it. Somehow I have managed to avoid being hit since 1986 and that was being rammed in the rear at a gas pump.
Jym’s comment reminds me of an article about antilean technology. Way back in the ’50s Citroen developed a system to cause a D sedan to corner level. It could even be set up to bank inwards. However, the engineers decided to drop the project, as they found that the increased comfort to the driver, and the improved roadholding due to keeping the tire treads flat on the road, encouraged the drivers to use up all the added margin of what should have been safety. Along with the advent of belted radial tires, which tend to lose grip suddenly, the cornering limit was raised so much that when it did run out, the car tended to lose grip abruptly and go out of control.
Citroen for a long time overdesigned its chassis with geometry, handling and braking suited for up to 150 mph but with drivetrains good for only 90-100 mph. In the same period other makers were building cars with 120-150 mph drivetrains but handling for 60 mph and brakes for 30.
Most car buyers don’t care about any engineering features except for powerful engines. A car that goes fast and accelerates briskly will sell, while emphasizing brakes and handling gets few sales. The only large market for safety is buyers for fleets, where the owner is a company that wants to reduce costs.

By: Jym

Jym — Fri, 12 Mar 2010 19:37:04 +0000

=v= Informative as this blog entry might be, it does not address the dynamic underlying our nation of chronic speeders. After WWII it became standard practice to overbuild roads, under the premise that this would add a margin of safety. As we have learned for the last half-century, though, what it really does is encourage motorists to speed.

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By: Andrew Smolik

Andrew Smolik — Wed, 03 Feb 2010 05:02:36 +0000

There is an article at http://www.safespeed.org.uk/speedlimits.html showing via a plot that crash risk is also at a minimum at the 85th to 90th speed percentile, which is a legitimate rationale for setting speed limits there: “Plenty of research has shown that the safest group of vehicles are travelling at or below the 85th to 90th percentiles. Research shows that crash risk alters with speed and this is shown by the red crash risk curve. At the 85th to 90th percentiles we tend to find drivers with above average skill and competence, and this is why their crash risk is the lowest. Above the 90th percentile we tend to find drivers exceeding safe limits and their accident risk increases as a consequence. Note that the ‘average’ driver at the 50% percentile has a greater crash risk than the 85th percentile driver. Below the 30th percentile crash risk is significantly increased and these speeds tend to be used by less skilled and competent drivers.”

Of course, it’s important to realize the caveat that such a relationship is only a correlation, not necessarily a cause, likely applicable only to motor vehicles — actually capable of reaching higher speeds — forming the typical majority of vehicles on the road; it ought not to be used as an excuse to discriminate against nonmotorized vehicles which simply are not capable of reaching higher speeds. It is likely attentiveness, lateral positioning, and positioning relative to other vehicles that matter a whole lot more than speed when it comes to minimizing crash risk; as the conclusion of the article states: “You can’t measure safe driving in miles per hour.” Those driving somewhat higher or lower than the average traffic speed may be safer because they are inherently not following or pacing other vehicles too closely within tight traffic packs; instead, they “pass or let pass” more safely.

By: Bob Sutterfield

Bob Sutterfield — Sun, 31 Jan 2010 14:23:02 +0000

Per CVC 22348(a), any person who “drives a vehicle upon a highway” is required to obey the maximum speed limit. Per 21200(a), cyclists are included among those drivers who are subject to this provision, unless someone argues that the maximum speed limit is one “which by its very nature can have no application” to cyclists.

If cyclists are subject to that provision, are cyclists’ travel speeds required to be counted in the “engineering and traffic survey” required to set that 85th percentile level?

Are there enough cyclists in the traffic mix to make a statistically significant difference in the speed distribution observed during such a survey?