Tesla CEO Elon Musk has said that he expects Tesla vehicles to have "Full Self-Driving" capability by the end of 2019. 

In a media call Thursday night to announce the base, $35,000 Model 3, Musk for the first time gave a concrete explanation of what he means by that.

He said that by the end of the year, Tesla's long-promised Full Self-Driving system will be "feature complete," but that it will still require drivers to remain engaged and pay attention, just as the company says is required with Autopilot.

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"I think we've been very clear on that when people buy the car, when they pick up the car, on what that means, that they still have to pay attention," Musk said in answer to a question about using the term.

Although the Full Self-Driving system will be "feature complete," Musk said he would not expect the system to be reliable until its cars have covered 1 billion or even 10 billion miles using the system, which could take years. Similar unreliability is what makes the company insist that drivers have to continue paying attention when using Autopilot and has led to a series of highly publicized crashes when they don't.

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Even then, it's likely that drivers will be required to keep paying attention, because federal highway safety regulators at NHTSA will have to be convinced of the system's reliability, Musk allowed, before it will be legal for drivers to leave the driving to the car entirely.

Currently, Tesla's Autopilot, which consists of adaptive cruise control and active lane control, as well as what Tesla refers to as Navigate on Autopilot, which will follow turn-by-turn directions from the navigation system on limited-access highways if a destination is selected. (Last year, Musk referred to this as "on-ramp to off-ramp capability.")

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The remaining challenges to implementing Full Self-Driving mode, Musk said, are the ability to recognize traffic lights and stop signs on city roads and to be able to make turns effectively on city streets. He said he has been testing a Model 3 with those features and says it does a "pretty good job" now.

Tesla's biggest rival in developing self-driving cars, Google's Waymo self-driving division, announced this week that its cars can recognize and respond to traffic cops and hand signals. It has been working on recognizing stop signs, stop lights, and making urban turns since before 2014.

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Tesla offers Autopilot on all of its new cars as a $3,000 option, and the company has gone back to offering buyers the option of pre-paying another $5,000 for the Full Self-Driving system when it becomes available. Autopilot is a prerequisite for Full Self-Driving. Even the new base Model 3 will be equipped with the hardware to run both systems, so they can be added later with nothing more than an over-the-air-software update and an extra payment—Musk called it "future proof." Adding Full Self Driving to an existing car later will now cost $7,000, plus $4,000 for Autopilot for cars that don't have it activated.

Last October, the company removed the pre-paid option for Full Self Driving Mode, but restored it on Thursday.