It could be a huge problem because of competing goals. The spammers won't care about traffic flow as their only goal will be to have cars on the road and visible, whilst people in cars will want to get places in a reasonable time.
Here's some real-world examples:
It's called the gas tax, and it does a good enough job charging people for their negative externalities that everyone seems to hate it.
Fuel taxes almost never tax heavy usage vehicles enough, compared to the amount of damage they do to the road.
A fuel tax also doesn't solve the problem grandparent post is trying to solve, excess usage of 'high value' roads.
If road usage, and road wear and tear, was evenly distributed then a fuel tax would work, but the reality is that is simply a convenient and fairly effective proxy solution. As technology improves a solution similar to grandparent, some form of usage tolling, will become feasible and provide a more effective outcome.
it probably wouldn't be that bad if government could track autonomous semis making deliveries to large shopping centers, but I wouldn't want it to have fine-grained data on who goes where and who delivers to their house.