The dashboard of a Tesla Model S, equipped with "Autopilot," during a test drive on Wednesday. (Photographer: David Paul Morris/Bloomberg)

With one over-the-air update Wednesday night, Tesla Motors has brought a new breed of self-driving car to American roads.

Tens of thousands of Tesla's all-electric sedan, the Model S, bought in the U.S. over the last year have already started downloading or installing "Autopilot" mode, one of the first great breakthroughs for making the kind of driverless magic seen mostly in Google-car demos.

With "Autopilot," the Tesla S can steer, change lanes and drive at highway speeds with little to no help from the human behind the wheel. It can parallel park, using its banks of cameras and sensors, and slow to a stop if the driver happens to drift asleep. In the next update, it may even be able to rouse itself from its parking space and pick the driver up.

It's called "Autopilot" for a reason: Like airplane pilots with takeoffs and landings, the driver will still be expected to handle much of the subtle and strange ballet that is modern driving. The human will still have to keep her hands on the wheel every few seconds, as a safety measure, and to meet state laws that demand a hand on the wheel. No naps in the driver seat.

[Tesla Model S P90D: A normal person drives the ‘best car’ ever made]

But the features it brings — as a software update, no less! — mark an incredible turning point for driverless technology, which could dramatically reduce the 33,000 deaths on U.S. highways every year, advocates of the technology say.

"It will get more and more refined over time," Tesla chief executive Elon Musk said Wednesday at a press event. "Eventually, we want it to automatically have your car put itself to bed in your garage."

When the Model S now is, say, cruising on the highway, its cameras, sensors and radar are already gulping in data on lane markers, the speed and location of other cars, and other important information. Before the update, the car could automatically slow down when, say, it sensed the car ahead hit the brakes, and it would also vibrate the steering wheel when the car seemed to drift out of its lane.

Turning on "Autopilot" instructs the car to take an even greater level of control, steering into curves and maintaining its speed. To change lanes, the driver can hit her turn-signal bar, called a stalk, and the car will speed up a little and drift over. The driver still has to check her blind spots before hitting the signal; if the car senses interlopers, it will keep going straight.

[Elon Musk vents about California’s lane markings confusing Tesla’s autopilot]

Parking also gets an upgrade. The car will now be able to, as Tesla says, "scan for a parking space, alert you when one is available, and parallel park on command." The car's back-up camera and sensors say exactly how many inches the car is away from the curb.

The Model S will give back control to the driver if she grabs or turns the wheel. Alternatively, it will sound a series of increasingly annoying beeps if the driver doesn't seem to be paying attention to the wheel.

If the driver doesn't snap to attention? The car flashes a warning for the driver to grab hold of the steering wheel and eventually slow to a stop where it's driving, firing off the hazard lights. (Other carmakers have similar backup plans but differing ideas on how best to jolt drivers awake.)

Consider it something like a diet driverless car. It's meant for the relatively predictable expanse of highway driving, and won't work below 18 mph. It won't always be able to navigate darting pedestrians and the many other confusing obstructions on the road, which for now will remain the human's domain.

Of course, that also means, as Tesla said, any accident when the car is in or out of "Autopilot" remains the liability of the driver. That may change as the cars become smarter and more assertive, but for now, driver beware.

The Model S P90D costs $140,000. It is also being called by many "the best car ever made". From its 17-inch touchscreen menu, to its startling acceleration thanks to "ludicrous mode", get a behind-the-wheel look at Tesla's newest car. (Jhaan Elker/The Washington Post)