TS&W Page 1, 1.0 Technical Relationships of Policy Consequences Concerning Pavements.

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Comprehensive Truck Size and Weight (TS&W) Study

Phase 1—Synthesis

Working Paper 3—Pavements and TS&W Regulations

1.0 Technical Relationships of Policy Consequence Concerning Pavements [ 1 ]

1.1 Background

Pavement-related effects of changes in truck size and weight regulations include the following:

Increased traffic loadings require thicker pavements which, in turn, increase the construction cost of pavements. There are, however, considerable economies of scale in designing new pavements for higher traffic loadings. In the AASHTO pavement design procedures used by many states, a given percentage increase in traffic loadings can be accommodated by a much smaller increase in pavement thickness and costs. For example, increasing a rigid pavement from 9 to 10 inches in depth will approximately double the traffic loadings that can be accommodated by the pavement.

For existing pavements, increases in traffic loadings would affect pavement rehabilitation costs in two ways. First, an increase in traffic loadings would shorten the time interval to the next resurfacing. Moving resurfacing expenditures nearer to the present would increase the real cost for resurfacing because of the time value of money. If the funds required to resurface highways sooner were not available to highway agencies, pavement condition would worsen and, as discussed below, highway users would be subjected to added cost and discomfort. Second, at the time resurfacing is required, higher traffic loadings would either increase overlay thickness or require more frequent resurfacing in the future. However, for asphalt pavements, milling the rough surface can delay the need for resurfacing.

Costs for routine maintenance might also be affected by changes in traffic loadings. A pavement in new or very good condition requires relatively little expenditure for maintenance. As pavement condition worsens, however, expenditures for activities such as filling cracks and patching potholes increase. The effect of an increase in traffic on costs for routine maintenance would be relatively insignificant if resurfacing programs were expanded so that there was no change in times between overlays and terminal serviceabilities. However, if resurfacing programs were not expanded, the maintenance workload could be much greater than it was before the increase in traffic.

If traffic loadings are increased and highway agencies do not increase pavement-related expenditures to compensate for the increase, then pavement condition will deteriorate, in turn forcing users to travel over worse roads. Changes in pavement condition affect highway users by increasing vehicle repair cost and decreasing speed and fuel economy. Driver and passenger comfort are also affected by pavement condition, although there is no generally accepted way to quantify these effects. Further, highway users may suffer time delays during pavement resurfacing, reconstruction, rehabilitation, and maintenance. Such user costs should be included in a life cycle cost analysis of every major investment in pavements.

[continued on next page, 1.2 Truck Characteristics Affecting Pavements. (a) Axle Weights ]

[1] Much of this discussion is drawn from TRB Special 1 Report 225, Truck Weight Limits: Issues and Options. That study, which was published in 1990, included an extensive review of the literature on pavements in relation to TS&W policy.

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