Both lack of regulation as well as weather, road condition, and road layout. Most of the Phoenix metro area is a grid, fairly modern as streets are constantly redone as we've grown so new-ish signs that are largely unobstructed by trees, etcetc.
As a motorcyclist with a 45 minute commute, I'm on the cautious side when it comes to self-driving tech on the roads, but I have to say that anecdotally it's been fine so far. I'd be willing to bet I see ~10 Waymo vehicles a day. I've had plenty of issues with normal cars/drivers, have never had nor seen an issue with Waymo.
As a motorcyclist with a 45 minute commute, I'm on the cautious side when it comes to self-driving tech on the roads, but I have to say that anecdotally it's been fine so far. I'd be willing to bet I see ~10 Waymo vehicles a day. I've had plenty of issues with normal cars/drivers, have never had nor seen an issue with Waymo.