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 transportation.”

Cupertino, Calif.-based Apple has been working on its car effort, code-named Project Titan, for several years, but has never publicly acknowledged it. The secrecy has made the exact status of Project Titan hard to pinpoint. The Wall Street Journal reported in September that Apple had restructured some elements of the project, which has had several hundred employees working on it, after placing it under veteran Apple executive Bob Mansfield in July.

Apple has looked at doing an entire car, but more recently its focus has narrowed to developing an autonomous-driving system and software. Focusing on software is more in line with the company’s historic expertise and fits its efforts to improve machine-learning capabilities that allow computers to adjust their behavior without being explicitly programmed. 

Apple’s five-page letter, dated Nov. 22, appears on the NHTSA website. It isn’t clear when it was posted.

“We’ve provided comments to NHTSA because Apple is investing heavily in machine learning and autonomous systems,” Apple spokesman Tom Neumayr said Friday. “There are many potential applications for these technologies, including the future of transportation, so we want to work with NHTSA to help define the best practices for the industry.”

The letter comes as the Obama administration rushes to put guidelines in place to help guide self-driving development. Alphabet Inc. ’s self-driving project had already collected more than 2 million miles of public-roadway testing and General Motors Co. this year competed a $1 billion deal to acquire Cruise Automation to jump start its autonomous vehicle program.

In the letter to NHTSA, Apple called on the regulator to work quickly to adopt new safety rules and look at ways to be more flexible with regulations. Mr. Kenner said by “improving regulatory flexibility,” the agency would foster more innovation and encourage “the development of life-saving technology.” 

Not surprisingly, the letter showed Apple’s autonomous-vehicle ambitions go beyond the U.S. Mr. Kenner encouraged the agency to work with international groups such as the United Nations Economic Commission for Europe and others to develop a “harmonized approach to automated vehicles.”

Write to Tim Higgins at and Tripp Mickle at