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I am skeptical of all analyses purporting to quantify road deterioration inflicted by vehicle types.  This is because I cannot imagine who would develop such an analysis without having an agenda that benefited from the outcome of the analysis.  If a private business owned the roads, I could see them having an interest in knowing the truth about the wear and tear caused by their vehicles.  Knowing the facts would be part of their business.  But otherwise, there would be inherent motivation to exaggerate the results, downplay them, or shift the blame from one vehicle user to another.  <o:p></o:p> The problem with any analysis of road damage by vehicle type is so complex that anyone can convincingly challenge it and come up with a different result.  On one hand, engineering facts and numbers don’t lie, but on the other hand, they can be made into an effective smokescreen.  <o:p></o:p> Certainly roads wear out with use and weather.  And they wear out faster as vehicle weights increase.  Beyond that, I don’t trust any conclusions.  You can account for the strength of the asphalt, concrete, and rebar, and the axle weight of the truck.  But this only opens the door to the analysis.  You have to go further and consider the strength of the base and the subgrade.  You have to consider the effects of the frost lifting the subgrade.  As the frost forms, it migrates moisture toward the surface of the subgrade, and the moisture freezes near the surface.  Then when the frost melts in the spring, the subgrade’s weight-bearing capacity becomes diminished from the over-saturation of water.  <o:p></o:p> Asphalt and concrete road surfaces develop cracks.  Concrete paving has joints.  The joints and cracks act like rail joints on a railroad.  That is, the joints and cracks in paving are weaker than the rest of the slab.  So the axle weight of a vehicle places a higher loading on the road base at the cracks and joints, just like railroad wheels place a higher loading on the ties at rail joints.  The higher loading of the ties at rail joints beat the joints down so their support allows the rail to fall below the proper track surface when under load.  The same effect happens with highway pavement and concrete slabs.   <o:p></o:p> The more the load bearing ability is beat down at the joints of railroads or highways, the greater the dynamic load at these joints.  When a wheel enters a low or soft joint, it is pulled downward from its normal line of travel by gravity.  Then when the wheel is raised upward against gravity as it comes out of the low spot, it places an enhanced loading on the rail or slab.  On a railroad track, this effect shows up as battered, mushroomed, and spalled railheads on each side of a joint.  <o:p></o:p> Two factors come into play here:  <o:p></o:p> 1)      As concrete joints and asphalt cracks lose their load bearing ability from this enhanced impact loading, the rate of continued loss will accelerate.   <o:p></o:p> 2)      The faster the highway vehicle travels, the greater the impact loading of the concrete joints and asphalt cracks.  <o:p></o:p> So this leads to two very interesting conclusions.  One is that a vehicle of any given weight will do more damage to a road that already has slab joint deterioration than it will do to a new road that has no slab joint deterioration.   <o:p></o:p> The second conclusion is this:  Suppose the road authorities could perfectly account for truck wear and tear to a highway, and that they had the tax rate perfectly set for the truck to pay its fair share of the wear and tear.  If that were the case, and if the truck exceeded the speed limit, it would be causing more wear and tear than what it is paying for.
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