The New Zealand govt solves this by taxing diesel vehicles with a per km charge that is based on different weight bands.  These are call Road User Charges (RUCs) and have to be purchased in advance before the vehicle sets out on the road.  Heavier vehicles pay higher RUCs, thus the small engined diesel station-wagons, etc. don't get penalised too much.

These charges also help in two ways:

  1. Diesel powered engines that are not in vehicles (generators, pumps, etc.) do not have to pay the charges, thus reducing the cost of operating these machines.

  2. It is a nice fair way to charge bio-diesel users for the damage that their vehicles still do to the roads.

Rick, nice to see some TOD input from NZ!  Are there any Peak Oil Awareness groups here in NZ? I'm in Hawkes Bay.
Duncan-Glad to hear from you. There is some PO activity going on in Auckland, though I'm not closely involved in that. My email is listed under my user information if you want to drop a line. A NZ mailing list would be a useful start.