He had tunnel vision.

The allegedly drunken man who drove a half-mile through a Muni tunnel, stifling train service during the height of the morning commute last month, might have been three times over the legal limit, said prosecutors and toxicologists.

Scott Mitchell, 40, of Sebastopol is scheduled to be arraigned Friday on charges that include drunken driving, speeding and failure to obey a traffic control device.

Prosecutors said Mitchell was wasted Jan. 19 when he raced into the Duboce Portal Tunnel at about 40 mph. His tunnel trip ended after his sport utility vehicle smashed into a buttress about 6 a.m., transit officials said.

Mitchell’s blood alcohol level tested at 0.20 more than three hours after the incident, Assistant District Attorney Omid Talai said.

That would mean it was likely 0.245 at the time he was driving, which is three times California’s legal limit of 0.08, said Dr. Damon Borg, a laboratory supervisor at American Forensic Toxicology Services.

But that doesn’t necessarily mean Mitchell didn’t know what he was doing that day.

“A lot of it comes down to tolerance,” Borg said.

maldax@sfexaminer.com