Increased loads decrease road life

On Sept. 18, Iowa Governor Kim Reynolds signed a proclamation that lifted weight restrictions for truck traffic on roads in the state to make it easier for farmers to transport corn, soybeans, hay, straw, and silage. The weight limit on Iowa roads for a semi truck is 80,000 pounds. With the proclamation, the restriction is now at 90,000 pounds.
Over the past several years, we in Iowa have been hearing about the deterioration of our roads and bridges. It was only July when the Iowa Department of Transportation released a map detailing the poor condition of most of Iowa's bridges. A national research group has said Iowa has some of the worst rural roads in the nation.
This made me want to find out if there is a correlation between heavy truck loads and the condition of our rural roads. I sought out Marion County Engineer Tyler Christian, who had plenty of answers.
"Any increased loading on county roads is just going to accelerate the deterioration of the pavement," Christian said.
Putting load restrictions on roads dates back to the 1960s, when the American Association of State Highway Officials (AASHO) devised a way to calculate load limits. What resulted was the establishment of the Equivalent Single Axle Loads (ESAL) system, to provide a standard for nationwide load limits.
An 80,000-pound load on five axles is typically 2.39 ESALs per axle. The 90,000-pound load on the same number of axles is 3.8-4.0. While the number of ESALs may increase only slightly, the actual damage done to the road can increase by 60-70 percent, according to Christian.

“That can always vary, too,” Christian said. “The damage done to the pavement increases significantly more than a linear factor than what the load increases.” 

To put it simply, the heavier loads decrease the life of a road's pavement. Trucks within the usual weight limit already put a strain on highways, as a single 80,000-pound truck traveling over a highway once causes the same amount of damage to the pavement as 800 passenger vehicles. 

Trucks are something county engineers have just dealt with, as it is also very important to Iowa's economy to allow the easy transportation of goods - including harvested crops. Other states are looking into increasing their load limits, but the difference here is that Iowa roads have not been designed to handle the heavier loads. 

The number of miles of road a jurisdiction is responsible for can affect every aspect of design and maintenance. For instance, Christian's Secondary Road Department is responsible for 850 miles of road within Marion County's borders. Because of the volume of miles to be maintained, county roads cannot be built to the same standards as interstate highways. 

Funding for county roads comes primarily from the State's Road Use Tax. This tax was increased in 2015 by 10 cents, much to the chagrin of many Iowans. Of the amount of tax collected at the state level, 24 percent is allocated to counties. It remains the largest source of funding for counties, who also receive a small amount of property tax collected from properties not within an incorporated city's borders. The fund this money goes into is known as the Rural Services Basic Fund. 

"I think the formula they have in place now works well," Christian said. 

In all, Christian's budget is around $7 million. However, there are still many Marion County roads that rely on gravel. Each year, $1.5 million is budgeted to meet gravel needs. 

"Heavy trucks absolutely play a factor in how much rock we purchase," Christian said. The County has a rock report, which tracks how much is used on a certain road over time. There are definitely parts of the county that have required more rock than others. 

"Traffic volume is our baseline, and we adjust from there," Christian said. Roads can also be affected by drainage, soil conditions, heavy trucks and other factors. 

Damage by heavy trucks is easier to locate on gravel roads, versus pavement. Pavement issues can take years to surface. Typically, the County likes to get 40-50 years of life out of pavement if it can. 

Christian added that, beyond being secure in the fact that ESALs increase and damage is done, it is difficult to quantify how much damage heavy loads cause. There is no way to really tell what kind of an effect the weight restriction being lifted will have, but there is no doubt that the governor's decision will negatively impact rural Iowa roads.