LANSING — Michigan's highest-in-the-nation gross truck weights are responsible for significant damage to state roads and bridges, experts say, despite years of denials from the Michigan Department of Transportation.

The issue is an important one as residents consider Gov. Gretchen Whitmer's plan to "fix the damn roads" by hiking the tax on both regular and diesel fuels by 45 cents per gallon but not changing Michigan's truck weight laws. The plan would raise an extra $2.5 billion a year, of which $1.9 billion would be spent on roads and bridges.

For years, MDOT has had a ready answer when motorists draw a link between roads that are ranked among the nation's worst and a truck weight limit of 164,000 pounds that is more than double the federal limit.

It's not the total weight of the truck that matters — it's the amount of weight carried by each truck axle, the department has insisted. And a 164,000-pound truck with the 11 axles that Michigan requires actually spreads the load more than a standard five-axle truck weighing 80,000 pounds, the federal limit.

That's not true for bridges, experts say, and only true for roads with respect to certain types of damage if the pavement the truck travels on is a smooth one. Send that same heavy truck bouncing down a road that is already rough — as so many Michigan roads are — and a different set of physics applies.

In interviews with civil and mechanical engineers and a review of academic literature, the Free Press found:

Even on a smooth road, studies show gross vehicle weight — not axle weight — is directly related to a type of damage called "rutting," which is a permanent depression in the pavement along the path the wheels follow, and which is related to road roughness. Conversely, for road fatigue and cracking, civil and mechanical engineers agree that axle weight, not the total truck weight, is the critical factor.

Trucks that bounce on rough surfaces create a "dynamic loading" effect that is significantly higher than what results from the weight of the truck when it is standing still or moving on a smooth surface. Engineers disagree over whether gross weight or axle weights take precedence when trucks start to bounce, but they agree both are part of the calculation.

When it comes to damage to bridges, it's all about the truck's total weight, not the axle weights, engineers agree. That's because a bridge bears the entire weight of a truck, regardless of how many axles the truck has. A 2016 U.S. Department of Transportation study estimated that raising the federal weight limit from 80,000 pounds to 97,000 pounds would necessitate $2.2 billion in bridge improvements to handle the extra loads.

Larry Galehouse, founder and past director of the National Center for Pavement Preservation at MSU and a civil engineer who worked more than 20 years at MDOT, told the Free Press that Michigan's high gross weight limits contribute significantly to road damage.

"When you get a road that's not smooth, you get dynamic loading, which is a truck bouncing up and down," Galehouse said.

Not just the axles, but the entire weight of the bouncing truck crashes onto the pavement, and "it has a tendency to really tear up the roads," Galehouse said.

Both the bouncing axles and the bouncing truck body can deliver hammer blows to a rough road, since doubling the load of an axle produces 16 times the damage, based on a rule of thumb used by civil engineers.

Galehouse, who believes axle weights become largely irrelevant when the body of a truck is bouncing, points to studies in which gross truck weight and dynamic loading are linked. But views on that point vary widely.

Experts say the road damage caused by a bouncing truck is almost impossible to calculate without a specific fact situation. That's because a huge number of variables come into play, including the roughness profile of the road, the speed of the truck, the spacing between the truck's axles, and the nature of the truck's suspension system and shock absorbers.

"There is no easy answer, and no one answer," said Steve Karamihas, a senior research associate at U-M's Transportation Research Institute who worked on a landmark 1992 study that examined the effects of heavy trucks on pavement performance.

The U-M study found that while truck axle weight is most important when it comes to cracking or fatigue, rutting, described as "the permanent deformation of the asphalt concrete layer caused by a vehicle," is "directly dependent on its gross vehicle weight."

The study also found that rough pavements experience damage at a rate approximately 50% higher than smooth pavements.

"On the roughest roads, fatigue damage may increase by 200% to 400%, depending on the type of road and truck properties," the report said.

Still, the study discounted the value of putting a cap on gross truck weight, saying that "to the extent that freight must be carried by trucks ... lower weight limits will only put more trucks on the road to meet commercial hauling needs," and the heavier truck with more axles will result in less rutting for each pound of cargo moved.

Karamihas, who has a master's degree in mechanical engineering, said he believes axle weights are more important than total truck weight when it comes to overall road damage, even on a rough surface. But he adds a qualifier, given the huge number of variables.

Despite considerable more research since his 1992 study, "I wouldn't say anyone has completely worked it out," he said.

Karamihas pointed to Karim Chatti, an MSU professor of civil and environmental engineering and director of the University Transportation Center for Highway Pavement Preservation (which is distinct from Galehouse's organization), as one of the nation's leaders on the effects of dynamic loading.

Chatti's work was cited by the U.S. Department of Transportation in its 2016 "Comprehensive Truck Size and Weight Limits Study," which rejected calls to increase the federal government's 80,000-pound gross weight limit for trucks, saying more study is needed.

Chatti told the Free Press he can't endorse the view that truck axle weights are more important than gross weight when a truck is bouncing on a rough surface, though he can't disprove it, either.

Since both the body of the truck and the axles are bouncing independently of each other, both the gross weight and the axle weights are important, said Chatti, who has a doctorate in civil engineering.

"The pavement community ... for a long, long time ignored this issue of dynamic loading and the effect of roughness," Chatti said. "I happen to be one of the people few people in civil engineering that have looked at the vehicle pavement dynamics."

The issue is important because no matter how good the design and how skilled the construction, roadways built over many miles will always have variability and weaker areas, Chatti said. That weak area may be well below the surface of the road, but will eventually affect smoothness. Once a rough spot develops, trucks that bounce over that spot will deliver more punishing blows than they would otherwise. That, in turn, results in more damage, which causes more bouncing, and a vicious cycle of rapid deterioration on stretches of pavement known as "hot spots," he said.

Chatti conducted a 2009 study for MDOT that found that on both asphalt and cement pavements, trucks with more axles caused less cracking than trucks carrying the same load with fewer axles. But the study also found that the amount of rutting caused by truck traffic was roughly proportional to the gross weight of the truck, even when the truck carried that load on more axles. Still, Chatti said two 80,000-pound trucks should cause as much rutting as one 160,000-pound truck, so in that respect, reducing weight limits might not help the roads if the result is more trucks.

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Chatti examined the impact of road roughness and dynamic loading in an earlier study, completed in 2001. That study, which focused on the standard 80,000-pound 18-wheeler, rather than the 164,000-pound truck also allowed in Michigan, identified thresholds at which roads are just beginning to turn rough and repairs could be made to significantly extend road life, before rapid deterioration resulting from the vicious damage cycle related to "hot spots" begins.

Ideally, maintenance should begin when a certain level of roughness can be measure, even if the road might appear fine to the naked eye, Chatti said. MDOT accepted the findings, but it's not clear the agency formally implemented them, possibly because of a lack of available maintenance funds, he said.

MDOT has long endorsed the value of early maintenance but has complained that in stretching its budget to fix badly crumbling roads it's often difficult to simultaneously keep good roads from turning bad.

In a departure from the view espoused by the former director Kirk Steudle, the agency isn't necessarily opposed to a lowering of Michigan's truck weight limits, Jeff Cranson, a spokesman for Director Paul Ajegba said Wednesday.

But regardless of the debate over the effects of dynamic loading, MDOT remains hostile to the idea that Michigan's high gross truck weight limits is a significant contributor to the poor condition of the state's roads, saying the heaviest trucks make up only a small proportion of the total commercial fleet.

Of about 150,000 commercial trucks registered in Michigan, just under 19,000, or a little over 12%, exceed 80,000 pounds, according to the Secretary of State's Office. Just under 12,000, or about 8%, weigh 130,000 pounds or more, the office said.

"It is a canard that Michigan’s road woes are because of truck weights instead of investment," Cranson said.

"I can provide ... plenty of photos of local streets that are crumbling on their own, without the benefit of any heavy trucks."

Overweight trucks in Michigan are another issue, some on the roads legally through the purchase of inexpensive special permits, others with drivers illegally attempting to avoid detection by the Michigan State Police and local enforcement agencies.

In 2017, MDOT issued nearly 109,000 permits for vehicles that exceeded normal vehicle weight or size limits, including permits for 96 vehicles that weighed more than 450,000 pounds, according to a House Fiscal Agency report.

The permits cost only $50 for a single trip and $100 for one that allows multiple trips with super heavy loads.

In 1982, Congress passed the Surface Transportation Assistance Act, which imposed the 80,000-pound weight limit across the entire interstate system, prohibiting states from setting lower or higher limits but "grandfathering" a few states such as Michigan that had earlier set higher weight limits.

When it comes to catching illegally overweight trucks, motorists frequently complain about passing freeway weigh stations that are closed more often than they are open.

The Michigan State Police says it focuses most of its staffing at busy stations on the Ohio and Indiana borders and has increasingly moved to a more mobile enforcement strategy involving automated scales beneath the freeways and troopers who patrol for overweight trucks using portable scales.

Karamihas said that if Michigan is going to have the highest gross truck weights in the nation, it's important for the state to make sure it is enforcing that limit through effective use of weigh stations and other methods. He said it's also very important to police and enforce reduced weight limits that are imposed on many roads during the spring thaw, when roads are most vulnerable to damage from heavy trucks.

Maine allows up to 100,000 pounds on all interstates, while South Dakota, Arizona, Utah and Nevada allow up to 129,000 pounds on a variety of routes, according to the Federal Highway Administration. Some states allow higher loads only on certain roads, such as the Ohio and Florida turnpikes.

Just as there's been a push by industry to increase the federal weight limit, many states are facing pressure to increase allowable weight limits on state roads. Virginia, for example, has been considering plans to increase its gross weight limit from 80,000 pounds to 91,000 pounds.

An argument for higher weight limits is that it could result in fewer trucks carrying the same amount of cargo on more axles.

Opponents say bigger trucks pose not only a risk to road quality, but to safety.

Contact Paul Egan: 517-372-8660 or pegan@freepress.com. Follow him on Twitter @paulegan4. Read more on Michigan politics and sign up for our elections newsletter.