I was reading the recipe on the back of a packet of cake mix my mum bought the other day (I was calling out the ingredients she needed) and found that it had a separate set of instructions for people living over 3,500 feet above sea-level. I thought this was odd but when I mentioned it she said a lot of products sold in the US have this on, especially cake and bread mixes. I pointed out that it was still stupid, just how much of the US is above 3,500 feet, not enough to warrant different instructions surely, and why not have instructions for other altitudes. So she told me to go find out how much of the US is above that magic number. A few days went by and I suddenly remembered to try looking it up. It's not like I could just type it into Google and bazam, the answer would appear.
After much hunting I was surprised to find that quite a lot of online recipes had extra instructions for those living over 3,500 feet. I did find some general instructions on cooking at altitude, it seems that aside from the temperature water boils at changing, some other ingredients act differently too (notably flour it seems).
Now, according to this article, 36% of North America is covered in mountains. Obviously that covers more than just the US, and while there seems to be different definitions of 'mountain' the general definition appears to be anything over 2,000 feet. I did find a topographic map here. It's far from ideal though. Taking a rough guess, I'd say 20-25% of the continental US is above 3,500 feet! No wonder they include the instructions.