Roads in Chandler are very regular (nice, wide grid), and the weather is clear and sunny the vast majority of the days. The particular geography helps isolate the number of factors the car needs to consider besides "just" driving.

From a NYT article[0],

>"The company did not say whether it was testing the driverless cars in environments considered challenging for autonomous vehicles, like bridges or tunnels, or more difficult conditions, like driving at night or in rain and snow — usually not a big concern in the dry Phoenix climate."

[0] https://www.nytimes.com/2017/11/07/technology/waymo-autonomo...