The other day, I was surprised to learn that my colleague M did not know that the numbers of the interstate highway system have meaning.  He then brought it up in a zoom meeting.  Of about 10 people, most my age or older, only one other person besides M and myself knew what the numbers mean.

For the record – an odd number (I95, I29) means the highway runs north south.  An even number (I94, I80) means the highway runs east west.  If the first number of a three-digit number is even (I494, I695) it indicates a loop around a city.  An odd first number (I395) means a spur into the center of a city.

I thought this was common knowledge, but it is apparently more esoteric than I thought.  What is something you thought was common knowledge, but turned out not to be?