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Experts Agree: 165 MPH is a safe speed limit September 4, 2014

Posted by David Bookstaber in Transportation.

Safety is relative. There’s no guarantee you won’t be in a collision if you drive on public roads, no matter what precautions you take. So society, via engineers and legislators, weighs costs, benefits, and risks and reaches designs and compromises that are generally accepted.

But they are grossly inconsistent. Consider: America is full of roads with no physical divider and speed limits of 55 MPH. I.e., it is not considered excessively risky to drive 110 MPH relative to cars in adjacent lanes, separated by nothing more than a double yellow line. If you have ever designed, approved, or driven on one of these roads you evidently agree.

On a divided highway with a speed limit of 65 MPH this is equivalent to 165 MPH.

Many Americans to whom I have pointed this out seem appalled by the equation. What about distracted drivers? Can regular cars even handle such speeds? A small mistake at 65 MPH becomes catastrophic at 165 MPH.

Amusingly, on the unrestricted sections of the German Autobahn the answers to those questions are on display every day. For one thing, every modern road vehicle has a speed limiter (a.k.a. governor) set by its manufacturer to ensure that it isn’t pushed faster than its design limits. Drivers who push their cars well into triple digit speeds don’t do so without care. And distracted drivers, knowing that they share the road with aggressive drivers in high-performance cars, learn quickly to stay out of the left lanes. Of course they have accidents there too, and speed certainly contributes to their severity. But allowing drivers to speed on suitable divided highways does not seem to create more accidents.

So why do Americans accept the risks of passing oncoming vehicles at 110 MPH, but shudder at the idea of overtaking at the same rate on a divided highway?


1. johnfpetersonj - October 4, 2014

Basic physics: The impact of two cars (of the same mass) going 50 mph is the same as the impact of a car going 50 mph crashing into an immovable object (such as a very large tree). Though a single car is less likely to stop as fast in a crash (because it probably will not crash into an immovable object, or it will slow down some before it hits an immovable object) as it will in a head-on crash, it is not right to say that a head-on crash is generally twice as bad as a single-car crash (particularly if you crash into a car smaller than yours).

Also, the chance of losing control of your car at 110 mph or causing an accident by driving twice the prevailing speed is much greater than the combined chance of two drivers losing control or causing an accident at 55 mph.

federalist - October 4, 2014

To clarify your first observation: You’re noting that there are plenty of single-car crashes that are not into practically immovable objects. Note also that most “head-on” collisions are what industry engineers call “frontal offset,” wherein a lot of the energy is dissipated as the cars rotate or deflect about the combined center of mass.

While technically a car is less stable and forgiving of driver error as speed increases, in practice (at least in my experience) drivers who speed, especially relative to surrounding traffic, tend to be even more attentive and less prone to error and distraction. I would wager the increased attention at higher speed more than offsets the decreased stability, but there must be studies that settle that question….

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