Jeep Grand Wagoneer
Above: the only acceptable SUV, from a design perspective


A week or two ago some drunk bros in a white Lexus SUV made fun of me for taking the bus. Whatever, I thought, we're about to tax them and their dumb car through the roof. (SUV design has been all downhill since the Jeep Grand Wagoneer; in particular, late-model Range Rovers look like Tupperware, IMO).

But now I feel sorry for them, since it looks like they're about to be wronged. Let me explain.

In his budget address, Rahm Emanuel laid out the rationale for the city's "SUV tax."

It is estimated that 80 percent of the damage to Chicago’s streets is caused by the small share of heavy vehicles like trucks and SUVs. So we are proposing a modest increase for heavy vehicles that do most of the damage.

If you drive a standard-size or smaller car, the cost of your city sticker will stay at 75 dollars. That means 75 percent of Chicagoans will see no increase. If you drive a heavier car or truck, it will go from 120 dollars to 135. Some of the additional revenue will go to fill an additional 160,000 potholes in 2012, nearly a 40% increase over this year.

Turn your pagers back to 2003, and you'll find the same proposal: an 20 percent increase on sticker fees for cars 4,500 pounds or more—"quietly" moved down from 5,000—from $75 to $90 (obviously, they've gone up since then).

As I was researching this, Steve Chapman beat me to the punch: he contacted the author of the study on which the rationale for the increased sticker fee is based, and found that the claim doesn't hold up. Which is exactly what Fran Spielman found when she looked into the issue for the Sun-Times back in 2003:

Engineering and transportation experts contacted Monday insisted that roads are typically designed for heavy trucks that weigh "at least double," if not 10 times what an SUV does.

They argued that there is virtually no difference between the road damage caused by a 5,401-pound Lexus LX-470 or a 5,070-pound Toyota Sequoia, and the wear and tear caused by a 3,950-pound Ford Crown Victoria, a 4,049-pound Cadillac Deville or a Mercedes Benz S-Class that weighs 4,200 pounds.

"That much extra weight is not going to cause any more damage than a regular passenger vehicle. You'd have to weigh what a truck weighs — at least double the weight of an SUV" to cause additional wear and tear, said Sidney Guralnick, a distinguished professor of civil and architectural engineering at the Illinois Institute of Technology.

Chapman basically got all the relevant info, but might as well not let my research go to waste, because it's kind of interesting.

He writes: "[I]t struck me as implausible that a minivan weighing 4,500 pounds would beat up the pavement appreciably more than a 3,000 pound sedan."

That didn't strike me as implausible, and for good reason, I initially thought; it's an assumption you see a lot. The accepted rule of thumb for road damage is this (PDF):

In the late 1950's the then American Association of State Highway Officials (now the American Association of State Highway and Transportation Officials) conducted pavement deterioration tests at Ottawa, Illinois. The measure of pavement deterioration used was the Present Serviceability Rating (PSR). The tests found that, with increasing axle load, pavements deteriorated at a rate that was roughly equivalent to the relative weight increase raised to the fourth power…. For most pavement distresses the relationship between axle load and pavement deterioration is less than a fourth power, and the overall relationship between axle load and pavement deterioration may be closer to a third power rather than a fourth power relationship.

So that would seem to make it plausible: if the effective damage goes up by the third or fourth power of a vehicle's axle weight, then the difference between a sedan and a minivan could be substantial.

Then I got sucked into the rabbit hole of roadway engineering. Turns out it's more complicated than that. In 2005, San Francisco looked into a similar fee structure, and produced a legislative analyst report (PDF) that seems to sum up a lot of the research I came across:

City pavements are considered “rigid pavements” due to use of a concrete base. When using the Fatigue Strength Method of design, the pavement will fail if it is subjected to repeated applications of heavy loads causing it to exceed its fatigue strength. Due to their relatively light weights, repeated applications of car and SUV loads will not cause a pavement to exceed its fatigue strength.

For instance, University of Michigan economist Richard C. Porter, writing in Economics of the Wheel: The Costs of Cars and Drivers: "Typically, a truck will carry 10 times as much weight per axle as a car, and hence that truck will do 1000 times as much damage to the road (per axle). For practical purposes, road damage is done by trucks, not cars."

Or the Alaska Flexible Pavement Design Manual (PDF):

Only medium and large trucks are assigned ESAL [equivalent single axle load] equivalency for design purposes. Automobiles, pickup trucks, and other relatively small vehicles have such small ESAL loadings that they do negligible damage to the pavement structure. An old rule-of-thumb is that pavement structural damage done by the passage of a single large truck is equivalent to that done by about 9,000 automobiles.

Or Bituminous Mixtures in Road Construction: "In determining the structural life of a road pavement, the effect of cars and similar vehicles is negligible…. It is therefore the heaviest axles in the stream of commercial vehicles that cause a disproportionately large amount of structural damage to a flexible pavement."

Or from "Techniques for Assessing the Socio-Economic Effects of Vechicle Mileage Fees" (PDF) from the Oregon Department of Transportation, which also produced the study the SUV tax is based on:

Short run marginal road costs from autos and other light vehicles are directly related to miles driven and, despite increasing variance in vehicle weights (and fuel efficiency), there is not much difference in the damage done to the road by different types of light vehicles (whereas there is a big difference in damage to the road done by heavy trucks of different weight groups and with different axle configurations). Indeed, Merriss (2004) notes that the “difference in pavement damage imposed by a 6,000-pound large SUV versus a 3,500-pound compact car is inconsequential as compared to the difference in (pavement) damage imposed by either of these vehicles versus a fully-loaded 80,000-pound truck” (p.2).

In other words, if Joe More wants to defend his "pimped-out Taurus" from higher sticker fees, there's plenty of ammunition.


Photograph: DVS1mn (CC by 2.0)