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Iowa Demonstration Project: Improvements to the 24th Street–I-29/80 Interchange in Council Bluffs
A key aspect of HfL demonstration projects is quantifying, as much as possible, the value of the innovations deployed. This entails comparing the benefits and costs associated with the innovative project delivery approach adopted on an HfL project with those from a more traditional delivery approach on a project of similar size and scope. The latter type of project is referred to as a baseline case and is an important component of this economic analysis.
For this analysis, the Iowa DOT supplied most of the cost figures for the as–built project. The assumptions for the baseline case costs were determined from discussions with Iowa DOT and FHWA Iowa Division staff and national literature.
It is likely that standard phase construction methods would have been used to build the bridge to insure that two or three lanes were kept open to 24th Street traffic. Completely closing the bridge to 24th Street traffic and reconstructing the entire bridge would have been the least expensive option in terms of construction costs, but it would have been unacceptable to the surrounding businesses that rely heavily on the interchange. Nevertheless, the phased construction method with traditional cast–in–place construction techniques serves as an appropriate baseline for comparison with the as–built phased construction method using innovative contracting and construction materials and techniques.
Conventional construction methods would have negatively impacted the 24th Street interchange with construction–related congestion for an estimated 16 months. Shortened construction time limited the impact on traffic flow to less than 6 months.
Table 3 presents the differences in construction costs between the baseline and the as–built alternatives. All of the as–built costs were taken from the actual contract bid provided by the Iowa DOT. Baseline costs were determined on the basis of a study of bridge replacement options for this interchange by HDR Engineering, Inc. and in consultation with the Iowa DOT engineering staff by noting whether the itemized as–built costs would have applied to the baseline case. Adjustments were made to the cost categories as necessary. The baseline cost estimate is inexact, therefore, and the information presented is a subjective analysis of the likely cost differential rather than a rigorous computation of a cost differential.
It can be estimated from table 3 that the adoption of the HfL innovations (as–built scenario) to build the 24th Street interchange bridge resulted in an increased construction cost (less incentives) of $1,377,398 (12 percent) when compared with the baseline scenario.
Generally, three categories of user costs are used in an economic/life–cycle cost analysis: vehicle operating costs (VOC), delay costs, and safety–related costs. The cost differential in delay costs and safety costs were considered different enough to be included in a comparative analysis of cost differences between the baseline and as–built alternatives.
The impact on traffic for the baseline case is based on using traditional contracting methods and cast–in–place construction. It is estimated that $1,560,135 was saved as a direct result of accelerating the construction to only a single season. The following provides a basis for this conclusion:
As discussed earlier in this report, this interchange has experienced above–average occurrences of crashes over the past several years. Costs associated with the crashes that could have occurred during the project construction are detailed below.
Assumptions and data supporting the cost analysis are as follows:
Ullman et al1 investigated the safety of work zones for various scenarios: (1) crashes during daytime and nighttime work periods when lanes were closed and work was ongoing, (2) crashes when work was ongoing but no closures were required, and (3) crashes when no work was ongoing (the work zone was inactive). They concluded that crashes increased 60 to 66 percent (an average of 63 percent) when a traffic lane was closed day or night. Given this information and considering the traffic volumes and hourly traffic variations on this highway and the expected construction schedules, table 4 presents the number of vehicles that would have passed through the work zone for the as–built and baseline projects.
Table 4 shows that the total volume of traffic exposed to crash risk was much lower for the as–built case than the baseline case. The estimated increase in crashes for the baseline case can be computed as the product of (1) the historical crash rate for each type of crash (number of crashes per million vehicles), (2) the total volume of traffic exposed to the risk, and (3) the risk escalation factor associated with work zones (= 0.63, as discussed earlier). This is computed for the baseline case as follows:
The elevated risk noted above was monetized by assuming unit costs from Council et al2 for the various types of historical crashes reported by the Iowa DOT. The following mean comprehensive costs per crash for a rural highway with a posted traffic speed greater than or equal to 50 mi/h (80.4 km/h) and an arterial highway with a posted traffic speed less than 45 mi/h (72.4 km/h) were used in the analysis:
Table 5 presents the difference in safety costs for the baseline and as–built cases. It can be computed from the table that the total expected safety costs for the baseline case would have been $834,329 ($738,580 + 95,749) as opposed to no costs for the as–built case. The $834,329 total is essentially the safety benefit of the as–built case.
Construction costs (less incentives) for the 24th Street bridge would have likely placed the as–built construction at $1,377,398 (12 percent) more than the traditional delivery and construction methods. However, delivering the project in only one season saved users $1,560,135 in delay costs and $834,329 in safety costs. Using the innovative HfL project delivery approach saved an estimated $1,017,066. In other words, the innovative approach to this $12.7 million project had an 8 percent cost benefit over traditional methods.
1Ullman, G.L., M.D. Finley, J.E. Bryden, R. Srinivasan, and F.M. Council, Traffic Safety Evaluation of Nighttime and Daytime Work Zones (NCHRP Report 627),National Cooperative Highway Research Program, Transportation Research Board, Washington, DC, 2008.
2These costs were based on F. Council, E. Zaloshnja, T. Miller, and B. Persaud, Crash Cost Estimates by Maximum Police–Reported Injury Severity Within Selected Crash Geometries (FHWA–HRT–05–051), Federal Highway Administration, Washington, DC, October 2005.