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Reliability (R) is the probability that the pavement section designed using the design <br />method will perform satisfactorily for the traffic and environmental conditions that <br />actually occur during the design period. Use the following values: <br />Street Classification Design Reliability <br />Arterials 90 Percent <br />Collectors 190 Percent <br />Neighborhood Collectors and Local streets 95 Percent <br />Overall standard deviation (S) accounts for the designer's ability to estimate the <br />variation in the equivalent axle loads. Use the following value: <br />S = 0.5 for flexible pavement <br />Subgrade support (Mr) is measured by the soil resilient modulus, the strength of the <br />soil. Follow the 1993 AASHTO Guide for correlations of the Mr to other soil properties. <br />Soil support values shall be determined from soil tests, however, an "R" value of 5 or <br />a Mr value of 3775 will be accepted for design purposes without verification by soil <br />testing. Soil support values may also be back calculated from falling weight <br />deflectometer (FWD) tests. <br />Drainage coefficient, m; is the effect of moisture in the layer of material. The following <br />values shall be used: <br />Crushed rock 1.0 <br />Sand, gravel 0.6 <br />Layer coefficient (a#) is the contribution of the pavement layer to the structural <br />number per inch of thickness. Use the following coefficients for pavement design. <br />Subbase coefficient (a3) <br />Crushed rock 0.08 <br />Lime -treated soil 0.11 <br />Cement -treated soil 0.11 <br />Base coefficient (a2) <br />Crushed rock 0.10 <br />In-place cement treated base (ICTB)* 0.16 <br />Plant mix cement treated base (CTB) 0.22 <br />Public Improvement Design Standards Manual, 2016 Page 2-32 <br />