Trucks and roads

I read today that the city of Newton MA is cracking down on large trucks driving through its streets. Commercial drivers often take shortcuts through cities and towns -- for example, in Newton, to get from Rte 9 to Rte 95. Trucks are often carrying excess, illegal loads, and also may have safety violations.

There is another reason to crack down on heavy trucks driving through city streets. Most people don't realize that the damage to paved roads done by vehicles is not proportional to the weight, but proportional to the fourth power of the weight. Consider the case of the 10,000 pound truck. Comparing this to a large (4000) pound car, the ratio is 10,000/4000 = 2.5. However, the wear and tear on the roads is is proportional to the fourth power of 2.5 which is a little more that 39. Thus, a 5 ton truck does nearly 40 times the damage as a 2 ton car. The owner of such a truck does not pay anywhere near 40 times the taxes that the owner of the car does. This is another form of subsidy for the trucking industry.

This phenomenon is well-known to civil engineers and is called the "Fourth Power Rule". For more details and further technical references, see my blog on this issue (Wednesday, October 7, 2009).