Perhaps charging based on as the crow flies distance could work. Although you would need someway to handle multi destination trips.
This could get complicated but I would allow a base mileage, set about half of the annual average, for free. Setting it at half encourages more than one person per journey. Then a sliding scale which becomes prohibitively more expensive once you hit double annual average. I'm not sure about trading but I would allow some way for people to forgo their free allowance for alternative benefits (perhaps LBS credits).
There should be an option for passengers to use their allowance as it would also encourage lift sharing and possibly even shared ownership. Passenger allowance should also be used for taxi/phc.
Annual allowance wouldn't just be straight mileage in my rules either. Journeys under 2ish mile would often cost extra.
Road storage charges should also be implemented at the same time to discourage people from owning more cars than they need/have space for. I pondered something along the lines of a one car free per household rule but suspect that would have some pretty unfortunate unintended consequences.