Google's self-driving cars to face their toughest test yet

"Distinct desert conditions" in Arizona will push vehicles to extremes.

One of Google's self-driving cars in Mountain View, California.

Google is taking its self-driving cars to a fourth city in an expansion of its ongoing development. The latest location for testing is Phoenix, Arizona, which is significantly more arid than any city previously used as a proving ground.

Speaking to Reuters, Jennifer Haroon, Google's head of business operations for the self-driving car project, noted that the desert conditions will further the company's understanding of "how our sensors and cars handle extreme temperatures and dust in the air." Modified Lexus SUVs have already begun to map the Phoenix area, checking for street layouts, lane markers, traffic signals and curb heights.

The move to Phoenix is the second new area for Google this year. In February, it began testing in Kirkland, Washington, which is significantly wetter than previous locations. Those previous locations were in California and Texas, though, so that's not saying much -- Kirkland is still dryer than the US average for inches of rainfall per year.

It's suggested that, in addition to providing a new set of environmental challenges for self-driving cars, Arizona is potentially more open to the testing than California. Google's home state recently proposed new rules that would require a wheel, pedals and a driver to be in self-driving cars -- all things that are not present in Google's current prototype.