Drunk driving fatalities have fallen dramatically since the early '80s. They continued to fall since the mid-90s, but that drop is less significant because non-DUII auto fatalities have fallen the same amount, suggesting that improvements in auto safety (which protect the drunk and sober alike) account for most if not all of the drop in the last two decades.
Still, if you go back to the early '80s, the decline of drunk driving fatalities has been an extraordinary success story. Is it because of tougher laws? Social stigma? Demographics? Change is alcohol consumption patterns?
You might find an answer here.