Lately, I’ve been writing a lot about Vision Zero. It’s an initiative a number of wealthy countries around the world are following to different degrees with the overall goal of reducing auto fatalities in their populations. One of the central tenets of Vision Zero is the belief that roadways need to be designed to protect different kinds of travelers from coming into contact with each other above particular speeds. For example, if you’re an unprotected traveler, like a pedestrian, any roadways you use that might put you in contact with cars (e.g., cross-walks) should not have traffic traveling any faster than 20 mph. These guidelines also exist for individuals traveling in cars who might come into contact with other cars, which is precisely what this article is about.
What can we do to increase our odds of surviving a head-on collision?
What makes 43 mph the limit for survivability?
But I know / heard of someone who survived a head on at 50/60/80 mph!
How do other safety organizations (i.e., the IIHS) feel about this figure?
Does this mean my car / SUV / minivan / pickup is only designed to protect my family at up to 40 mph?
How much more severe is a crash at 70 mph than one at 40 mph?
It’s important to remember that the forces in a collision quadruple when speeds are doubled, rather than simply doubling, because kinetic energy increases with the square of velocity. To put this in practical terms, a crash at 80 mph carries 4x the energy as a crash at 40 mph, even though the speed is only 2x as fast. As noted above, your vehicle is only designed to protect you from a crash with an equal or lighter mass vehicle at 40 mph. Let’s use that as a baseline, where your vehicle handles 100% of tested forces while allowing you to survive the crash with minimal injury. At these speeds, your odds of survival in a vehicle with a “good” frontal score are close to (not quite, but close to) 100%.
A 70 mph crash carries more than 2x as energy, or precisely 306% as much energy as the 40 mph crash (100% of tested forces). In my observations from studying crashes, once you get up to around 300% of the forces your vehicle was designed to handle, your odds of survival drop down to around 25%. To put it another way, in a head-on crash at 70 mph involving 2 vehicles with 4 people in the front seats, only 1 of the 4 people involved is likely to survive.
This is the risk we run every time we drive at 70 mph in an environment with a possibility of head-on collisions (i.e., every undivided highway in the country).
If crashes are this much more dangerous above 40 or 43 mph, then why do we have speed limits at 55, 65, or 75 mph?
That’s a great question. A basic answer is because our society (as well as virtually every other around the world) prioritizes speed over safety when it comes to auto travel. Additionally, most people aren’t aware of the dramatically increased risks that come from higher speed limits until it’s too late (you can’t advocate from beyond the grave). However, it’s important to remember, too, that speed limits beyond 43 mph can be tolerated with low risks of severe injury or death as long as the risks of head-on collisions are eliminated, which is possible through good road design. The challenge is to bring that good road design to a country that’s plagued with poor road design.
What do you mean by poor road design?
Poor road design, per Vision Zero, in this context refers to roadways with an opportunity for head-on collisions that permit or encourage vehicles to travel beyond 43 mph. In other words, two-lane undivided highways with 50, 55, or 60 mph (or higher!) speed limits are roadways that shouldn’t exist, but do throughout the US (and globe).
What does this mean for my family and I? Do we have to drive at 43 mph everywhere?
Not necessarily. But as noted earlier, I’d certainly try to avoid driving on any undivided roadway with a posted speed limit above 40 mph. The risk increases exponentially with the speed of traffic. And given the propensity of people to speed in this country, a road with a 40 mph PSL will most likely already be dealing with 45-50 mph traffic. You can do the math for higher speeds.
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