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.