While zero incidents is naturally what they should be aiming for, it’s more of a goal for continuous improvement, like it is for air travel.

As far as I know, proper self driving (not "autopilot") AVs are pretty close to zero incidents if you only count crashes where they are at fault.

When another car runs a red light and smashes into the side of an autonomous vehicle at 40mph... it wasn't the AV's fault. Those crashes should not be counted and as far as I know they currently are in most stats.

What liability can/should we place on companies that provide autonomous drivers that will ultimately lead to safer travel for everyone?

I'm fine with exactly the same liability as human drivers have. Unlike humans, who are motivated to drive dangerously for fun or get home when they're high on drugs or continue driving through the night without sleep to avoid paying for a hotel, autonomous vehicles have zero motivation to take risks.

In the absence of that motivation, the simple fact that insurance against accidents is expensive is more than enough to encourage these companies to continue to invest in making their cars safer. Because the safer the cars, the lower their insurance premiums will be.

Globally insurance against car accidents is approaching half a trillion dollars per year and increasing over time. With money like that on the line, why not spend a lazy hundred billion dollars or so on better safety? It won't actually cost anything - it will save money.