It was just six inches.

That was what made the difference at 4:40 a.m. yesterday as Gilberto Cantu, a truck driver from Texas, approached the New Jersey entrance of the Lincoln Tunnel in his big rig, loaded with bathtubs, toilets and plumbing fixtures. The truck was 13 feet 6 inches high. The tunnel has a height limit of 13 feet. Six inches can make a big difference.

Mr. Cantu drove the entire 1.5 miles of the tunnel from Weehawken, N.J., to Manhattan, tearing his way under the Hudson River in the tunnel’s center tube and peeling back the roof of his tractor-trailer as if it were a tin can. No one was injured, but an undetermined number of decorative tunnel ceiling tiles were ripped off.

It was unclear why Mr. Cantu did not heed warnings from flashing signs and a loudspeaker in New Jersey, said Steve Coleman, a spokesman for the Port Authority of New York and New Jersey, which operates the tunnel. “There were enough bells and whistles going off that this should not have happened,” Mr. Coleman said. “He told the officers he didn’t know where he was going.”

Mr. Coleman said that accidents of this kind were almost always averted. When a too-tall vehicle enters the toll plaza, an electronic sensor is tripped, several stoplights are activated and police officers at the plaza use a loudspeaker to order the driver to stop.