How does this account for all the upkeep on the residential roads where a large truck almost never travels. Sure, large trucks may account for the vast majority of wear on roads where they travel, but I imagine the majority of the roads in a city rarely see large trucks, and still need regular upkeep.

Without some sources to back up your claims, I'm not sure if it's accounting for that or not. Can you supply your sources so we can look into exactly what that statistic means?