Thinking out loud about relatively few black DUIs:
 
African-Americans are now largely urbanized and most live in or near
center cities. Because of their more central location, licensed
African-American drivers probably make (on average) fewer trips and
shorter trips. Fewer and shorter trips translate into less exposure to
police speed traps, sobriety checks, etc. By both custom and necessity
(i.e., suburban or urban fringe location), white motorists spend more time
in their cars, which means greater exposure for those who drink.
 
Another hypothesis: Those African-Americans who are most heavily
dependent on drugs and alcohol are also the poorest, and hence can't
afford cars.
 
David Courtwright
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