Armstrong entered his plea by mail on Friday under a plea agreement with prosecutors, closing the case and avoiding a court appearance. Two other charges, failing to report an accident and speeding too fast for conditions, were dismissed under the agreement, which was first reported by the Aspen Daily News.
Court records show Armstrong paid USD 238.50 to cover court fees and a USD 150 traffic fine.
Armstrong's girlfriend, Anna Hansen, initially told police she had been driving home from an Aspen Art Museum party December 28 when she lost control of Armstrong's GMC Yukon on icy roads, hitting the cars.
Detectives later interviewed Hansen, who eventually told them Armstrong was driving but the couple had decided to let her take the blame. She also retracted her statement about Armstrong being unable to drive because he had been drinking. None of the charges against Armstrong involved driving under the influence.
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"We've had our family name smeared over every paper in the world in the last couple of years and honestly, I've got teenagers, I just wanted to protect my family," Hansen told police, the reports state.
Deputy District Attorney Andrea Bryan said it is fairly common for people to reach plea deals in such cases and to enter pleas by mail. Careless driving can be punished by between 10 and 90 days in jail and a fine from USD 150 to USD 300 but jail time in such cases is rare, she said.
Prosecutors reserved the right to seek restitution for the damaged cars but Bryan said she believes Armstrong has already done that voluntarily.