Apple Inc. took its biggest step toward acknowledging it is working on autonomous vehicles, writing a letter to U.S. regulators offering feedback on proposed guidelines for the emerging technology.

The letter from Steve Kenner, Apple’s director of product integrity, to the head of the National Highway Traffic Safety Administration offered no details on the project, but said Apple “is investing heavily in the study of machine learning and automation, and is excited about the potential of automated systems in many areas, including...