More on road damage and other costs of transport.

January 1, 2016

I did a round of noodling in a spreadsheet to try to get a feel for how different ways of carrying people or cargo damage roads. The formula for damage is the sum of the cubes of the wheel loads; crudely, the gross weight of a vehicle cubed, divided by the square of the number of wheels. This can lead to some pretty non-intuitive results — city buses in particular are non-intuitively bad for roads .  Overall the most surprising thing is that even when results are calculated either per-passenger or per-pound, you end up needing a log scale to display them meaningfully; heavy vehicles are that bad for roads.

I’m not quite sure what to do with this information; there are other things to optimize for besides road damage.  For example, it would not surprise me in the least if a semi-trailer were more fuel-efficient per-pound than a human on a bike; bikes are efficient, but we do have to eat to run them and food is quite often energetically costly.  Similarly, though bicycles are far safer for pedestrians than cars and trucks, per pound of cargo they will lose most of their advantage against delivery vehicles (the comparison is quite tricky; a cargo bike can easily carry 100 lbs in addition to its rider, and 250 is not out of the question, and such large bicycles are even less likely than “normal” bicycles to travel at pedestrian endangering speeds.  Contra that, a human pedaling a loaded cargo bike has an incentive to not stop because it takes quite a lot more energy to bring that larger weight back up to speed).

Note that a single-occupancy vehicle still fails badly on efficiency and safety metrics, but trucks of various sizes need not.


2 Responses to “More on road damage and other costs of transport.”

  1. psteckler Says:

    Do tire widths, diameters, and pressures figure here? I’d suppose that damage is inversely proportional (roughly) to each of these, since they increase the size of the contact patch where pressure is applied to the road surface. A bus is heavy, but its weight is distributed over fairly large surface area.

    • dr2chase Says:

      They all figure in a little bit, but load-per-wheel dominates. The wear effects occur over a pretty large scale when the entire roadbed flexes (and eventually flows and/or cracks) — spreading out the rear axles on an 18-wheeler helps a little bit, for example. And do note, I’m plotting this on a log scale; the differences in damage are large. (I got this info from “Road Work” by the Brookings Institute and by reading what I could find on-line to check their assumptions/claims.)

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