Recently, SFMTA posted an interesting and pertinent infographic for drivers and bicyclists in San Francisco. Obviously, the faster a car is going when it hits someone the more bad news bears the result. But I couldn’t help but take a second look. Why is it seemingly right at 20mph that our fatality rates start to spike upwards?

These stats are rounded to fit within the ‘whole person’ or to the nearest 10% mark. See sources for more precise numbers, which ballpark risk of death at 20 mph nearer to 5%.

The human body is a remarkable feat of evolution. We live in an unpredictable environment from tree branches falling to kids jumping on us. Our bodies have been designed to withstand the impacts associated with our environment. Now, I can’t simulate what impacts cavemen encountered from chasing mammoths, but there is one basic impact everyone will most likely sustain at least once in their life. We all fall over.

With a brief thank you to Newton, here are the estimated speeds at impact an average 5’6″ person would encounter playing at the park:

• Falling over while standing – 13 mph
• Falling over while walking – 16 mph
• Falling over during a light jog (10 minute miles)  – 19 mph
• Falling out of a 12 foot tree – 19 mph

Provided we’ve evolved to survive playing in the park, the human body will do okay with impacts of less than 20mph. Above 20mph, it becomes increasingly difficult to find naturally occurring high speed impacts. Coincidentally, the mortality rate from impact with higher speed cars dramatically increases. Using world records to increase natural impact speeds we find:

• Falling over while running a 4 minute mile – 28 mph
• Usain Bolt tripping and passing out during his world record 100m run – 36 mph

We just don’t encounter a 40mph impact naturally, so why would we evolve to survive them? Now, this is not a rigorous laboratory analysis, but as a driver routinely frustrated by speed limits it helps me understand what really matters. My personal actionable insight is:

When children or happy drunk people on Mission are involved, treat 20mph as the maximal speed limit.

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Simulation assumptions and references:

• Person does not break their fall (which is a fair assumption, because when you faint or fall awkwardly backwards, you hit like a rock)
• Point of impact is the head
• SFMTA tweet (it is more conservative than the following primary documents)
• Impact speed and a pedestrian’s risk of severe injury or death.
• Effects of Vehicle Speed on Pedestrian Fatalities
• AAA in-depth report

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Update:

This post was written before the tragic event of a high speed truck killing pedestrians in Nice. My heart goes out to those who lost loved ones, those who are still fighting to breathe, and those who have seen what can not be unseen. To donate and offer support for a full recovery, please visit here. I will leave this post up in the hope, data will help make the world a safe place.