Freight trucks cause 99% of wear-and-tear on US roads, but only pay for 35% of the maintenance. This $60B subsidy causes extra congestion and pollution, and taxpayers pay the bill.
It seems obvious that the heavier the vehicle, the more damage it does to roads over time. A 40,000 pound big rig probably does a bit more damage than your average 3500 pound consumer vehicle, right? It turns out that vehicle road damage doesn’t rise linearly with weight. Road damage rises with the fourth power of weight, and this means that a 40,000 pound truck does roughly 10,000 times more damage to roadways than the average car !
In other words, one fully loaded 18-wheeler does the same damage to a road as 9600 cars. According to the American Trucking Associations (ATA), the trucking industry represents 11% of all vehicles on the road in the US, while paying 35% of all highway taxes. But if trucks represent 11% of vehicles, their heavy loads cause them to do 99% of all road damage!  The trucking industry paid $35 Billion in highway taxes in 2005, according to the ATA. Since most of the $100 Billion in highway taxes paid goes to maintenance (and US infrastructure maintenance is far behind), this implies that the trucking industry receives a $60 Billion annual subsidy from other drivers.
What are the negative effects of this subsidy? Since the trucking industry doesn’t pay the true cost of its road usage, it benefits relative to rail and other forms of transport. Freight rail lines are privately owned and maintained in the US, so they don’t receive a similar subsidy. As a result, more truck traffic ends up on highways than the market would dictate, leaving the taxpayers poorer, the air dirtier, and the roads more congested.
 Here’s some information on US pavement equations, including the statement of the fourth power law. Here’s another statement of the same, which also shows that on weaker surfaces, damage rises with the 6th power of the load.
 In order to calculate the damage done by trucks versus other vehicles, let’s assume that a fully loaded truck does the same damage to the roadway as 9600 cars, as mentioned above. In that case, then 11%, or 0.11 * 9600 = 1056. This is a measure of total damage done by truck traffic. Meanwhile, car traffic does 89% * 1 or 0.89 in damage. So the total damage is 1056 + 0.89 or 1056.89, of which 1056, or 99.9%, is done by trucks.
Perhaps half of all trucks are actually traveling empty. If an empty truck weighs 20,000 pounds, then it puts 4000 pounds onto each of its five axles, versus 2000 pounds on each axle for a car. The truck will do 2^4 more damage than the car, or 16 times more damage. So let’s add the totals back up: 5.5% * 9600 + 5.5% * 16 = 529. 529 / 529.89 = 99%. In fact, even if all big rigs in the US traveled empty, they would still do two-thirds of all damage to US roads!