The question is, does one drink affect a person’s ability to drive? I doubt it, but it probably depends on the individual and the drink.
People can enjoy a drink or two without getting buzzed. The purpose of marijuana is to get buzzed.
According to tests that check reaction times and judgment, yes, one drink affects the ability to drive. You can't drink without getting buzzed and that is the purpose of drinking. In fact it seems to affect people's judgment so much that even when they're sober they believe alcohol has no such effects.