Driverless cars are a more common sight on Phoenix-area streets than in most other parts of the country for a number of reasons. One is that the conditions are friendly: not much rain, ice or snow, few potholes and a lot of wide suburban streets.

Arizona also has one of the nation's most permissive regulatory environments for self-driving technology, Ars Technica reports. Gov. Doug Ducey signed an executive order in 2015 directing state regulators to promote the technology.

Among other things, the order said, "... agencies of the State of Arizona with pertinent regulatory [jurisdiction] shall undertake any necessary steps to support the testing and operation of self-driving vehicles on public roads within Arizona."

Waymo, Alphabet's self-driving car company, is running trials of a self-driving taxi service in Chandler. Uber's self-driving cars are common on the region's roads, and other companies have cars out and about here as well.

Waymo's software is able to drive reasonably well on wide, straight roads in good weather, so the company decided to focus on a driverless taxi service in the Phoenix metro area.

Not every driverless vehicle company is on board with testing in a place like Phoenix. In a blog post, Cruise CEO Kyle Vogt wrote: "By testing in densely populated areas we expose our software to unusual situations at a much higher rate, which means we can improve our software at a much higher rate."

Cruise is owned by GM, and it has elected to test its cars in a place with a few more tricky situations: San Francisco.

"Our vehicles encounter challenging (and often absurd) situations up to 46 times more often than other places self-driving cars are tested," Vogt wrote.