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Pages 41-56

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From page 41...
... 41   C H A P T E R 5 5.1 Median Barrier Guidelines The main reason for using a median barrier is to minimize the chance of a vehicle fully crossing the median and striking or being struck by a vehicle in the opposing lanes of traffic. Likewise, median barriers are only considered for medians where roadside barriers are not needed for clear-zone reasons (e.g., shielding is not needed for either fixed objects or terrain features)
From page 42...
... 42 Selection and Placement Guidelines for Test Level 2 Through Test Level 5 Median Barriers Applying the NCHRP Project 15-65 methodology explained earlier, the frequency of KA crashes for an unshielded median (OUTCOMECMC) and the shielded median (OUTCOMEBAR+CMC)
From page 43...
... Guidelines 43   Figure 23. Guidelines for median barrier need determination and material selection.
From page 44...
... 44 Selection and Placement Guidelines for Test Level 2 Through Test Level 5 Median Barriers 40,000 vehicles/day, as shown in Figure 23. On the other hand, a 50-ft-wide traversable median on a controlled-access highway with bi-directional traffic volume in the design year of only 10,000 vehicles/day is better left with no median barrier.
From page 45...
... Guidelines 45   Recalling Equation 2, the OUTCOME for terrain and the OUTCOME for longitudinal barrier shielding terrain with and without fixed objects are as follows: ( )
From page 46...
... 46 Selection and Placement Guidelines for Test Level 2 Through Test Level 5 Median Barriers )
From page 47...
... Guidelines 47   risk of a KA crash, even for foreslopes as steep as 2:1. There is no question that a rollover is a more severe crash than a crash with a W-beam guardrail.
From page 48...
... 48 Selection and Placement Guidelines for Test Level 2 Through Test Level 5 Median Barriers Figure 24 defines the variables used. Notice that the lateral distance to the back of the feature (WB FO)
From page 49...
... Guidelines 49   If a row of multiple narrow fixed objects is less than this distance, the multiple narrow fixed objects can be considered one object where the length of the fixed object is the sum of all the spacings and the length of the objects. For example, a row of five utility poles spaced 200 ft apart is considered a single 800-ft-long (i.e., 200[5 − 1]
From page 50...
... 50 Selection and Placement Guidelines for Test Level 2 Through Test Level 5 Median Barriers Figure 26. Evaluating shielding for multiple fixed objects.
From page 51...
... Guidelines 51   like a bridge pier. For example, the lower-left portion of Figure 25 indicates that a 10-ft-wide canal (WB FO – WF FO =10 ft)
From page 52...
... 52 Selection and Placement Guidelines for Test Level 2 Through Test Level 5 Median Barriers where BCR = The benefit–cost ratio of the barrier alternative with respect to the null alternative. ORALT/NULL = The outcome reduction is the estimated difference in the annual frequency of fatal and serious injury crashes for the shielded median (ALT alternative)
From page 53...
... Guidelines 53   hand, if a cable median barrier were installed on a four-lane divided highway with 45,000 veh/day and a 45-ft-wide median, Table 13 indicates that 0.0002 fewer KA CMCs can be expected annually. In this case, the cable median barrier is risk-beneficial although it may or may not be cost-beneficial.
From page 54...
... 54 Selection and Placement Guidelines for Test Level 2 Through Test Level 5 Median Barriers highway agencies will determine appropriate values for the rate of return (i) and design life (n)
From page 55...
... Guidelines 55   ( ) = +     = +     = − +     = /> • • • • • • • • • BCR OR C VSL DC AP MC BCR OR C VSL DC AP MC BCR 1 0.75 0.33 12,300,000 125,000 0.0446 2,500 0.1 1 ALT/NULL KA ALT i,n ALT ALT NULL KA ALT i,n ALT 5.3.2 Cost-Effectiveness Analysis Cost-effectiveness analysis is very similar to benefit–cost analysis but instead of monetizing benefits, the outcome itself (i.e., the annual reduction in KA crashes)
From page 56...
... 56 Selection and Placement Guidelines for Test Level 2 Through Test Level 5 Median Barriers AADT increases to 60,000 veh/day, the number of KA crashes avoided is 0.0021 from Table 13, and the ICER is: = +   = • ICER 125,000 0.0446 2,500 0.0021 $38 million per KA crash avoidedCABLE This is just a little less than the KA crash cost, so it is likely a reasonable expenditure. Notice that these were also the conditions that resulted in a BCR = 1.

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