The rate at which a road wears depends on various factors, including climate, amount of traffic, and the size and weight of the vehicles using it  The only land transportation to Rittland's seaport is via a divided highway, one side carrying traffic to the seaport and one carrying traffic away from it
The side leading to the seaport has worn faster, even though each side has carried virtually the same amount of traffic, consisting mainly of large trucks
The volume of traffic to and from Rittland's seaport has increased beyond the intended capacity of the highway that serves it.