Joe Simpson's DUI -- Rolling with a .12 ... Allegedly

Joe Simpson was driving with a .12 blood alcohol level when he was popped last week for DUI -- law enforcement sources tell TMZ.

TMZ broke the story ... Simpson was busted August 4 in the San Fernando Valley. We're told he was stopped at a sobriety checkpoint on Ventura Blvd. Although we don't know where Simpson had allegedly been drinking, we're told the checkpoint was two miles from Casa Vega, a Mexican restaurant frequented by Jessica Simpson -- she's a regular. BTW, they make a mean margarita.

Joe's .12 is well above what is considered legally drunk in California -- .08.