In order to determine the economic impacts of Michigan’s cable median barrier program, a benefit/cost (B/C) economic analysis was conducted. The B/C ratio is calculated by dividing the annual benefits (from crash severity reduction) by the annualized costs to install and maintain cable median barriers. It should be noted that the analysis does not include 2013 cable barrier installations because no after crash data was available for such installations, and, as such, the total mileage included in the analysis is 302.9 miles. The benefits were calculated using the expected average annual target crashes (and sum of injuries) for the before period obtained from the EB analysis and the average annual target crashes (and sum of injuries) observed in the after period. The benefits are calculated by multiplying the reduction (or increase) by the cost for each injury level, and the benefits were calculated for both economic and comprehensive costs (as shown in Table 26). It should be noted that the costs for PDO/C crashes and K/A injuries were blended using weighted averages. This is consistent with the methodology used by MDOT for economic analyses of safety initiatives. These blended costs, along with the results of the benefit/cost analysis are shown in Table 27. It should be noted that the total average annual
number of crashes does not match the total average annual number of injuries because it is possible to have multiple injuries in one crash.
Table 27. Summary of Benefit/Cost Analysis
Injury Severity Expected Annual Crashes/ Injuries After Installation (from EB estimate) Observed Annual Crashes/ Injuries After Installation Blended Economic Costs of Crashes/ Injuries ($) Blended Comprehensive Costs of Crashes/ Injuries ($) PDO/C 496.8 1233.4 8,900 6,548 B 77.4 79.4 23,400 58,700 K/A 49.7 30.5 278,878 894,186 Economic Factors Annualized Amounts Installation Costs $3,159,789 Maintenance Costs $1,115,034
Economic Crash Cost Savings (Benefit) -$1,248,025 Comprehensive Crash Cost Savings (Benefit) $12,227,714
Benefit/Cost Ratio (Economic Costs) -0.29
Benefit/Cost Ratio (Comprehensive Costs) 2.86
In order to annualize the total installation costs, an appropriate discount rate and analysis period must be determined. MDOT recently used a discount rate of 2.7 percent for an economic analysis of their highway program (52), however the Federal Highway Administration (FHWA) recommends using discount rates ranging from 3 percent to 7 percent (53). Accordingly, a discount rate of 3 percent was adopted for the B/C economic analysis of cable median barriers in Michigan which is close to the 2.7 percent recently used by MDOT but also falls within the FHWA recommended range. A discount rate of 3 percent was also used in a past B/C economic
analysis of cable median barriers in Wisconsin (54). An analysis period of 20 years was chosen, which is conservative as this is less than the typical service life of a roadway (25-30 years). A 20-year analysis period was also used in the economic analysis of cable median barriers in Wisconsin (54).
With a discount rate of 3 percent and an analysis period of 20 years, the capital recovery factor (CRF) which is applied in order annualize the initial costs of installing the cable median barriers was found to be:
CRF (i=3%, n=20 yrs) = 0.0672
Therefore, the annualized cost of installation was ($47,020,662.95 x 0.0672) = $3,159,788.60
The annual maintenance costs were determined by multiplying the total average annual number of crashes in the after period by the average cost per cable barrier repair after a crash:
Annual Maintenance/Repair Costs: 1,314 crashes x $848.58 per repair = $1,115,034.12
The total annual cost for the cable barriers was then found by summing the annualized installation costs and the annual maintenance/repair costs:
Total Annual Cost: $3,159,788.60 + $1,115,034.12 = $4,274,822.60 per year
The B/C Ratios were then calculated:
B/C (Economic Crash Costs) = -$1,248,025/$4,274,821 = -0.29 B/C (Comprehensive Crash Costs) = $12,227,714/$4,274,821 = 2.86
When considering economic crash costs, the B/C ratio was less than 1.0, indicating the reduction in severe injuries did not outweigh the costs of installation, maintenance, and increase in PDO and minor injury crashes. However, when the B/C ratio was calculated assuming comprehensive crash costs as recommended by the NSC for the purposes of a cost-benefit analysis (51), the resulting B/C ratio was 2.86-to-1. Ultimately, these results indicate that the installation of cable median barriers has proven cost-effective through the substantial reductions in fatal and incapacitating injuries when comprehensive crash costs are considered (as
recommended by the NSC).
4.10 Cable Median Barrier Installation Guidelines
One of the primary emphases of this study was to develop guidelines to assist the
Michigan Department of Transportation (MDOT) in the prioritization of candidate locations for the installation of cable median barrier. State agencies generally install median barrier on the bases of: (a) historical data for median-involved crashes; or, (b) segment-specific data for traffic volume and median width. In the latter case, guidelines have been developed such as those presented in the AASHTO Roadside Design Guide (3). AASHTO recommends barrier installation on roads with median widths less than 30 feet and an annual average daily traffic (AADT) volume greater than 20,000 vehicles (3). AAHSTO also suggests that barrier
installation be considered on roads with medians of up to 50 feet and similar traffic volumes. Barrier installation is considered optional on roadways with AADT of less than 20,000 vehicles or with median widths beyond 50 feet.
Recent research suggests that barrier installation may be warranted across a wider range of median configurations (24). The results of these studies, coupled with state-specific concerns such as high levels of annual snowfall, motivated the development of guidelines for barrier installation in the state of Michigan. For the purposes of this project, six primary factors were considered as screening criteria for assessing the suitability of high-tension cable as a median barrier alternative:
Average daily traffic (ADT); Median width;
Number of lanes;
Lateral offset of the barrier from the travel lane; Annual snowfall; and
Horizontal curvature
Using these criteria, guidelines were developed such that a stepwise procedure can be utilized to: 1. Estimate the expected annual number of target (i.e., median-involved) crashes for a given
freeway segment where no barrier currently exists;
2. Estimate the expected annual number of target crashes following cable barrier installation; and