Because she was driving without an Illinois license, Cavallari was escorted to the Evanston police station to post $150 bond.
The wife of Chicago Bears quarterback Jay Cutler was driving with a California license. But Illinois law requires those living in the state for more than 90 days to get an Illinois license.
The 26-year-old former star of MTV's "Laguna Beach" and "The Hills" was stopped Friday while crossing into Evanston from Chicago.
Police would not say how fast she was going.
Afterward, Cavallari tweeted, "Who knew having a California drivers license in Illinois was a CRIME to be escorted to the police station to post bail?!!"