Would Heavier Trucks Cause More Potholes?

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With an untold number of potholes on American highways, roads and streets, there are concerns that a pending piece of federal legislation might add to the count. The bill is H.R. 763, the Safe and Efficient Transportation Act, which would allow an increase in single-vehicle truck weights from the current limit of 80,000 pounds to 97,000 pounds.

At first glance, this seems to be a proposal to place greater strains on roads and bridges, further damaging our already crumbling infrastructure. The breakdown of roads and bridges is a function of several different things – freeze-thaw cycles, moisture, high temperatures, time and wear and tear, the worst of which is heavy vehicles. But supporters of the bill claim its provisions would not cause new damage to infrastructure, and would additionally alleviate traffic congestion and possibly reduce fuel consumption and greenhouse gas emissions.

The provisions of the Act have been incorporated into the broader American Energy and Infrastructure Jobs Act by the U.S. House Committee on Transportation and Infrastructure as of January 31, 2012. Congressman John Mica (R-FL), chairman of the House Transportation and Infrastructure Committee, posted his rational for pushing this bill on The Hill’s Energy and Environment blog on March 20, 2012.

The Coalition for Transportation Productivity (CTP), a group lobbying in support of this legislation, notes that heavier vehicles under provisions of the Act would have an additional sixth axel, which spreads the weight out over four more tires. Under current rules, five axels and 18 tires carrying 80,000 pounds means about 4444 pounds per tire. With six axels and 22 tires, 97,000 pounds places 4409 pounds per tire on roads – a slight reduction of pressure on pavement at the moment of tire-to-asphalt contact.

Trucks cause more damage to infrastructure than cars

Note that the trucks will not increase in size. The increase is in how much freight weight can be contained within single-trailer vehicles. But that still strikes us as a dynamic, something different. Would the additional 17,000 pounds lead to pavement deterioration any faster than with the current standard of lighter trucks?

A report, “Effect of Truck Weight on Bridge Network Costs – Report 495,” published in 2003 by the National Cooperative Highway Research Program (NCHRP, by Gongkang Fu, in cooperation with the National Research Council, Transportation Research Board, American Association of State Highway and Transportation Officials, and the U.S. Federal Highway Administration) suggests some concerns are well placed.

The report looks at a study begun in 1982 on two bridges in relatively close proximity in the Bay Area of northern California. One bridge carries I-880 and allows trucks of all legal weights (up to 80,000 pounds), and the second bridge (I-580) runs parallel but only allows vehicles with gross vehicle weight (GVW) at or below five tons (10,000 pounds), which limits it to much lighter trucks and automobiles). These bridges provide good points of comparison because they are built with the same design and engineering, environmental conditions for both are the same and no deicing agents are used on either because the area almost never has freezing temperatures. This isolates the single biggest factor that would lead to pavement deterioration on the two spans, which is traffic.

Despite the fact the I-880 bridge deck is about 15 percent thicker – 7.5 inches vs. 6.5 inches – the bridge carrying heavier vehicles clearly and repeatedly required a significantly larger number of repairs over the decades the two were studied. “It is concluded that the difference in the two decks’ condition was due to the different truck loads carried … these two routes have had similar total annual average daily traffic (AADT) over these years, but very much different truck traffic.”

From this we can broadly conclude that vehicle weight – not frequency of traffic – has a deleterious effect on pavement. This does not definitively say, however, that a difference between 80,000 pounds and 97,000 pounds will noticeably matter. But the more weight = more potholes equation seems logical. But in an advocacy packet published by the CTP, research on how the bill would affect Interstate and non-Interstate roads in the state of Wisconsin, taxpayers would save between $1.1 million and $10.19 million in pavement expenses if vehicle miles traveled were reduced by way of six-axel, 97,000 pound vehicles.

Benefits of bigger trucks include additional user fee

But truck traffic is inevitable in a commercial system that has migrated freight from rail to surface transportation over the past several decades, as previously reported on Pothole.info.

Which is the basis for an argument in support of this bill. Proponents state that the advantages include fewer vehicles on the road, increased highway safety, an overall boost to the economy and even environmental benefits. Here is how that would work:

  • Fewer vehicles means less congestion – The coalition supporting this bill says heavier freight-filled trucks “will simply allow an individual company to use fewer trucks to deliver a given amount of products. Enabling our manufacturers, growers and producers to be more efficient is the best way to ensure our prosperity in the years ahead.”
  • Improved highway safety – CTP also bases its argument for safety on the idea that there will be fewer trucks on the road, or, more specifically fewer truck vehicle miles traveled. In other words, fewer trucks traveling fewer miles should translate to fewer accidents. The coalition cites member MillerCoors, which could reduce its fleet by 2,000 trucks and log one million fewer vehicle miles per week.

One opposition group, Advocates for Highway and Auto Safety (AHAS), which enumerates an average of 4,676 deaths per year in truck-involved crashes, argues that heavier vehicles are more deadly. An argument is sometimes made that, given the basic principles of physics, braking distances would be longer with 97,000-pound trucks. The CTP, however, cites research that shows the stopping distance for a five-axel 80,000-pound vehicle is 240 feet, while a six-axel vehicle weighing 97,000 pounds would require 241 feet.

But AHAS points out that total vehicle miles traveled will likely increase over time, as happened since the 1980s when freight weights were increased from 73,000 pounds per vehicle. Their argument may hold water, but for a slightly different reason: Because population increases generally lead to freight increases, therefore overall freight will be greater in the future. Add to that how heavier vehicles will enable truck shipping to be economically advantaged over rail, therefore the total number of vehicles using the roads still would likely increase for two reasons – population increase and shipping economics – in the long run.

  • A boost to the economy, benefits to consumers and businesses – This idea works off the general concept of shipping efficiency. Currently, many trucks travel great distances with partially empty vehicles due to weight limitations. With fuller trucks, fewer drivers have to be paid to haul more goods, savings that might be passed on to consumers and businesses. A single large shipper such as International Paper expect to shave $70 million off of transportation costs, while the transition to different trucks would boost jobs in truck manufacturing.
  • Environmentally beneficial – CTP argues that fewer-even-if-heavier trucks ultimately lead to greater fuel economy and lower emissions. They cite a study by the American Transportation Research Institute, which found that six-axel, 97,000-pound vehicles get 17 percent more ton-miles per gallon. Also, the U.S. Department of Transportation claims that this weight limit increase would result in reduced consumption of diesel fuel, a drop by 2 billion gallons annually.

Of note, Canada, Mexico and most European countries already allow larger vehicles than those in the U.S.

None of this changes the fact that our roads need fixing – and this promise of fuel efficiency, while a net plus for many reasons, would reduce revenues for the Highway Trust Fund, the key source of highway maintenance funding. Which is a huge concern that has yet to be effectively addressed.