More than a half-million overweight trucks are allowed onto the nation's roads and bridges -- an increasingly routine practice that some officials say is putting dangerous wear and tear on an already groaning infrastructure.

1 TRUCK = 9,600 CARS?

The weight limit for nearly all interstate highways is 40 tons. Yet one government study said that one 40-ton truck does as much damage to the road as 9,600 cars. And state permits frequently allow vehicles to exceed that amount -- by 2 tons in Texas and sometimes by as much as 85 tons in Nevada.

THE NUMBERS

An Associated Press review of state permits in all 50 states found that more than half a million overweight trucks were allowed to traverse the nation's bridges and highways at will in the past year. Those were permits good within a state's boundaries for an entire year. While 10 states do not issue yearlong permits, all states hand out shorter-term permits good for a few days, weeks or months. Those add up to more than 1.8 million additional overweight trucks on the road.

RULES VARY BY STATE

State policies vary greatly, with some much more lenient than others. Many states will not issue permits for loads that can be easily split up and carried at safer weights. "It's one of the most befuddling policies we deal with, that we spend millions of dollars to build roads ... and the state comes along and for a pittance gives out a permit to allow trucks to destroy those roads in a matter of months or years," said Don Lee, executive director of the Texas Conference of Urban Counties. As for why states issue overweight-load permits, many officials said they have no choice -- they are simply carrying out the laws. Critics say the laws have been written to benefit powerful local industries, such as logging in the West, or oil in Texas.

A FEE SYSTEM

Many states charge fees ranging from $12 to $1,000 for overweight-load permits, depending on the excess weight allowed. In theory, those fees are supposed to offset the damage done to the highways. However, Darrin Roth, director of highway operations at the American Trucking Association, said it is not fair to put all the blame on legally overweight trucks because permit loads are only a tiny proportion of total traffic.

ASSOCIATED PRESS