Median Cross-Section Design for Rural Divided Highways (2014) / Chapter Skim
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5 C H A P T E R 2 2.1 AASHTO Median Design Guidelines The AASHTO Roadside Design Guide (2) and the AASHTO Policy on Geometric Design of Highways and Streets (commonly referred to as the Green Book)
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6greater than 9 m (30 ft) but less than 15 m (50 ft)
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7 Figure 2-2. Guidelines for median barriers on high-speed, fully controlled-access roadways from 2006 AASHTO Roadside Design Guide (2)
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8A roadside barrier may be required either to prevent errant vehicles from colliding with a fixed object in the median or to prevent vehicles from overturning when traversing the slope. Placement locations "b" and "d" in Figure 2-3 are at the edge of the inside (or median)
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9 2.2.1 Encroachment Studies Early median safety studies sought to determine and quantify factors that caused vehicle encroachments into the median area on divided highways. In the early 1960s, Hutchinson and Kennedy (4)
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10 enter the median area because another vehicle unexpectedly merges into the occupied lane. Hutchinson and Kennedy also examined the relationship between ADT and the percent of vehicles that crossed into the median.
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11 were the primary focus in the study were median width and median cross section. For the routes studied, variables such as pavement width and shoulder width remained constant.
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12 Relationships were developed between the type of collision and the relative effects of the median width. The type of accident most affected by the increase in median width was headon collisions.
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13 of the variation in CMC frequency. Interpretation of the ADT parameter (1.77)
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14 median barrier crashes. The outlined region also contains B/C ratios that exceed 1.0.
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15 Variable NB RENB NM Constant –1.551 (0.181)
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16 the economic analysis. The severity distributions for each of the four crash types are shown in Table 2-5.
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17 produced B/C ratios greater than 10.0. As such, divided highways in Zone No.
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18 Miaou et al.
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19 state's median design policy so that no more freeways could be built with median widths less than 21 m (70 ft)
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20 • The Florida Highway Patrol should have a line item on their accident report forms that clearly indicates a crossmedian crash. • The benefit of using shoulder rumble strips along shoulders adjacent to median barrier walls should be evaluated.
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21 frequency and severity. During the data collection period, it was estimated that between 300 and 750 cross-median collisions occurred on Florida highways.
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22 improvement projects.
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23 2.4.1 Cable Barrier In December 1996, the Oregon Department of Transportation installed two sections of cable barrier along I-5 between Salem and Portland to reduce cross-median collisions. Sposito and Johnston (26)
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24 words, this states that the severity index of a crash decreased after cable barrier was installed in the median. Davis and Pei (29)
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