Chart of the Day: Interstate Highway volumes vs. Interstate Loading

Rural Interstate highway volumes versus loading over time.  Source:  FHWA

Rural Interstate highway volumes versus loading over time. Source: FHWA

This chart, from the Federal Highway Administration’s 2011 Highway Statistic series (most recent year available) shows the changes in both rural Interstate highway traffic volume and loading since 1970. Loading here refers to the weight of vehicles. The 2002 peak in volume is noted on the chart (Urban Interstates peaked in 2007 per FHWA figures), but notice how loadings continued to increase, even through the 2008-09 recession. This suggests that, even with the peak in overall vehicle travel, truck volumes and/or truck weights have continued to increase. This is important to consider in light of Interstate pavement conditions and, on some corridors, capacity concerns.

Adam Froehlig

About Adam Froehlig

Adam Froehlig, aka "Froggie", is a Minneapolis native who grew up studying the state's highways and bicycling the Minneapolis parkways and beyond. A retired US Navy sailor who worked as a meteorologist and GIS analyst, he is now losing himself among the hills and dirt roads of northern Vermont. He occasionally blogs at Just Up The Hill.

10 thoughts on “Chart of the Day: Interstate Highway volumes vs. Interstate Loading

  1. Walker AngellWalker Angell

    Fascinating chart. It’s amazing that with that much change in auto traffic that loading was not more affected. Do you know what the average weight of cars vs trucks is? 25:1?

    Do truck drivers pay taxes/fees appropriate to the wear & tear they cause? For that matter, do car drivers? Many trucks have placards that they pay $xx,000 per year in road taxes but that doesn’t seem to jive with what MNDOT and USDOT indicate.

    1. Adam FroehligAdam Froehlig Post author

      Legal limit for trucks in most states is 80,000 lbs. According to EPA figures (presumably as it’s related to fuel economy, which EPA monitors/standardizes), the average vehicle weight in 2012 for cars and light trucks was around 4,000 lbs. So figure a 20:1 ratio for purposes of this discussion.

  2. Wayne


    Yarr, I didn’t know there be pirates at the FHWA, matey. Good to see they use excel charts like the rest of us schlubs, though.

    Pedantic jokes aside, I don’t think the fees commercial trucking pays have kept up with the cost of the damage to roads they cause. It’s yet another way that highways are subsidized and we give tax money to business interests.

    It also kind of makes it that much more amazing how well freight railroads can compete when we’re so heavily subsidizing long-haul trucking, effectively stacking the deck against the railroads. Even with a thumb on the scale, trucking can’t even begin to compete with railroads cost-wise for many types of goods. If we subsidized freight rail the same way we did highways (or maybe stopped subsidizing highways to put them on equal footing), we’d probably never have to hear about all these delays we’ve been having these year, because the railroads could invest in capacity expansion.

    1. Alex CecchiniAlex Cecchini

      It’s hard to say if you’re right or not. Long-haul trucking certainly receives a softer subsidy in the space given to them in urban environments to allow turning movements, wider lanes, etc. But on an interstate or primary arterial level, the cost of making a road more durable does not increase much at all to accommodate a much larger weight. Yes, vehicles do more damage at an exponential rate, but we could build a road that’s 2x as durable for a very marginal cost adder (

      However, building a second, third, etc lane to handle congestion increases costs much more linearly. So, freight does most of the damage on our roads, but vehicles require the space. Hard to say exactly who isn’t paying their fair share, and this is especially tough to calculate when you factor congestion externalities from drivers onto the trucking business (freight is only 7% of urban area VMT nationwide – but trucking companies pay for the delay in labor and/or pay extra to have drivers work in off-peak hours/overnight).

      Also, current policies do the opposite of what they should. Ex charging per axle when more axles spreads the weight out, reducing road damage.

      I’m with you on stopping the subsidy of one form of transport over another. I just think that a bigger cost-driver in our road network comes from personal vehicle travel demands.

      1. Wayne

        Great points! I was mostly thinking of urban impacts (as I am wont to do), and hadn’t even considered the space requirements.

        Speaking of being urban-focused, I remember looking up the designated truck routes in Minneapolis recently because I was annoyed with the ridiculous amount of truck traffic on University Ave around Hennepin/1st, and I was surprised to see just how many streets are designated for trucks in town. I’d figure there would be some main arterial connectors in industrial areas that go straight to the highway, but they are basically all over town going everywhere.

        I understand smaller trucks need to make deliveries to commercial areas, but why do we need to accommodate semi trucks on so many city streets? It’s so unsafe to have them mixing with urban mixed-mode traffic, especially when they’re blazing through built-up urbanized areas with lots of pedestrian traffic going 40mph or more.

  3. David MarkleDavid Markle

    This graph makes clear an important distinction. Does anyone have good data on urban arterial streets? Sensible urban planning should take heavy street loading into consideration, for example, in planning LRT routes and bicycle lanes, and in finding better ways to provide businesses with the commercial service they need.

      1. Matty LangMatty Lang

        Absolutely. Perhaps we should also look into shifting major freight transfer points outside of the urban core and only allowing smaller “last mile” trucks on urban core streets?

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