The driver thought he might have fallen asleep.
The result was a Tuesday morning truck accident that closed one eastbound lane of the U.S. 50-63 Expressway for nearly four hours.
Jefferson City police said Joseph Troxel, 43, Homedale, Idaho, lost control of his eastbound 2010 Freightliner tractor-trailer unit about 10:15 a.m.
Police reported the truck ran off the right side of the 2100 block of the highway and into the guardrail, then continued east against the guardrail until it ended.
The truck then traveled down a grass embankment and into a wooded area south of the highway.
With help from the state transportation department, one lane of the highway was closed and marked, while a local towing company removed the truck from the wood line.
Police said Troxel wasn't using a seat belt, but wasn't injured.
No other vehicles were involved.