Just a couple of weeks after releasing new self-driving features for its Model S vehicles through a software upgrade, Tesla will place new constraints on its Autopilot system to further limit when it can be used.

Tesla CEO Elon Musk discussed the Autopilot issue during a question-and-answer session after the company reported its third-quarter financial results yesterday.

Tesla The Model S now has automatic steering, lane-keeping capability and an automatic lane-change feature.

Asked whether he'd seen videos of drivers taking their hands off the steering wheels of Model S sedans and allowing the Autopilot feature to take control, Musk said he thinks changes must be made for safety.

"There's been some fairly crazy videos on YouTube.... This is not good. And we will be putting some additional constraints on when Autopilot can be activated to minimize the possibility of people doing crazy things with it," he said.

According to Musk, nearly 1 million cars have already installed the over-the-air software upgrade to the Model S Autopilot feature, which includes a beta version of Autosteer and Auto Lane Change.

Autosteer doesn't turn the Model S into a fully self-driving vehicle; it's more akin to an enhanced adaptive cruise control and lane-keeping system. But it does automatically maintain distances from other cars around the Model S.

Autosteer uses a variety of metrics -- including steering angle, steering rate and speed -- to determine the appropriate maneuvering operation.

The Auto Lane Change system is engaged when the driver uses the turn signal, moving the Model S into the adjacent lane after determining that it's safe to do so.

Creative Commons Lic. The Tesla Model S instrument panel and tablet-like infotainment center.

Model S drivers have taken to posting videos to YouTube showing how their vehicles can drive hands-free -- a practice Tesla discourages.

Tesla has been tracking data related to accidents involving Model S P85D vehicles using Autopilot and, while it's still early, Musk described the findings as "very positive."

He did, however, acknowledge that reports of Autopilot errors should come as no surprise, because it's a beta program that will need to "learn over time."

"The system is getting better with each passing week," he said. "I think it will start to feel quite refined within a few months."

Musk said Tesla has received no reports of Autopilot causing accidents and added that the technology has, in fact, prevented accidents.

"This is still early, but it's a good indication. So it appears to be quite beneficial from a safety standpoint, and I believe some of our customers have posted videos to this fact," Musk said.

Unlike traditional car manufacturers, which tend to bundle upgrades for introduction in a new model year, Tesla is constantly engineering upgrades to its existing fleet and to vehicles being manufactured.

The company's philosophy is to continually make improvements, "so every week there are approximately 20 engineering changes made to the car," Musk explained. "So model year doesn't mean as much. There are cases where that step change may be a little higher than normal as, for example, with having the Autopilot camera, radar, and ultrasonics. But we try to actually keep those step changes as small as possible."

While Autopilot's features allow for driver assist functions today, Musk said he expects Tesla to produce a fully autonomous vehicle within about three years.

In 15 to 20 years, it will be "quite unusual" to even see a car rolling off an assembly line that's not fully autonomous, he said.

"And for Tesla, it will be a lot sooner than that," Musk said, adding that cars without full autonomy will be seen as having a negative value. "It will be like owning a horse -- you're really owning it for sentimental reasons."

Google, which has been building and testing its own self-driving vehicles, released a report this week indicating it would be safer to ensure that drivers cannot take control of self-driving cars.

In its report, Google stated that it has spent time considering features that would allow its autonomous cars to "hand off" control to the driver. But the company said such functionality would be problematic because driver reaction time to hazards was shown to be markedly slow.

Google referenced a study by the Virginia Tech Transportation Institute that found drivers required somewhere between 5 and 8 seconds to safely regain control of a semi-autonomous system.

In August, the National Highway Traffic Safety Administration published a study that revealed that some participants took up to 17 seconds to respond to autonomous vehicle alerts and retake control of the vehicle.

"There's also the challenge of context -- once you take back control, do you have enough understanding of what's going on around the vehicle to make the right decision?" Google stated. "In the end, our tests led us to our decision to develop vehicles that could drive themselves from point A to B, with no human intervention."

"Everyone thinks getting a car to drive itself is hard. It is. But we suspect it's probably just as hard to get people to pay attention when they're bored or tired and the technology is saying, 'Don't worry, I've got this... for now.'"