A BAC of 0.04 is measurably impaired though. Mostly in terms of higher brain functions and reaction times making it both less obvious and often within a persons 'normal' range.

Something to remember is people generally drink late in the day. So, the accident statistics mix drinking and tired data. Tired driving makes things far worse, but it's hard to test for as accidents and being pulled over both tend to wake people up. Pot on the other hand is more often a daytime drug which is going to mess with the statistics.