Evaluation of the 13 Controlling Criteria for Geometric Design (2014) / Chapter Skim
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Pages 49-61
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From page 49... ...
49 S E C T I O N 4 This section of the report presents the results of research performed as part of NCHRP Project 17-53 to expand knowledge of the traffic operational and safety effects of the 13 controlling criteria. These research results have also been incorporated in the summary of traffic operational and safety effects presented in Section 2 of this report.
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50 is nearly zero. By contrast, previous research conducted by Potts et al.
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51 reasonable, accompanied with design guidance emphasizing the importance of bicycle and heavy vehicle considerations. 4.3 Safety Effects of Bridge Width on Rural Two-Lane Highways Existing guidance suggests that the known safety effects for lane and shoulder width (i.e., the safety effects of lane and shoulder width specified in the HSM)
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52 observed differences are not statistically significant. Thus, there is no evidence that there would, in general, be any documentable safety benefit from widening a rural two-lane highway bridge with a roadway narrower than its approach.
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53 where Speedcurve = Speed of vehicle on horizontal curve (mph) Speedapproach = Speed of vehicle on tangent approaching curve (mph)
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54 speed and curve speed increases. In other words, drivers have to reduce their speed to navigate the curve more for sharper curves than for flatter curves.
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55 rural four-lane divided roadways and rural freeways are the following: N exp b b ln AADT b L I b ln 2 5730 R I (41)
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56 These results imply the following CMFs for horizontal curvature: • For fatal-and-injury crashes on rural four-lane divided highways, CMF exp 0.87L 0.22 ln 2 5730 R (43)
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57 Prior to statistical modeling, the parameters of interest were assessed for extreme values (both high and low) ; this was done using a combination of plots of crash rates per MVMT versus selected parameters and distributions of the individual parameters.
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58 It is disappointing that the percent-grade variable was statistically significant in the model for property-damageonly crashes, but not for fatal-and-injury crashes. With this inconsistency, it does not appear that the models presented in Tables 61 and 63 can be used to represent the effect of percent grade on crashes for rural multilane divided highways.
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From page 59... ...
59 Parameter Estimate Standard error 95% Lower confidence limit 95% Upper confidence limit Chi-square Significance level Fatal-and-Injury Crashes per Mile per Year -- Type 1 Crest Vertical Curves and Level Roadways Intercept –6.96 1.40 –9.70 –4.21 n/a n/a ln(AADT) 0.76 0.15 0.48 1.05 25.35 <.001 Vertical curve length (mi)
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From page 60... ...
Feature present Number of sites Combined length (mi) Number of crashes Average AADT Exposure (MVMT)
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From page 61... ...
61 half of each table are for Type 1 crest vertical curves with stopping sight distance greater than the AASHTO stopping sight distance design criteria, and crash rates in the lower half of each table are for Type 1 crest vertical curves that have stopping sight distance less than the AASHTO design criteria. The data shown in Table 66 indicate that crash rates for all crest vertical curves with stopping sight distance less than AASHTO stopping sight distance criteria are, on average, 27 percent higher than those for all curves with stopping sight distance greater than AASHTO stopping sight distance criteria for vertical curves (1.38 versus 1.09 crashes per million vehiclemiles of travel)
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