Why am I being charged with both a DUI and driving with excessive blood alcohol content?

The traditional offense is "driving under the influence" of alcohol (DUI) or, in some states, "driving while intoxicated" (DWI). In recent years, however, a large majority of states have also enacted a second, so-called "per se" offense: driving with an excessive blood-alcohol concentration (BAC), which is set at 0.08% in all 50 states now. And in some cases, both offenses are charged. The defendant can even be convicted of both, but can be punished for only one.

Both laws were considered necessary because some people, notably heavy drinkers, can appear to be sober and pass field sobriety tests (FSTs) even at a high BAC level. Others on the other end of the scale may be below 0.08% BAC, but are intoxicated and their driving is adversely affected. The law wants all of them off the road.

If the case involves a refusal to submit to chemical testing, of course, only the traditional DWI/DUI offense will be charged, because officers normally are not allowed to offer opinions as to what the blood alcohol level was if no test was administered.