The concept of reliability as applied to pavement design can be defined as the probability that the pavement will perform as intended under the design traffic loading, and other crucial design inputs (material properties, environmental factors, etc.).

Reliability for pavements designed using the department’s recommended design programs is related to maintaining serviceability at or above the specified minimum (terminal) serviceability index throughout the desired performance period or design life. The department generally uses a reliability of 90 to 95% for rigid pavements and higher volume flexible pavements. Lower reliability, consistent with managed risks, may be appropriate for low to medium volume flexible pavements. For any given pavement structure, the serviceability at the end of this period is typically assumed to have a normal distribution with a mean and standard deviation.

The critical design inputs will have some variability associated with them; the assigned modulus values for the subgrade and other materials used in the structure, the construction process (in-place densities, layer thickness, etc.), traffic load predictions, and the design equation itself all have variability. Generally, the higher the reliability of a designed structure, the thicker the structure will be. Otherwise, the thickness must be offset using higher quality or stabilized materials. Since the assigned confidence level adjusts the calculations to account for material variability in the agglomeration of all inputs, including traffic, select an average modulus (based on specific material type, considering seasonal variation) for all inputs.