For many drivers who commute long distances, the prospect of owning a self-driving car — where a driver takes his hands off the wheel and feet off the gas — has been an elusive dream.

But on Thursday, Elon Musk, chief executive of Tesla, took a big step in that direction when he announced that the maker of high-end electric cars would introduce autonomous technology by this summer. The technology would allow drivers to have their cars take control on what he called “major roads” like highways.

Mr. Musk said that a software update — not a repair performed by a mechanic — would give Tesla’s Model S sedans the ability to start driving themselves, at least part of the time, in a hands-free mode that the company refers to as autopilot.

But some industry experts said serious questions remain about whether such autonomous driving is actually legal and are skeptical that Model S owners who try to use autopilot would not run afoul of current regulations.