Pavement designers use estimates of the volume and mix of vehicles anticipated to use a road in designing a road pavement. Calculations convert current and forecast average daily traffic, or ADT, on a road segment to an equivalent single axle load, or ESAL, that equals the load of a single pass of an 18,000-pound truck axle load. The Iowa Department of Transportation’s pavement design standards include procedures for estimating the ESAL for two-lane and urban multi-lane roads. Follow these steps to estimate the ESAL for two-lane, two-way asphalt roads.
Assemble current and 20-year forecast of ADT data and the percentage of trucks. Forecast traffic volumes will come from a formal traffic forecasting process, or from growth factoring current traffic volumes.
Select a design year ADT. The Iowa DOT procedures suggest the design year ADT should be the current ADT plus 70 to 80 percent of the 20-year forecast increase in ADT.
Calculate the number of trucks in the design year ADT by multiplying the design year ADT by the percentage of trucks. For example, if the design year ADT is 2,000 vehicles per day and the percent of trucks is estimated at 10 percent, the number of trucks in the design year is 2,000 x 10 percent, which equals 200 trucks per day.
Apply an ESAL factor to the number of trucks. The Iowa DOT provides a range for this factor--0.40 for low-volume roads to 0.50 for heavy-truck routes.
Compute the number of ESALs per day by multiplying the number of trucks in the design year by the ESAL factor. For our example, if the appropriate ESAL factor is 0.50, the number of ESALs per day is: 200 trucks per day x 0.50, which equals 100 ESALs per day.
Annual ESALs are computed by multiplying the daily ESAL by 300 for low-volume roads or 365 for high-volume roads.
Compute the 20-year ESAL by multiplying the annual ESALs by 20. This number is then input into pavement design calculations.
Calculations for multi-lane urban roadways are more complex. These calculations involve determining the loadings in a “design lane” of the road. Axle load calculations consider a broader range of truck sizes and weights. Adjustment factors differ for asphalt and concrete pavements. Pavement design computer programs are available for speeding calculations and for evaluating design alternatives.