This freeway in Los Angeles is the biggest test of Tesla's autopilot feature

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The 405 freeway.

However, CEO Elon Musk tweeted that one major hurdle remains, specifically related to the "autosteer" function, which enables a Tesla vehicle to handle highway driving more or less autonomously.

"Final corner case," Musk wrote, "is dealing with low contrast lane markings (faded white on grey concrete) while driving into the sun at dusk."

Final corner case is dealing with low contrast lane markings (faded white on grey concrete) while driving into the sun at dusk

- Elon Musk (@elonmusk) July 31, 2015

A few weeks ago, on a call with reporters to introduce a suite of new performance features for the Model S sedan, Musk made a reference to this problem. As it turns out, when he drives from his home in Bel Air to SpaceX HQ south of Los Angeles (he's also CEO of SpaceX), his route takes him on the 405 Freeway (sometimes called the "San Diego Freeway").

This stretch of road is one of the most heavily traveled in the country, with wear and tear that means its has to be more or less constantly repaired, maintained, and periodically shut down for massive structural improvements.

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Los Angeles residents have a love-hate relationship with the 405. More hate than love, but as someone who used to drive the 405 north at dusk every day, I can confirm that one develops a sort of grim bond with the freeway.

It's interesting that it has now become the toughest test of Tesla's autopilot technology. Almost as interesting as the fact that Musk is the main test driver for his cars.

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Musk and the Model S.