Identification and Evaluation of the Cost-Effectiveness of Highway Design Features to Reduce Nonrecurrent Congestion (2014) / Chapter Skim
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Pages 40-52
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From page 40... ...
40 The objective of the safety analysis was to estimate, in quantitative terms, the safety effectiveness for each treatment of interest. Design treatments to reduce nonrecurrent congestion have two potential effects on safety for the highway facilities on which they are implemented: • Design treatments may have a direct effect on crash frequency or severity if they affect the speeds or lateral positions of vehicles.
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41 Tables 5.1 and 5.2 can be used by users of the Analysis Tool to determine the direct effects of changing outside or inside shoulder width on safety. The design treatments to which these effects potentially apply are as follows: • Accessible shoulder 44 Inside shoulder width 44 Outside shoulder width • Drivable shoulder 44 Inside shoulder width 44 Outside shoulder width • Alternating shoulder 44 Inside shoulder width 44 Outside shoulder width Development of Congestion– Safety relationship The reduction of congestion through application of design treatments or intelligent transportation system improvements has been widely thought to have a positive effect on safety, but this relationship has not been well quantified in previous research.
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42 analysis purposes, the freeway system was divided into directional segments, usually extending from one interchange to the next. The sections were selected so that a given detector would be representative of the traffic conditions for all crashes within that section.
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43 15-min volume during a particular hour, as is commonly used in HCM procedures. As specified in the HCM, six LOS categories are assigned by traffic density ranges as follows: • LOS A: 0 to 11 passenger cars per mile per lane (pc/mi/ lane)
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44 within each of the 18 LOS categories for each metropolitan area. The resulting pairs of data points are plotted by severity level and metropolitan area in Figures 5.1 and 5.2.
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46 upward)
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47 Table 5.6. Crash Type Distribution for Seattle and Minneapolis–St.
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48 Figure 5.3. Crash rate versus density for combined Seattle and Minneapolis–St.
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49 known because it is, by definition, equal to 1.00. As shown in Figure 5.5, these five percentile values can be plotted to estimate the cumulative TTI curve.
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50 where PDOCRsubset i is the PDO crash rate (crashes/MVMT) for subset i.
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51 estimation of Crash Distributions by hour of Day Chapter 23 of the 2000 Highway Capacity Manual provides a methodology for estimating freeway operating speed (3)
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52 where NChour i is the predicted total number of crashes for hour i (crashes/year) , and Length is the length of the freeway segment (mi)
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