Crossing the street — what are the odds?
I needed to go to the store last weekend, and I thought I’d walk. It isn’t far and walking is good for you. I did it all the time when I lived in a city.
But to get to the store I have to cross one big street. There’s a light and a crosswalk, but it’s one of those streets where the drivers don’t expect pedestrians. And there’s always cars turning left and right even when there’s a cross signal. It’s a complicated wide busy intersection.
I’ve crossed this street before however, and I’ve never had a close call. There’s a lot going on, but I try to be super careful. I feel the risk. I wouldn’t want my kid to have to cross this street.
So I’m thinking, what if I had to cross this street every day for, say 10 years. I’d be crossing thousands of times. What are the odds that I’d be hit by a car, have really bad luck just once in all those chances?
Let’s say the odds of being hit are one in 100 million. If I cross 5 thousand times, my odds of being hit are about one in 20 thousand. I’m a little uncomfortable with that, but it feels like an acceptable risk for something spread out over 10 years.
But I think the odds are probably somewhat worse than that, maybe even as high as one in 10,000. Which would mean I have a pretty good chance of being hit over 10 years. Which negates all the health benefits of walking.
And if I shouldn’t be crossing that street every day for 10 years, I probably shouldn’t be crossing at all without a good reason.
So the problem is that you can’t rely on intuition to decide whether something like this is safe. Intuition isn’t very good at distinguishing between 1 in 10,000 and 1 in 10,000,000. Intuition isn’t very good at summing odds over 10 years. You have to think it out.
Now all I have to do is find out what the odds really are.
Comments
2 Responses to “Crossing the street — what are the odds?”
Leave a Reply
The CDC has some excellent information on how many people died in the US from various causes.
161 deaths in 2006: V03 “Pedestrian injured in collision with car, pickup truck, or van”
418 deaths in 2006: V05 “Pedestrian injured in collision with railway train or railway vehicle”
4262 deaths in 2006: V43 “Car occupant injured in collision with car, pickup truck, or van”
631,250 deaths in 2006: “Heart Disease”
Imagine that we talked about something with a time proportional to how many of our neighbors that something has killed.
Then for every minute we talk about getting hit while walking across the street, we would talk for 30 minutes about car accidents, and 65 hours about heart disease.
David,
You’re probably right. The health benefits of walking probably out-weigh the risk of being hit crossing that street.
But that 4000-to-one ratio of deaths due to heart-disease vs car-smack is not a slam-dunk argument in this case. Of the ~2000 people in my neighborhood, only about 100 ever seem to walk anywhere at all. (I’m guessing. I walk around most days, at various times, and I see the same 20 people over and over.) And nobody ever crosses that street. So very very few people ever get the opportunity to get killed as a pedestrian.
At least in the suburbs.
Let’s say that instead of crossing that street I had to swing on a rope across a deep chasm every day. I’m reassured because the CDC tells me that only 2 people/year are killed doing this. But later on I find out that only about 200 people in the entire country ever swing across chasms at all, and many of them only a few times a year.
So I guess I’m saying this is an exceptional activity, and I don’t know how risky it is because so few people partake. Maybe crossing that street is as risky as skydiving? If you’re a skydiver, what’re your odds every time you jump? 0.00009? Or 0.0000001? There’s a big difference. I’ll take the later, but not the former.