Highway Safety: Speed Limits

So far, we’ve agreed states have a responsibility to provide safe highways. I think we’ve also agreed speed limits are one of the major ways of providing safe travel.

How are speed limits set?

Many things go into setting a limit. Traffic, road entries and exits (access), type of environment (expressway, city street, young family neighborhood), even vehicle capabilities are included.

In the space of a blog it’s impossible to talk about every aspect so this discussion will be limited to interstate highways, that is expressways. They are consistent across state borders, built to standards set by the federal government. The standards are available from AASHTO (http://www.transportation.org/) as part of their publication, A Policy on Design Standards -- Interstate System
Standards were developed as part of the Federal-Aid Highway Act of 1956. Nineteen fifty-six. Yes, with limited modifications, the interstate highways you drive today are designed according to 50 years old standards. That includes the speed limits on those expressways.

Well, if 70 was good enough then, what’s wrong with it now?

Seventy is no longer the safest speed. Not the safest because it is too slow for your safety.

Huh?

Time for more background.

Highway safety engineers have known for decades that the safest speed limit is the 85th percentile speed. That’s the speed 85% of all drivers drive at or below when given an unmarked speed limit. As with many speed related things, the reason for this is complicated. However, we can make a simple but true statement that covers most of it:

Too low a speed leads to driver inattention.

I heard that. You’re wrong, it’s not bull shirt.

If you want a personal example of the dangers of a too-slow speed limit,, take a drive on I-20 East into Alabama from Mississippi. There’s a 20 plus mile stretch with a 50 mph limit. No construction equipment, no barrels, no workmen, just a nice, fresh blacktop section where the 50 mph signs are still up. If you drive it at 50, you’ll begin suffering highway hypnosis before you’re half way through. Unless you’re in panic because everything, including 18 wheel trucks, is blowing past you going 20 to 30 mph faster.

Wonder why 70 is no longer the safest speed? Take a look at the car of 1975. Compare it to the 21st century models.

In 1975 tires were bias ply. They were short lived and far more prone to going flat. Today, tires are radial, often have 40,000 mile guarantees they outlive, and provide far more adhesion (grip) in all kinds of weather than those old tires did even on a sunny day.

The cars themselves are far more controllable. In 1975 most cars wallowed like a boat on a choppy sea. By the 21st century every car sold had much improved suspension and ride control (shocks and struts), far better than anything available when the standards were set.

These two factors alone, radial tires and improved ride control, increased safe speeds over identical surfaces by as much as 30% for every car sold. In some cases, such as the 550i, vehicles are so improved even a beginner could drive safely at nearly twice the posted speed. For those familiar with the Jackson, MS area, the north entrance and south exit to I-220 are marked at 50 mph. In that car, even a normal drive can negotiate those ramps at 80 to 90 mph safely – barring other traffic of course.

For those that can’t get to Alabama, I-220, or don’t believe their own experiences, let’s look at some speed related statistics.

You may have heard of Autobahns. Those real high speed, sometimes unlimited speed, interstates in Germany. I’ll bet you’ve also heard how dangerous and deadly they are.

Not so according to statistics from the International Road Traffic and Accident database (IRTAD). Here are the numbers for 2006:

Injury Accidents per 100,000 Population.
Germany 408
USA 647 (2004, US 2006 numbers were not available)

Deaths per Billion KM
Germany 7.8
USA 9.4

Let’s also look at a report dating back to 1992 for the U.S. Department of Transportation, Federal Highway Administration. The report, titled Effects of Raising and Lowering Speed Limits (http://www.ibiblio.org/rdu/sl-irrel.html), makes a couple of very interesting statements. I quote.

Accidents at the 58 experimental sites where speed limits were lowered increased by 5.4 percent

And

Accidents at the 41 experimental sites where speed limits were raised decreased by 6.7 percent.

There we go. For 15 years, states have known setting speed limits too low could kill you. For at least as long, the policy makers have known of the improvements in cars After all, they do drive.

A few states have acted on this information, if in an unofficial way. One state has an 80 mph policy on interstates. The speed limit sign may say 70 but you won’t get a ticket unless you’re over 80.

Why haven’t the others acted? I suggest it’s a conflict of interest. They know, because their staff engineers told them, higher speed limits are safer. They don’t act because they want the ticket revenue.

What can we do about it?

I doubt educating our legislators will help. They’re the ones who get and spend the money from those tickets.

Our best bet is probably a young law firm, eager to make a name and a few bucks for themselves. A class action suit on behalf of traveling salespersons and others whose livelihood is affected by time lost to a ticket hungry state.

If they do it right, the results will be 3 speed limits. 80 for cars and smaller trucks, 70 for anything with a trailer or over some heavy weight, and, at last, a minimum speed because we all know how dangerous it is to come up on some moron doing 35 when you’re traveling at 70.

Lawyers, where are you?


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First post in the series.