IMHO, it better to tax normalized_areanormalized_weightnormalized_congestion . For example, if vehicle is 30% spacer than average, has 50% more weight per space than average, and uses road at peak hour when road usage is 50% more than average, then it should pay 1.3 x 1.5 x 1.5 = ~3x tax. If owner want to reduce tax, then it should use smaller, lighter car, and use it at off-peak hours, e.g. 0.9 x 0.9 x 0.5 = 0.4.