REV05249 - Laserfiche WebLink

Home Browse Search

II 1 CX~~~1} reduced to a terminal v~e (e.g., p, equal to 2.5 or 2.0). If the~ffic is underestimated, the actual time to p, will probably be less than the predicted performance period, thereby resulting in increased maintenance and rehabilitation costs. The maximum performance period to be used in designing for a particular pavement type should reflect agency experience. The performance period and corresponding design traffic should reflect real-life experience. The performance period should not be confused with the pavementlife. The pavement life may be extended by periodic rehabilitation of the surface or pavement structure. In the case of a new roadway a traffic assignment is based on: 1) Historic records of traffic volumes on comparable types of roadways. 2) The percentage of single unit and combination trucks. The equivalent loads derived from the many traffic prediction procedures represent the totals for all lanes for both directions of travel. This traffic must be distributed by direction and by lanes for design purposes. Directional distribution is usually made by assigning 50 percent of the traffic to each direction, unless available measured traffic data warrant some other distribution. In regard to lane distribution, 100 percent of the traffic in one direction is often assigned to each of the lanes in that direction for purposes of structural design if measured distributions are not available. For purposes of pavement structure design, it is necessary to estimate the cumulative number of 18-kip equivalent single axle loads (ESAL's) for the design (performance) period. The number of ESAL's may or may not be proportional to the average daily traffic. Truck traffic is the essential information required to calculate ESAL's; it is therefore very important to correctly estimate future truck traffic for the facilify during fhe design p f truck traffic on a pavement can be dramatic. Tests have shown that a si gle-unit, fully loaded, 000 pound truck can cause pavement damage equivalent to that cawed by 6,000 automobiles. his illustrates why carefully made estimates of expected traffic are critical to proper pavement d sign. The accepted traffic analysis procedure currently'used-his under review by both Federal Highway A ministra ion H an the Department. Some changes may result, but for this edition of the Design Guide the parameters used to determine the traffic volume for design purposes is based on current methods. Based on these current methods, several important parameters need to be determined: • Design Traffic - is determined based on the traffic volume on comparable types of pavement and types and numbers of trucks expected to operate on the roadway over the performance period of the facility (usually twenty but sometimes ten years). • Average Daily Traffic (ADT) -the amount of daily traffic determined for the first year p{us the projected average amount of daily traffic for the last year of the design period divided by two (ADT for the midpoint of the design period). .~-.