The analysis included a test to determine the availability of redundant capacity of the roadway network - using the SARPC MATS model for the year 2035. This analysis was performed on a series of selected links to represent various travel patterns within the study area. While neither the scope nor the budget permitted redundancy testing of every link in the system, the project team was able to assess redundancy of certain types of links (for example, links that connect housing and commercial areas) and then extrapolate the redundancy to the other links of the same type. The approach included the following steps:
Findings from the analysis were used to develop a method for determining a score for redundancy for each link in the system. Those links whose loss resulted in LOS E or F conditions on the surrounding network were identified as having no system redundancy and therefore were assigned a score of 3. Those that had a medium effect were scored a 2 and those with little effect were scored a 1. Figure 12 depicts those network links that were tested to determine system resiliency.
A number of assumptions were needed to conduct the analysis. The MATS travel demand model produced daily trips, representing travel patterns in the study area; however, the question of system redundancy is more pertinent in the peak hours when the highest volumes are using the area's roadways. A methodology for conducting the test was discussed with SARPC staff, and the specific process for determining redundancy was developed.
The redundancy test made the following assumptions: the peak hour capacity for each link was equal to 9% of the identified daily capacity from the model; and the peak hour demand figure was equal to 10% of identified daily volume. These assumptions were identified in the forecasting model documentation, Mobile Area Transportation Study - 2030 Transportation Plan - Model Documentation.
Figure 12: Network Links Selected for Redundancy Test
Figure 13 shows the before and after conditions, depicting peak hour level of service calculations for the roadway network in the area around I-10 near McDonald Road. The loss of link capacity on the interstate in this example results in substantial delays in the remainder of the network in the area of the link tested. This link was identified as having limited to no system redundancy available and therefore was identified as a more critical link for this measure than other links in the system.
Findings from this analysis led to a development of an understanding of the capacity of various segments of the roadway network and identified those segments at a general level where loss of a segment could be absorbed by the remaining network.
Figure 13: Before and After Redundancy Test of I-10 South of McDonald Road