Pay CA road tax on miles driven outside of CA
I participated in the pilot and tried two different approaches: a) report miles monthly and pay per mile, b) buy miles in advance and report less often. There was a form to report miles driven outside of CA, but I think the minimum was 1,000 miles so it didn't come into play when I took a trip to AZ.
It's a bit of a tricky problem, particularly the privacy aspect.
With regard to weight, I've been told by two different director's of public works that the differences between two cars is negligible, trucks and buses are the bigger problem. Which is kind of ironic since fuel taxes are used to subsidize mass transit, which includes buses which in turn do more road damage.
WRT privacy, the reality is that our cars are constantly tracked by Tesla and my guess is that the data is just a subpoena away from being in the hands of the government. My BMW i3 is also tracked, but only when it starts/stops, not in motion.
Assume all EVs are going to be tracked in a similar manner as a "feature", the car companies could actually report where and how much we drove on an monthly basis, setting us up for paying taxes in multiple states if we drive in multiple states. Or they could turn the raw data over to the government and let them do the calculations. The only benefit would be that it's after the fact, the government wouldn't be tracking you in real time.
Early in my career I had to write programs to extract this kind of data for aircraft to pay property taxes on when our airplanes were in a given state, both while in the air and on the ground. So this is not at all unprecedented.
And the reality is that a Federal tax is inevitable as a replacement for the Federal gas taxes which both repair interstate highways and subsidize mass transit.
The bottom line for me is that a tax based on miles and weight (probably non-linear and as it really applies to road wear) seems entirely reasonable, but the actual measurement and collection method is going to be contentious.