This section deals with the use of alcohol and controlled substances by drivers of commercial vehicles; and the testing requirements set forth by the FMCSA. http://www.fmcsa.dot.gov/rulesregs/fmcsrhome.htm

You must not report for duty or attempt to operate a vehicle if you are under the influence of alcohol.  Your blood alcohol concentration may not be 0.04 or higher.  Your employer may not knowingly allow you to operate a vehicle under the above conditions.

You may not perform driving duties within 4 hours of consuming alcohol, nor may your employer allow you to do so.

You may not consume alcohol for 8 hours after an accident, or until a post-accident alcohol or controlled substance test has been administered.

No driver shall refuse to submit to a required alcohol or controlled substance test.  If he does refuse, his/her employer shall not allow him/her to continue to drive or perform any safety-sensitive functions.

The same rules and restrictions apply to the use of any controlled substance other than those prescribed by a physician.  If a driver is using a prescribed medication, they must not, in any way, interfere with the safe performance of his/her duties.

An employer shall require pre-employment testing for alcohol and controlled substance use.

Post-accident testing will be done as soon as possible following an occurrence involving a commercial vehicle.   These tests apply to each driver who was performing safety-sensitive functions (driving), if the accident involved fatalities, bodily injury, issuance of moving violation citations, or any damage to the vehicles involved. 

*In other words, you're in deep kimchee if you are involved in an accident and have even a hint of alcohol or drugs in your body, around your body or within a two-block radius.  DON'T DO IT!!  All you have to do is listen to the news to know that the driver of an eighteen-wheeler is going to be guilty until proven innocent any time there is an accident.  So CYA, drivers, save your recreational stuff for the appropriate time and place.

U.S. Department of Transportation
Federal Highway Administration