Got to this question late. We had this happen [1] during our first unmanned run completely unintentionally. We were testing our 1st gen emergency and diagnostics systems, and the truck ground to a halt a few minutes into the test. Our teleoperation center lost complete power (freak outage, no weather or anything) so the teleop station disconnected, even though our UPS was still working along. We've made a few changes to support this case a bit more, but the broad strokes worked in that case. Long story short, the truck will stop itself. There are many different ways we try to do that more safely depending on road conditions, and those are outlined in the VSSA linked elsewhere.

Link below is a dramatized account of the events, couldn't find anything less editorialized, sorry!

[1] https://www.caranddriver.com/news/a19177797/self-driving-tru...