The traffic on a road is composed of applications of axle loads of various weights and configurations. The weights range from less than 2,000 lbs on a single axle to over 50,000 lbs on multiple axles. To determine the load effect on pavement during a particular design period, the designer must obtain an estimate of the weight distribution of various axle configurations over the design period.

One method of estimating the traffic effect is to use the Average Daily Traffic (ADT) and/or the number of daily heavy trucks (HCADT) expected. The two-way ADT and HCADT are the traffic factors used for the current Soil Factor and R-value flexible design procedures. To determine the design lane traffic for a two-lane road, the total ADT is divided by two unless some other factors are known. For four or greater numbers of lanes, the traffic must be proportioned by lane.

Portable weigh-in-motion devices can be used to accurately measure ADT for a given roadway. Researchers at the University of Minnesota and Mn/DOT are developing a new method for assessing traffic on low-volume roads. Contact Dr. Gene Skok at skokx003@tc.umn.edu or (651) 779-5525 for additional information.

The equivalent single axle load (ESAL) calculations requires an estimation of the total traffic in the design lane. Most ADT values are two-way. The second factor required to calculate ESALs is vehicle type distribution. Vehicles are classified using ten vehicle types, which can be found in the Mn/DOT Road Design Manual Table 7-5.03A. Mn/DOT Road Design Manual Table 7-5.03B lists typical distributions for rural, metro, and local roads.

For information on calculating ESALs for use in pavement design, click here.

The Mn/DOT Traffic Office can provide assistance with collecting traffic data.

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