I always hated that "cup" measure in the US... Anywho, I don't think the volume of coffee one drinks in a day is relevant without stating how concentrated it is

I always hated that "cup" measure in the US... Anywho, I don't think the volume of coffee one drinks in a day is relevant without stating how concentrated it is. I'm assuming 6 oz of americano coffee doesn't have quite the same effect as a 6 oz cup of esspresso (oh Lord.) I know that, technically, a glass of beer should contain the same amount of alcohol as a shot of vodka or a glass of wine in a bar. Of course in practice it's probably not exactly the case, but when someone tells me "drinking 2 glasses before driving isn't such a good idea," I'm supposed to be able to tell how much 2 glasses is regadless of the alcohol consumed. Isn't it the same in the article ? Shouldn't a cup of esspresso contain the same amount of coffee as a (medium ?) americano ? If not, how do these 2 coffee dilutions compare ?