Explained: Calculating Vehicle Impacts on Road Surfaces

Meg Dunn explains how civil engineers calculate the wear and tear of vehicles on the roadway. "Roads need to be maintained. But some vehicles cause more wear and tear than others, requiring additional maintenance," explains Dunn to set the stage for the discussion.

Dunn lays out the complex set of factors that can influence the state of street surfaces, such as construction materials, vehicle frequencies, vehicle weight, and vehicle speed. When comparing the impacts of various types of vehicles, Dunn suggests the Generalized Fourth Power Law, "which is a rule-of-thumb way to determine the level of damage caused by a particular load" and offers "some sense of how all of the different vehicles on the road compare in terms of how much damage they do to the surface of the street."

Dunn then breaks down the impacts of a sample of vehicle types on roads—everything from a nine-ton big rig to a Toyota Highlander to a Smart Car. Among Dunn's findings: "It would take 410 average sized cars traveling on a road to equal the level of damage caused by one 18,000 pound big rig truck. But it only takes 21 average cars to cause the level of damage created by a single hummer."

The agenda behind the post, however, is to point out the small amount of impact on streets perpetrated by people on bikes. After laying out the math for the impact of bikes, Dunn produces the following assessment: "every time you leave your car at home and take your bicycle out instead, you’re saving the City, and yourself, a car-load of money."