Apple's testing vehicles will carry employees between its various Silicon Valley offices. The new effort is called PAIL, short for Palo Alto to Infinite Loop, the address of the company's main office in Cupertino, Calif., and a few miles down the road from Palo Alto, Calif.
Apple's in-house shuttle service, which isn't operational yet, follows Waymo, Uber and a number of car companies that have been testing driverless cars on city streets around the world.
Apple has a history of tinkering with a technology until its engineers figure out what to do with it. The company worked on touch screens for years, for example, before that technology became an essential part of the iPhone.
But the initial scale of Apple's driverless ambitions went beyond tinkering or building underlying technology. The Titan project started in 2014, and it was staffed by many Apple veterans. The company also hired engineers with expertise in building cars, and not just the software that would run an autonomous vehicle.
It was a do-it-all approach typical of Apple, which prefers to control every aspect of a product, from the software that runs it to the look and feel of the hardware.
From the beginning, the employees dedicated to Project Titan looked at a wide range of details. That included motorized doors that opened and closed silently. They also studied ways to redesign a car interior without a steering wheel or gas pedals, and they worked on adding virtual or augmented reality into interior displays.
The team also worked on a new light and ranging detection sensor, also known as lidar. Lidar sensors normally protrude from the top of a car like a spinning cone and are essential in driverless cars. Apple, as always focused on clean designs, wanted to do away with the awkward cone.
Apple even looked into reinventing the wheel. A team within Titan investigated the possibility of using spherical wheels — round like a globe — instead of the traditional, round ones, because spherical wheels could allow the car better lateral movement.
But the car project ran into trouble, said the five people familiar with it, dogged by its size and by the lack of a clearly defined vision of what Apple wanted in a vehicle. Team members complained of shifting priorities and arbitrary or unrealistic deadlines.
There was disagreement about whether Apple should develop a fully autonomous vehicle or a semiautonomous car that could drive itself for stretches but allow the driver to retake control.
Steve Zadesky, an Apple executive who was initially in charge of Titan, wanted to pursue the semiautonomous option. But people within the industrial design team including Jonathan Ive, Apple's chief designer, believed that a fully driverless car would allow the company to reimagine the automobile experience, according to the five people.
A similar debate raged inside Google's self-driving car effort for years. There, the fully autonomous vehicle won out, mainly because researchers worried drivers couldn't be trusted to retake control in an emergency.
Even though Apple had not ironed out many of the basics, like how the autonomous systems would work, a team had already started working on an operating system software called CarOS. There was fierce debate about whether it should be programmed using Swift, Apple's own programming language, or the industry standard, C++.
Mr. Zadesky, who worked on the iPod and iPhone, eventually left Titan and took a leave of absence from the company for personal reasons in 2016. He is still at Apple, although he is no longer involved in the project. Mr. Zadesky could not be reached for comment.
Last year, Apple started to rein in the project. The company tapped Bob Mansfield, a longtime executive who over the years had led hardware engineering for some of Apple's most successful products, to oversee Titan.
Mr. Mansfield shelved plans to build a car and focused the project on the underlying self-driving technology. He also laid off some hardware staff, though the exact number of employees dedicated to working on car technology was unclear.
More recently, the team has grown again, adding personnel with expertise in autonomous systems, rather than car production.
Apple's headlong foray into autonomous vehicles underscores one of the biggest challenges facing the company: finding the next breakthrough product. As Apple celebrates the iPhone's 10th anniversary, the company remains heavily dependent on smartphone sales for growth. It has introduced new products like the Apple Watch and expanded revenue from services, but the iPhone still accounts for more than half of its sales.
In April, the California Department of Motor Vehicles granted Apple a test permit to allow the company to test autonomous driving technology in three 2015 Lexus RX 450h sport utility vehicles. There will be a safety driver monitoring the car during testing.
While many companies are pursuing driverless technology and see it as a game changer for car ownership and transportation, no one has figured out how to cash in yet.
With expectations reset and the team more focused, people on the Titan project said morale has improved under Mr. Mansfield. Still, one of the biggest challenges is holding onto talented engineers because self-driving technology is one of the hottest things in Silicon Valley, and Apple is hardly the only company working on it.