Relative Importance within Each Decision Parameter

section is compared to another. For example, when does a section become more important in terms of traffic volume? Is a section with 1,500 vehicles per day equally as important as a section with 4,000 vehicles per day, and how much more important is a section with 15,000 vehicles per day compared with a section that has only 750 vehicles per day? The same questions arise for each one of the decision parameters and represent the third level of the hierarchy in Figure 4.1.

To make these comparisons, each section must be compared to every other section for each decision parameter. Normally the AHP is applied to a fairly small number of competing alternatives, say 15 alternatives. A matrix of this size is easily completed in a short period of time, however to apply the method to a pavement network, the number of comparisons would be quite large and completing the matrix through personnel interviews by individually comparing each section to every other

section is not feasible, economical, or reasonable. Case in point, the pavement network used in the study includes the on-system roadways in Robertson, Leon, and Freestone counties in the Bryan district, consisting of 2349 sections. These section versus section pairwise comparisons are made through the use of logic statements, resulting in a 2349 element vector. This vector is the ranking of each section in regard to that specific decision parameter. Ultimately these vectors will have the decision parameter weights applied and carried through to create a 2349 element vector that is the project selection number or ranking of each section based on the research method. By using logic statements to compare each section, there is no further need to test consistency. The AHP uses consistency calculation to ensure that human comparisons do not deviate to a point that makes the comparisons invalid for additional use, however by using logic statements coded in computation tools, consistency can be assumed.

All of the section information regarding each decision parameter must be stratified so that logic statements can be written. These stratifications of the data are illustrated in Table 4.6.

Table 4.6. AHP Weight Associated with Minimum Comparison

AHP

Weight Visual Distress (DN) Current ADT (veh/day)

Current FM Truck ADT (trucks/day

Current Non-FM Truck ADT (trucks/day) 1 DN = 0.2629 veh/day ≤ 1000 trucks/day ≤ 160 trucks/day ≤ 1225

2 0.2629 < DN ≤ 0.433 NA NA NA

3 0.433 < DN ≤ 0.603 1000 < veh/day ≤ 2000 160 < trucks/day ≤ 320 1225 < trucks/day ≤ 2450

4 0.603 < DN ≤ 0.773 NA NA NA

5 0.733 < DN ≤ 0.943 2000 < veh/day ≤ 7000 320 < trucks/day ≤ 480 2450 < trucks/day ≤ 3675

6 0.943 < DN ≤ 1.113 NA NA NA

7 1.113 < DN ≤ 1.283 7000 < veh/day ≤ 10,000 480 < trucks/day ≤ 640 3675 < trucks/day ≤ 4900

8 1.283 < DN ≤ 1.45 NA NA NA

9 1.45 < DN 10,000 < veh/day 640 < trucks/day 4900 < trucks/day AHP

Weight Condition Score (CS) FM Ride Quality (IRI)

Non-FM Ride Qualtiy

By using the term minimum comparison in Table 4.6, it is meant that this sets the initial AHP weight of a section. For example, if a section has a Condition Score of 40 and is compared to a section with a condition score of 100, the section with a 40 would receive an AHP weight of seven. What is not displayed in Table 4.6 is the weight assigned when a section with a Condition Score of 40 is compared to a section with a Condition Score of 75. This is done with a logic statement by merely subtracting the two sections’ weights when each is compared to a minimum, thus receiving the weight in Table 4.6. It is important to note that one must be added to the result of this subtraction to preserve the fact that the AHP begins at one rather than zero.

The visual distress number is a unique parameter in the sense the weights had to be applied to the various distresses considered at the district level and each section must

compete against every other section in regards to how important more or less distress was and how did that importance change as multiple distresses were evaluated at the same time. To solve this type of problem, the same hierarchical approach was used as with the project selection number and the AHP was applied to determine a distress number that could be used as the visual distress component. This is discussed in further detail, but before explaining the creation of the distress number the stratification of the other parameters is discussed.

The first point to mention is that ride quality and truck ADT are evaluated differently for the type of roadway. This division was based on a suggestion by the Bryan decision maker and was made using the fact that many of the FM systems with the Bryan district contain a pavement structure that consists of flexible base with a seal coat as the riding surface. The assumption is made that fewer trucks will do more damage to a pavement structure such as this than the same amount on a more extensive pavement design. For ride quality, the basis for the division is that applying a hot-mix asphalt riding surface allows a contractor more control over deviations in the surface and thus roughness should be distinguished differently between the two. The district holds to this thought for the application of monetary penalties, at a higher IRI value for roadways having only a surface treatment as the riding surface. The further stratification of ride quality was done based on the ride specification in the Standard Specifications and additional TxDOT guidance on smoothness defining an IRI of 1 to 59 as very smooth, 60 to 119 as smooth, 120 to 170 as medium rough, 171-220 as rough, and 221-950 as very rough. The Bryan decision maker had never thought of truck ADT in a

sense of when to determine if a section is more or less important. Therefore the truck ADT breakdown was based on the data available within the analyzed network. The maximum AHP weight of nine was assigned to all truck volumes in the highest two percent. The further assignment of weights proceeded linearly by placing the truck volume in equal bins for weight assignment. This assignment of weight based on truck ADT could be adjusted based on the damage caused by trucks to a certain pavement design if the information were readily available and trustworthy. This type of flexibility within the AHP makes it attractive for the decision making process, particularly with the knowledge that these parameters will change as more information becomes available or as research warrants.

Current ADT was broken down based on information from the Bryan decision maker as to how traffic is currently viewed and handled for preservation projects.

Condition Score was stratified based on the current descriptions used by TxDOT to describe scores. Lastly, maintenance costs were placed in equal bins and a linearity of importance was assumed to the point that a nine was assigned when the cost exceeded 98 percent of the costs experienced by other sections. It is hard to verify the validity of the assumption, but the point should be made that the new method is trying to capture how decision makers currently make decisions and intuitively it can be stated that if a section has more maintenance money spent on it than another section it is more important from a preservation standpoint and the linear assumption captures this thought. The intermediate AHP values were used for maintenance cost to close the gaps between dollar amounts, eliminating broad cost amounts receiving equal weight.