He was arrested for suspicion of driving under the influence:

Mel Gibson was stopped early this morning for speeding on the Pacific Coast Highway in Malibu, Los Angeles County Sheriff’s spokesman Steve Whitmore said.
When Gibson’s interactions with the Sheriff’s deputy raised suspicions that he was driving under the influence of alcohol, he was arrested, Whitmore said.
“We arrested Mr. Gibson on suspicion of driving under the influence, and we took him into the station without incident and we released him and gave him a citation,” he said. “We will now prepare the case.”
[…]
In a 2004 interview for ABC News, he told Diane Sawyer: “I would get addicted to anything. Anything at all, OK?” When Sawyer asked him more specifically if it was alcohol, he said “Yeah, yeah mostly. It was, yeah.”
Saying addiction was his “flaw,” he acknowledged to Sawyer that he’d driven under the influence of alcohol in the past. While working on the film “Mrs. Soffel” in Toronto in 1984, he reportedly ran a red light and drove into another car after consuming enough alcohol to put him over the legal limit.
“I used to drive inebriated. I mean, this is, this is the height of careless stupidity. And when you think about that kind of insanity and that you, I look back at that now and I go, what was I thinking?” Gibson told Sawyer.

OOPS!
Read the rest here.