Loads increase while quality of bridges and roads deteriorates.

Loads increase while quality of bridges and roads deteriorates.

WASHINGTON (AP) — Just like Americans themselves, the nation’s roads and bridges are carrying much more weight today.

Added to an aging and deteriorating highway system, it means more safety problems, delays and repair costs for drivers, experts said.

In just a decade, from 1995 to 2005, the weight load on urban highways increased by half. Since 1970, the weight carried on rural highways has gone up nearly 71⁄2 times, according to Federal Highway Administration statistics.

And it’s not just more traffic. It’s heavier trucks.

The number of tractor-trailer truck miles driven on American roads has more than doubled to more than 145 billion miles a year since 1980, increasing faster than the rate of smaller trucks or cars, according to federal statistics.

Some engineering experts speculate that the wear and tear over the years of heavier loads could be one of the factors that triggered the collapse of the interstate bridge in Minneapolis last week.

The problem

More weight wouldn’t be a problem if the highway system were regularly and adequately maintained because well-kept roads and bridges can handle the added weight, said University of Texas civil engineer professor C. Michael Walton.

The American Society of Civil Engineers gave the nation’s roads a “D” grade and the bridges a “C” grade in 2005. One of the factors was the increased weight, said John E. Durrant, the society’s managing director of engineering programs.

The problem boils down to basic engineering. When engineers design bridges and roads there are two factors to balance: load, the force weighing on the structure, and resistance, the ability to withstand that force.

What’s happening is that loads are increasing while time, weather and fatigue weakens resistance.

The collapsed Minnesota bridge had no weight limits but was categorized as structurally deficient, one of over 73,000 such U.S. bridges.