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Alphabet is expanding its testing of self-driving cars to the Phoenix, Arizona metro area, the company said on Thursday, making it the fourth U.S. city to serve as a proving ground for the autonomous vehicles.

The company's Google unit has conducted driverless vehicle testing for six years in Mountain View, California, where it is based, and expanded testing to Austin, Texas last summer. In February, Kirkland, Washington, which is home to significant wet weather, was added as a testing site.

Test drivers use a Lexus SUV, built as a self-driving car, to map the area prior to a journey without a driver in control, in Phoenix, Arizona April 5, 2016 in a photo provided by Google. REUTERS/Google/Handout via Reuters REUTERS

Major automakers, and technology companies led by Google, are racing to develop and sell vehicles that can drive themselves, but they have complained that safety rules are impeding testing and ultimate deployment of such vehicles.

Most of Google's 1.5 million miles of autonomous vehicle testing has taken place in California. But it has publicly sparred with the state since December when California proposed rules requiring a steering wheel, brake pedals and a licensed driver in all robot test vehicles on the road.

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"Arizona is known as a place where research and development is welcome, innovation can thrive, and companies can set up roots," said Jennifer Haroon, head of business operations for the Google Self-Driving Car project. "The Phoenix area has distinct desert conditions, which will help us better understand how our sensors and cars handle extreme temperatures and dust in the air."

Google said its test drivers recently began driving four Lexus RX450h SUVs around the Phoenix area to create a detailed map of streets, lane markers, traffic signals and curb heights.

A Google self-driving car struck a municipal bus in Mountain View in a minor crash on Feb. 14. Google has said it bears "some responsibility" for the incident, the first crash that appears to be fault of the self-driving vehicle. The company said it made changes to its software after the crash to avoid future incidents.

The National Highway Traffic Safety Administration said in January it is working on new guidance on self-driving vehicles to states and automakers that they hope to release by July. NHTSA will hold the first of two public hearings on the planned self-driving guidance on Friday in Washington.

In February, NHTSA said the artificial intelligence system piloting a self-driving Google car could be considered the driver under federal law, a major step toward winning approval for autonomous vehicles on the roads.

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