Actually, Rolla Harger invented the Drunkometer in the 1930s, which tested alcohol levels by having someone blow into a balloon, mixing that exhaled air with a chemical solution which would change color, and using a mathematical equation. It wasn't perfect, but it was the first test police had to detect alcohol on someone's breath when suspected of drunk driving. Robert Borkenstein took the Drunkometer, improved on it, and in 1954 created the breathalyzer.
Keep in mind as well that there weren't all that many cars up until the 1920s. Prohibition was in effect from 1920-1933, during which time alcohol consumption declined. By the time Prohibition ended, we were in the Great Depression, followed by WWII, both of which of course had an impact on car sales, the availability of gas to even drive, as well as alcohol sales. A huge number of American families didn't own cars until the 1950s, by which point the Drunkometer and then the Breathalyzer were in use.
Also, the sheer number of vehicles on the roads now vs then makes it far more likely for an impaired driver now to crash into another vehicle and hurt or kill someone else. While I don't dispute that marijuana has medicinal benefits for some people, it still causes slower reaction times as well as other kinds of impairment, which can be dangerous in situations that require a clear head, like driving, a police officer handling an armed suspect, performing surgery, flying a passenger plane, etc.
Frankly, I don't care what people choose to do in the privacy of their own homes, but there needs to be a way to accurately measure impairment for things like driving and certain jobs so that others aren't put at risk.