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The Gulf Coast Study, Phase 2

Appendix B Methodology Applied to Test System Redundancy


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:

  1. Identification of links in the network that function as important connectors and represent a cross-section of roads of the same functional classification and general location. Links were chosen through a series of iterative steps, including:
    1. a) Identification of major roads servicing key facilities or economic centers.
    2. b) Identification of links both within and outside of the more heavily developed area, with the area east of University Boulevard being considered more developed than the area west of it.
    3. c) Identification of links that are representative of specific types of links (for example, links that are part of the arterial grid, segments that link housing and commercial areas, etc.)
  2. For each selected network link, testing of the loss of that link by removing the capacity to travel that link.
  3. Determination of whether the remainder of the network can function effectively, in terms of volume over capacity ratios during peak periods, or whether the impact is such that the remaining network could be considered to be at a condition where travel would be significantly affected.
  4. Extrapolating the results of the tested links across the entire network to determine where redundancy exists. For example, the redundancy test for the representative link connecting housing and commercial areas indicated that this link was/was not highly redundant. Therefore, other links that connect housing and commercial areas in the same geographic area were also given a designation of highly redundant.

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 12 is a map of Mobile County and the roadway links in the county. Removed links in the redundancy test are indicated by pairs of aqua-blue dots denoting the nodal endpoints. There are 18 pairs of nodal endpoints.

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 consists of two maps of the study area. The map on the left shows the Mobile highway system before the redundancy test, while the map on the right shows the highway system after the redundancy test. The V/C ratio for the highway segments is indicated by different colors. The V/C ratio is calculated as Peak Hour Volume divided by Peak Hour Capacity. Red indicates a V/C ratio of greater than one, or where the peak hour volume exceeds the capacity. A V/C ratio of 0.9 to 1 is indicated by orange. A V/C ratio of 0.7 to 0.9 is indicated by yellow, and a V/C ratio of less than 0.7 is indicated by green. The maps illustrate that after the redundancy test, the red and orange highway segments increase , primarily in the southwest quadrant of the county.

Figure 13: Before and After Redundancy Test of I-10 South of McDonald Road

Updated: 3/27/2014
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