while we're at it, let's make the congestion pricing proportional to some power of vehicle weight so that it also reflects the cost of road maintenance.
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.
Weight or axles, but fuel tax isn't the way to do this. I'll leave it as an exercise for the reader to come up with the sedan MPG necessary to beat a 40,000 lbs. semi that gets, say, 3 MPG. (And, BTW, semis haven't done as poorly as 3mpg for a long time.)
I'm saying we should tax vehicles in proportion to some integer power of vehicle weight, since the road damage is superlinear with respect to vehicle weight. a big truck that weighs ten times as your Cayman does much more than ten times the damage. since we're trying to fairly distribute the costs of transit, the operators of these vehicles should be paying the lion's share of maintenance costs.