Tell US DOT that bicyclists’ safety counts.

Vehicle Weight and Road Damage

Heavy trucks obviously cause more road damage than cars, but how much more? According to a GAO study, Excessive Truck Weight: An Expensive Burden We Can No Longer Afford, road damage from one 18-wheeler is equivalent to 9600 cars (p.23 of study, p.36 of PDF).

The study assumed a fully loaded tractor-trailer at 80,000 pounds, and a typical passenger car at 4,000 pounds. That’s 20 times difference in weight, but the wear and tear caused by the truck is exponentially greater.

Food for thought: a bicycle and rider at 200 pounds is the same 20 times less heavy than a 4000 pound passenger car. Similarly, the wear and tear caused by that bike and rider would be exponentially less than a passenger car’s.

Virginia has already figured out that it’s cheaper to move trucks off our highways and onto trains, than to support those trucks on our roads. Let’s also think about getting motorists out of their cars. Wide shoulders, wide outer lanes and bike lanes, and off-road paths and trails for bicyclists may seem like extra expense, but they’re cheaper than supporting the car trips they can eliminate.

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7 comments… add one

  • A question and an error:

    Question: A bike has 2 wheels. A car has 4. My understanding is that the higher pressure of a bike tire is irrelevant since the pavement locally spreads that weight over the size of the contact patch, but that the separation of wheels was relevant? In that case, since a car’s load per wheel is likely to be only 10 times that of a bike, it would do only 1000 times the pavement damage. Is it not so?

    Error: that’s not exponentially. Polynomially, perhaps? Exponentially greater would be if pavement damage were proportional to, say, c^w for some constant c and wheel weight w. Here, using my numbers above, if c=2, the car would cause 2^1000 (or about 10^300) times more damage than the bike. Given how much damage a bike causes, even a car would shatter the planet, and adding 4 pounds of groceries (one pound per wheel) would double the amount of damage done (2^1001 = 2 * 2^1000).

    Otherwise, good pointer. I had been looking for a study like that.

  • Great reference for ALL cycling advocates.
    I’m not sure it matters, but the article focuses on the number of “axles” not “wheels” so a two axle bicycle has the same number as a car but a 5 axle truck may have 18 wheels, so under the “wheel makes a difference” theory, the weight would be spread out over those 18 wheels. Under the govt’s analysis, it would appear that the number of wheels doesn’t matter, only that the weight is spread over x-number of axles…

    Steve Magas

  • The damage in proportion to weight is only polynomial to the 4th power. Saying it increases exponentially implies that the size of the exponent increases with weight.

  • To quote from the report,

    “Conclusion: Heavy and overweight trucks are a major cause of highway deterioration. The damaging effects by these vehicles….make it clear that these trucks are the principal cause of traffic-related deterioration on the highways….. Because of the exponential impact of excessive weight on the highway, a small percentage of overweight trucks will significantly decrease serviceable life of the Nation’s highways.’ (p.33)

  • Any way you care to crunch the numbers, one thing is clear, if we can reduce the number of cars and trucks on the road, we all win.

  • For those interested in the math, see Pavement Interactive’s page on equivalent single axle load.

  • The article does a great disservice when it states:

    That’s 20 times difference in weight, but the wear
    and tear caused by the truck is exponentially greater.

    As shown by the link to the equivalent single axle load in the Jan 4, 2013, the wear and tear on roads is related to the 4th power of the relative loads. Thus, if on car A, if the load is 1,500 pounds per axle and on car B, it is 3,000 pounds per axle, the road damage by car B is not twice as much, but about 2^4 = 16 times as great.

    I hesitate to use this equation to the relative damage of a bicycle to a car, since the experiments did not cover that issue. My gut feeling is that the pounds per square inch is not the major factor to look at. I think that it could be computer modeled by a grad student and then field tested.

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