I find it ... we'll say, weird ... where they've chosen to trial these vehicles already.

Is there some reason other than "we were able to get permits there more easily[0]" that I'm missing? I wasn't in Phoenix long enough to speak intelligently about that city, but I've done Austin and San Francisco on a few occasions in the distant past.

Both are high-traffic-with-pedestrians, much of it high-density and frequently changing traffic patterns ... except that most of them are varying degrees of "hellish" a whole lot of the time. Meanwhile you (likely) have ongoing maintenance of (likely many) high-traffic roads, detours, squeeze-around-work-crews all competing with emergency services.

Considering GM's HQ, I'm surprised they didn't elect to work these through Metro Detroit. We have all of that craziness at lower (and pretty predictable) traffic levels mixed in with roads that barely qualify as paved and some which are perfectly paved except for the tire-sized 2ft chasm that nearly borders the other tire-sized 2ft chasm. Figure out how to get these things to perfect the "pot-hole video game", understand the "Michigan Left that's also an Intersection[1]" survive more than a few months of trips from Metro Airport to "anywhere in Northern Macomb County" without dings, punctures or busted shocks then move to something more challenging. And we're a bit more tolerant of weirdness around "car sh!t" around here, especially since many in my area have jobs directly supported by the local autos.

I did notice that none of those cities are well known for large amounts of snowfall ... still, I must be missing something because it seems like there'd be much less disruptive places to refine your product. In places like these, I'd be handing the emergency vehicles a computer with a few buttons -- one of those things gets in the way, it's "Disable It" "GTFO" "Force it to Pull Over" maybe with fallback of a Joystick/Computer device to shove it out of the way. It almost sounds like they are providing some way that an emergency vehicle can control these things "on request" the way that reads. At least with a "three button/fallback joystick", mixed with "a little more adequate sensing/driving" work would make them no worse than a driver with the radio too loud who isn't checking their rear-view and would give them more power over the equivalent "machine driven" car than they have over the "d!ckhead driven" car.

Curious if anyone knows why, specifically, those cities were the test-beds?

[0] And one is also our HQ, which might be the only one that I can understand an exception (a desire to be close to your implementation solving the unique problems of that region that you probably understand better than most).

[1] This is a case where "traffic is U-turning to make a left" but the end of the U-Turn meets up with a road where traffic is turning right onto the road. In this case, "the law" is that "left hand turners yield the right of way" just like at any other intersection, but the vast* majority of time, the left-hand turning traffic mistakes their "green light" for a "green arrow" and assumes the usual rules of "go when it's green". It's so bad that they put signs* at many of these intersections reminding left-hand turners to yield on green. Follow the law, likely get hit. Follow the flow, and you'll likely get stuck there when you have the right of way. On a bad day, punch it with a hand on the horn if they move. :o)