I was driving south in an 80-mph zone on [XXX] between Brookings and Sioux Falls. I had my cruise control set to 82 and had just completed a lane change after passing a semi truck. With no change in the speed limit, the car shifted the set MPH to 30mph and engaged the braking system sharply. I was able to override by pushing the gas, but the mph setting was still set. The experience was a more extreme event than other similar instances Ive had (e.g., car shifting from a 30mph to 15 because of a school zone but not school hours so sign not relevant) because of the speed, proximity to other cars, and the absence of a change in speed indicator. I believe this glitch is a major concern for safety and appreciate your time in reviewing my concerns. INFORMATION REDACTED PURSUANT TO THE FREEDOM OF INFORMATION ACT (FOIA), 5 U.S.C. 552(B)(6)