I’m European and I only went to the US this year (first and last time), and it’s true I was weirded out by the amount of billboards. But still, the same billboard usually stays within your line of vision for a little while.

And if they really count it in such a crazy way by knowing what’s in everyones field of vision all the time, that’s a crazy approximation from their side.

I think it’s likely they instead measure how mamy ads come through the web to your device, which is different from how many you actually see. Like an ad at the bottom of a website you don’t scroll through and stuff like that.