§ 3.4.9. Design Constraint Data

These input parameters are used to identify specific constraints on the pavement designs generated by Municipal Flexible Pavement Design and Municipal Rigid Pavement Design. These constraints can be used to ensure the consideration of only practical design alternatives within specific financial and confidence requirements.

A.

Confidence Level

Many of the design factors (as well as the actual design models in Municipal Flexible Pavement Design and Municipal Rigid Pavement Design) have a certain amount of variability and uncertainty associated with them. Thus, depending on the extent of these variations, there is a corresponding amount of uncertainty in the program design predictions. To ensure that this inherent variability is considered within the program, a confidence level factor is incorporated. The confidence level is similar to a factor of safety since it provides increased reliability in the design. For example, a pavement design with a 90% confidence level has a statistical 90% probability of lasting the period for which it was designed. The recommended input value for all streets is 90% (or a Confidence Level Index of five). Figure 3-8 in Appendix H of this manual provides an illustration of the effect that confidence level can have on required slab thickness or structural number.

B.

Length of Analysis Period

This input variable defines the minimum time which a design must last. The City Street Design Policy requires a minimum 20 year analysis period. Caution is recommended when considering longer analysis periods, since a great disparity between initial construction design strategies and predominantly stage construction alternatives could develop. This is discussed in more detail under "Discount Rate."

C.

Minimum Time to First Overlay

This design constraint ensures that no design strategy will be generated in which an overlay is placed too soon after initial pavement construction. The City of Austin design policy for the minimum time to the first overlay is 20 years. A program design option is for the first overlay at ten years with a maximum of two overlays prior to the expiration of the 20th year. This option may be used for a public street in a private development; however, the developer shall be required to post the cost of any overlays) needed prior to the 21st year with the appropriate road authority. The funds shall be escrowed to ensure overlays at the design time intervals.

D.

Minimum Time Between Overlays

This constraint ensures that the timing between two consecutive overlays is practical and meets the City of Austin budget constraints. The minimum value for this constraint is five years.

E.

Maximum Thickness of Initial Construction

This constraint limits the thickness of initial pavement construction. The total pavement thickness includes the thicknesses of all layers down to the natural soil subgrade. Strategies with layer combinations which exceed this maximum are not considered by either program. Underground utilities are an example of a factor which can influence this constraint.

F.

Maximum Funds Available for Initial Construction

This constraint ensures that no designs will be considered in which the total initial construction cost exceeds a certain user-specified amount. This is typically not a problem since very few city street designs are generated in which the initial cost is prohibitive. Thus, a value of $50 per square yard is normally used.

G.

Discount Rate

This economic factor accounts for the estimated difference between present costs and costs incurred in the future. Any future cost is brought back to net present value using the discount rate and the projected year in which the cost is incurred. A value of 5% is used by the City for street design projects.

The value of the discount rate may be estimated based on the projected interest rates and the time value of money to the City. Consideration of inflation is not recommended since it generally affects the City's yearly budget as well as the contractor's construction cost. In other words, the City will be paying inflated costs using inflated dollars.

A consideration which should be recognized, however, is that the discount rate can have a very significant effect on stage construction versus predominantly initial construction strategies. For high discount rates and/or lengthy analysis periods, the costs associated with future activities (when brought back to new present value) can be a small fraction of the present cost for the same activities. The result is an emphasis on the selection of stage construction strategies or those which have heavy construction deferred until the future. Although such alternatives may actually be the best, it is important to realize that their costs have much more uncertainty than costs associated with initial construction strategies.