The thing that I never see mentioned in this debate about who should pay for roads, etc. is that road wear is proportional to the FOURTH POWER of the vehicle weight( http://en.wikipedia.org/wiki/Road ), i.e., all the damage is being done by trucks.  I suppose at some point they will charge bicyclists too for the road wear as well!
Of course, it could be argued that all of us, including cyclists, are benefitting from trucks' use of the roads.  That's how most of us get our groceries, not to mention how your local bike shop gets bikes and bike parts.
This is true, but of course the road wear is a very good reason to move long distance inland cargo traffic from road to rails where available. I don't see much of this being done, rather the opposite, at least here in the Northern Europe.
(Although here in Finland much of the road wear comes from studded winter tires, which are allowed for six months a year, and only allowed for light vehicles.)
Studded tires and chains aren't allowed in this area.  Which of course means we have fleets and fleets of snowplows, and tons and tons of salt put on the roads each winter.  Which probably cause as much damage as the snow tires would.  

Our whole distribution system has moved away from the railroads.  The Wal-Mart model is built on cheap energy.  Their cavernous stores are out in the boonies, their distribution centers are far from the stores, the ports are far from the distribution centers.  Trucks are used as "rolling warehouses."  It's going to get ugly if energy prices keep rising, or if there are actual shortages.    

When doing pavement design, we do not even count cars.  It is basically all trucks.  We use ESALs or Equivalant Single Axle Loads, to balance it out. When I took pavement design (many, many moons ago), it took 9,950 cars to equal the damage of one semi-truck.  Now Minnisota DOT uses 0.0007 for a multiplier (see table 2, http://www.lrrb.gen.mn.us/apg/esal.htm)

Of course, this only addresses damage to existing roads.  Capacity is another matter.  We multilane roads because of all the traffic, not because of the damage to the roads.

BTW, one of the most common complaints I hear is bicyclists do not pay for anything.  In 2004, Florida DOT said it cost $205,508 per mile of bike lanes.  This ain't cheap.
http://www.dot.state.fl.us/planning/policy/pdfs/TransCost.pdf

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.