How do map makers properly scale a map?

ok so this question popped into my head the other day when I was driving. I was on a highway and I could see about a quarter to a half mile away, another road going parallel with the one I was on. Between the two roads was rugged changing terrain, farms etc. I was wondering how do map makers figure out the exact distances between the two roads? They surely dont measure the distance between the roads by walking between the two at every point.