In a retrenchment of one of its most ambitious initiatives, Apple has shuttered parts of its self-driving car project and laid off dozens of employees, according to three people briefed on the move who were not allowed to speak about it publicly.

The job cuts are the latest sign of trouble with Apple’s car initiative. The company has added resources to the project — code-named Titan — over the last two years, but it has struggled to make progress. And in July, the company brought in Bob Mansfield, a highly regarded Apple veteran, to take over the effort.