The Case of Knowing Just A Little

I joke that I know enough about [that thing] “just to be dangerous.” However, it’s often not a joke, and that is particularly true when it comes to public policy.

I follow the Streets Blog Network for their take on all things non-vehicular. I recommend you follow them, too. Alas, just like any other news organization, or news aggregation organization, sometimes they fall short of the mark.

Like today, when they’re making the case that America has fallen behind on its infrastructure maintenance (true) because of a bad decision to build asphalt roads on “packed dirt” (not really true).

It’s true that a majority of roadways are built in the United States with asphalt-and-aggregate poured in layers over a base of compacted material, however that material is not typically “dirt” except in the most general sense1 The base layer in question is engineered to have a specific compaction and moisture content, a particular thickness based on the projected traffic volumes and truck percentage, and sometimes is combined with materials such as cement or lime to bring its material properties up to specification. Calling it “dirt” is doing the vast majority of state and local DOTs a grave injustice.

It’s also true that asphaltic pavement is a cheap(er) way to build roadways, and that it has a reduced life compared with portland cement concrete pavement. Whether it is cheaper over the entire lifecycle of the pavement is a question that can only be answered on a case by case basis. You have to account for the initial cost of construction, the maintenance, and the rehabilitation/reconstruction of the pavement in question. This will be different depending on location. For example, are you building in Florida, or Minnesota? These have very different climate and geotechnical conditions, both of which affect the cost of pavement placement and maintenance.

The only part of this article that is generally true is the comment that U.S. roadway maintenance has been deferred due to cost considerations. To add to that, in a lot of states it is because maintenance budgets do not get the same attention as new construction, and federal dollars are easier to get on new projects than on maintenance. Note also what I said earlier about news aggregation services. Streets Blog got this from Gizmodo and rehashed it with a brief synopsis. Unfortunately that synopsis missed the mark. Gizmodo’s article is a good description of the roadway situation both now and in the past.

  1. note that sometimes, this is actually the case. A thin layer of asphaltic concrete is laid over a base of rolled dirt, but this is for very low volume, low use roadways. []
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