A problem with means based charges is that it gives no incentive to people covered by the system to alter their behavior. I think a better way would be to have

A problem with means based charges is that it gives no incentive to people covered by the system to alter their behavior. I think a better way would be to have the charges apply to everyone and add a system that directs (some of) the collected money to people less well off. That way, if they need to, they can still afford to drive in the city, but if they can avoid it, it benefits them.