U.S. 40 is one of two main detour routes around a large rockslide onto Interstate 70 in Glenwood Canyon.
The Colorado State Patrol said the woman was the only person injured in the incident that happened just after 7 a.m. on the highway between Steamboat Springs and Hayden in northwest Colorado.
The coroner identified her as Karen Lynn Evanoff, 55, of Craig, a passenger in the 2004 Buick. The unidentified driver was unhurt.
The vehicle was traveling on U.S. 40 near Milner, a CDOT spokeswoman said.
"This is a very, very rare location for a rockslide," CDOT spokeswoman Mindy Crane said.
Crane said she hesitated to even call the incident a rockslide.
"It was a rock. One rock," Crane said.
Crane said a check of CDOT records back to 1998 did not indicate any other rock incidents in that area.
A CDOT maintenance crew was on scene, Crane said, and a geologist was headed to the area from Denver to survey the situation.
U.S. 40 remained open following the incident.
(Copyright 2010 by The Associated Press. All Rights Reserved.)