The argument was made that “55 Saved Lives”, using data that showed the number of traffic fatalities declined when the speed limit was dropped by an executive order issued by President Nixon after the Arab Oil Embargo in 1973 as a means of reducing fuel consumption. What those statistics used didn't show, and for good reason, was that the fatality rate itself didn't change, meaning the number of motorists killed per millions of passenger miles traveled. What caused the drop in fatalities was the drop in the number of miles being driven by the American public. With a decrease in the number of miles traveled the number of traffic fatalities also decreased.
That was then and things have changed considerably.
For one thing, cars and trucks are much more fuel efficient than they were 40-some years ago. They are also a heck of a lot safer, between structural changes affecting crash survivability, better safety design and equipment such as 3-point seatbelts and airbags, and better tires and braking systems. That maske much of the “55 Saves Lives” mantra even more obsolete.
Around 1985 the NMSL was modified to allow states to set the speed limits in their highways up as high as 65MPH. Then Congress got around repealing the NMSL in 1995. The hew and cry form the Safety Nazis was loud and long, with claims that “blood would cover our streets and highways” with the higher speed limits. The reality – the number of traffic fatalities dropped.
Some states raised the speed limits on their highways back to what they had been before the NMSL. Some almost did so. (My home state of New Hampshire has a maximum speed limit of 70MPH on the Interstates, though in some sections it used to be as high as 75MPH.) Some of the western states have no daytime speed limit on their highways other than “safe and prudent for the conditions”.
But are the speed limits set according to what this post by Erik calls Traffic Engineering 101? The answer is, unfortunately, no.
Every year, traffic engineers review the speed limit on thousands of stretches of road and highway. Most are reviewed by a member of the state’s Department of Transportation, often along with a member of the state police, as is the case in Michigan. In each case, the “survey team” has a clear approach: they want to set the speed limit so that 15% of drivers exceed it and 85% of drivers drive at or below the speed limit.It all comes down to this: people drive at the speed at which they feel comfortable regardless of the posted speed limit.
This “nationally recognized method” of setting the speed limit as the 85th percentile speed is essentially traffic engineering 101. It’s also a bit perplexing to those unfamiliar with the concept. Shouldn’t everyone drive at or below the speed limit? And if a driver’s speed is dictated by the speed limit, how can you decide whether or not to change that limit based on the speed of traffic? The answer lies in realizing that the speed limit really is just a number on a sign, and it has very little influence on how fast people drive. “Over the years, I’ve done many follow up studies after we raise or lower a speed limit,” [Lieutenant Gary] Megge [of the Michigan State Police} tells us. “Almost every time, the 85th percentile speed doesn’t change, or if it does, it’s by about 2 or 3 mph.”
… Years of observing traffic has shown engineers that as long as a cop car is not in sight, most people simply drive at whatever speed they like.
Our town had a number of people that demanded that a approximately 1.5 mile portion of one state road that passed through our town have the speed limit lowered from 50MPH to 35MPH because of one intersection towards one end of that 1.5 mile stretch. The intersection was on the other side of the top of a hill and getting out was sometimes problematic with cars coming over the rise and down the hill at 50MPH. So their solution was to lower the speed limit almost a mile and a half away just to lower the speed at that one intersection.
At the public hearing held by our town selectmen, a number of people spoke in favor of lowering the limit. I was one of the few who spoke out against it and for one very good reason, that being that it would be universally ignored because that stretch of highway was four lanes wide with an unobstructed view, and driving that stretch at 35MPH would appear to be ludicrously slow. Not too many people liked my comment, but more than a few nodded their heads in agreement.
In the end the public hearing had no effect. The state DOT denied the request, stating the lower speed was too low for the road design and would likely see only a small percentage of motorists adhering to the lower limit. The rest would still drive up and down that state road at 50MPH just as they always did. The DOT understood the principles of Traffic Engineering 101 and used it to justify their rejection.
They did the right thing.
Now if they would only apply Traffic Engineering 101 to the Interstates and state highways and reset the speed limits to what they were before NMSL made driving so miserable for everyone.