3.5.2 Traffic Loading
Perhaps the most important step in designing a pavement is the estimation of the
design traffic. Overestimation of the design traffic results in a thicker pavement than
necessary with associated higher costs. Underestimation of traffic results in a thin
pavement that will fail prematurely, resulting in higher maintenance and user costs. If
the proposed pavement will be used to replace an existing pavement, the design traffic
could be a projection of the existing traffic. If the proposed pavement is a new location,
the design traffic will have to be estimated on the basis of the proposed use of the
pavement. For design purposes, all traffic is equated to an equivalent 18-kip (80-kN)
single-axle load, or ESAL. Each vehicle in the expected design traffic volume is converted
to an ESAL by an equivalency factor. The equivalency factor is a function of the axle
loading, pavement thickness, axle configuration, and terminal serviceability. As discussed
in Art. 3.6, the terminal serviceability is an in