The incremental costs of building stronger pavements, wider roads and higher design speeds
can be assigned to vehicles according to their weight, size and speed. The incremental costs
of increasing roadway capacity should generally only be assigned to peak-period trips that
contribute to congestion. Some roadway costs, such as planning, law enforcement and
lighting costs are not clearly related to a particular vehicle attribute, and any remaining costs
are considered common costs that can be prorated based on other costs or allocated based on
mileage.