Things I Didn’t Know

Via The Infrastructurist:

“A commonly cited statistic is that a 70 ton tractor trailer does as much damage to a roadway as 10,000 passenger cars.”

Could this be true? Roughly calculating that a 70-ton trailer would be 20 times the weight of the average car, it seems a mismatch, to say the least, that the damage done by the truck would be 10,000 times greater. Unless there is some serious non-linear action going on, some threshold of massive deterioration which trucks routinely cross — but then one wonders if the economics wouldn’t shake out towards building stronger roads, or making trucks smaller. One also wonders why tolls for trucks would be that much higher.

Anyone see any real data?

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This entry was posted on Friday, April 24th, 2009 at 4:53 pm and is filed under Things I Didn't Know. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

15 Responses to “Things I Didn’t Know”

  1. Erik Says:

    I don’t know about that 10,000 number, but it is highly nonlinear.

    Page 8 here has some charts showing the relative contribution of a number of factors to fatigue (for a single trip). The black bars represent the relative contribution of a particular factor over its range of typical values. Increasing bar length means that the fatigue caused by the smallest value is more and more different from the fatigue caused by the largest typical value.

    I also vaguely remember that the way fatigue/damage accumulates is considered to be pretty nonlinear with weight but I can’t remember what the justification is exactly.

    Qualitatively, it’s not hard to imagine that for a light vehicle, the threshold load to cause permanent damage might never be exceed and thousands of trips could cause negligible damage, while one passage from a massively-heavy vehicle could cause enormous damage in one trip.

  2. Jay Jardine Says:

    This is likely derived from the Equivalent Single Axle Load factors used in flexible (asphalt) and rigid (concrete) pavement design. As shown in the tables, depending on the weight of the vehicle and the number of axles, you can get hugely varying magnitudes of relative impact. The relationship (based on observed wear rates) is indeed non-linear (to the power of four).

    This does have huge implications for who is subsidizing who in terms of wear-and-tear and is a strong argument in favour of weight-based tolling for trucks and time-based tolling for passenger cars.

  3. downfall Says:

    See the link for a discussion on equivalent single axle loads (ESALs).

  4. Matt Says:

    Road damage is proportional to the fourth power of the weight of the vehicle.

    The average US car is 4000lb (1800kg), and the average European car 1175kg.

    Presumably, you’d need to take account of the number of axles — a 70 ton tractor has 2, but a 70 ton tractor + trailer 4.

  5. Jack Says:

    If I remember correctly the forth power relationship came from the AASHTO road test back in the 1950′s. An interesting experiment. Matt is correct in that you do need to take into account axles. Engineers use, among other variables, something called and Equivalent Single Axle Load (ESAL). Estimating the number of ESALS a road needs to withstand is a large part of pavement design.

  6. Greg Says:

    Here’s a nice explanation of the origin and nature of the 4th power law:

    Apparently it comes from an extensive study done by AASHTO in the late 50′s and early 60′s and is still considered fairly definitive. Note the line “Therefore, as a rule-of-thumb, the damage caused by a particular load is roughly related to the load by a power of four (for reasonably strong pavement surfaces).” So apparently we are *already* talking about a well built road here. It may be even worse for lighter duty pavement!

    And yes, we’re talking about per axle loading.

    Either way, it makes clear that there’s probably a substantial externality that I haven’t seen mentioned much for the shift away from passenger cars to light trucks that has occurred in the US over the last few decades. I wonder how big that number is?

  7. Kevin Says:

    What vehicles are weighing 70 tons? Semis in the US, at least single trailer semis, need permits to weigh over 40 tons. Is this talking about doubles/triples or something?

  8. skh.pcola Says:

    Where is a 70-ton tractor trailer typical? The maximum legal weight for a tractor-trailer rig in the US is 80,000 pounds, although heavier loads are permitted with additional regulations. Even so, there are axle weight restrictions on all loads, with heavier loads requiring more axles. A typical 40-ton rig has 5 axles and 18 tires. Here’s a trailer designed for heavy loads that has 9 axles and 36 tires:
    The tractor that would haul that trailer would typically have 4 axles and 14 tires.

    Additionally, there are regulations that dictate how closely spaced those axles may be, so as to distribute weight properly to protect bridges and roads:

    Road damage is caused primarily by poorly-built roadways. Just as airport runways are designed and constructed to support certain types of aircraft and weights, so, too, are roads. I’ve seen roads deteriorate over the span of a year because of poor construction. It’s not trucks and the 40-ton weight limit that is at fault.

  9. Andrew Says:

    As far as I can tell, the oldest stretches of pavement on parkways like the NJ-NY Palisades Parkway could be over 50 years old. Try that on a road that carries trucks.

  10. Pete W Says:

    Some observations regarding trucks and road damage:

    1) In the State of Washington, trucks have more axles.

    2) In Southern California, the truck lanes (outside two) are thicker and wider, but still manage to deteriorate quickly.

    3) There are several weigh stations in Southern California and they routinely catch trucks improperly loaded.

    4) I watched a residential road crack when a fully loaded 53′(?) dump truck rolled through.

  11. Lee Watkins Says:

    Also depends on the roadway. Trucks come down the 50′s era brick here in Baltimore all the time – and being near the port some of them are quite heavy. Most of the bricks lay loosely on top of a bed of sand, and are looking just fine despite the abuse. Very few of the bricks have any cracks, and it would be a simple matter to replace cracked bricks, but in practice they will only take the bricks and lay cencrete or (more recently) asphalt patch.

    There also are a number of streets with cobblestones (Belgium Blocks), which are not affected in the slightest. In the last 10 years they have been paving right over a lot of the bricks, but the pavement has already peeled off many sections of brick they did 8-10 years ago – this year they’ve been using stimulus money to re-pave over the bricks and a few of the cobble-stone streets down town, which I find questionable considering that oil prices are going back up again. I wonder if there have been any studies on trucks and concrete paver stones? They have also been using stimulus money to replace all the concrete sidewalks with clay brick sidewalks.

    So they are switching brick and Belgian block streets to concrete and asphalt, meanwhile switching the concrete sidewalks to brick. I’m not sure how this is an improvement. It seems to me the old way worked better.

  12. morgan scott Says:

    Whats the stats for bicycles vs. cars. vs. trucks? How about 10 billion bicycles = 10,000 cars = 1 “70 ton Tractor Trailer”? Sorry so ignorant, bravo to all the wonky commentary. But bicyclists should really pay for their own infrastructure, all that striping is eating up taxpayers dollars for education. Put tolls on roads and bike lanes, so the poor will just have to sit in their apartments and get more obese. Leave that problem for the health system to solve. Apologies for the snark, but someone had to do it?

  13. aaron Says:

    Just visit Detroit. Plenty of data on shoddy construction and overloaded trucks.

  14. Jim Morris Says:

    Mr. Vanderbilt,

    I am reading Traffic and really enjoying the book. I am on the section dealing with parking right now. I am especially irritated by people who hold up the line of traffic in the Walmart parking lot, waiting for someone to get in their car and back out because they are too lazy to walk a few extra yards. I always go in the main side entrance, then u turn into a parking space near the entrance, heading back out…I walk farther but I have easy entrance and exit. My question is this…are you aware of any statistics that address how much gas is wasted in the U.S. due to drive thru’s…at banks and fast food places, etc? I am a librarian and have not had any luck finding statistics on this. Thanks, Jim Morris

  15. Grant Johnson, PE, PTOE Says:

    It all depends on the thickness of the asphalt and the sub base aggregates. We have a term TI (traffic index) to categorize roadways, because depending on what kind of traffic is intending to use them, the roads have different construction standards. Neighborhood roads are thinner, have low TI’s, and can’t take heavy loads day in and day out. Heavy trucks would flex the thin pavement too much (often just 2″ of black asphalt laid on compacted dirt), crack it, and pot holes form. But freeway systems on the other hand are often built not with just asphalt, but often with a top layer of 12″ thick reinforced PCC concrete on top of a firm compacted 18″ subbase of rock, all together which doesn’t flex much (and lasts a long time).

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