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APPENDIX H
NCHRP 3-74 Appendix H Selection of Speed Reduction Treatments at High-Speed Intersections Secondary Analysis Kittelson & Associates, Inc. Page H-1 Appendix H Secondary Analysis of 85th-Percentile Speeds This appendix presents the results of the 85th-percentile speeds analysis: ⢠By intersection approach (to match level 2 analysis of means). ⢠Across all intersection approaches with the same treatment (to match level 3 analysis of means), summarized in Exhibit H-2. In each case, the dependent variable of interest was the paired difference between the after minus the before 85th-percentile speeds at each location at each site. ANALYSIS BY INTERSECTION APPROACH Exhibit H-1 presents the results of the treatment effect on 85th-percentile speeds, separately for each intersection approach (across Locations A, B, C, and D). Here, the results are based on a paired t-test at each site where each location contributes a single paired difference. Columns 5 and 6 show the estimated speed difference in 85th- percentile speeds and its standard error. The last two columns show the results of the paired t-test with the last column indicating statistical significance of the treatment: a p- value of 0.05 or less (highlighted in yellow and bold) will indicate a significant effect at that intersection approach at the 5-percent significance level. Exhibit H-1. Treatment Effect on 85th-Percentile Speed by Intersection Approach Treatment Type Site No. Intersection Approach Number of Locations Estimated Speed Difference in 85th Percentiles (mph) Standard Error (mph) T- Statistic P- Value EB 4 -2.5 0.6 -3.95 0.029 OR1 NB 4 -1.0 0.3 -3.26 0.047 NB 3 -0.5 0.1 -4.13 0.054 OR2 SB 3 -0.7 0.1 -5.36 0.033 OR3 EB 4 -0.6 0.2 -3.81 0.032 OR4 WB 3 -1.2 0.2 -5.75 0.029 EB 3 0.4 0.1 3.30 0.081 Transverse Pavement Markings OR6 WB 4 -0.5 0.6 -0.72 0.526 TX1 SB 4 0.0 0.2 0.13 0.905 NB 4 0.1 0.2 0.28 0.796 Rumble Strips TX2 SB 4 -2.0 0.8 -2.43 0.093 TX4 WB 3 -1.4 0.7 -1.94 0.191 EB 2 -1.8 0.4 -4.50 0.139 Dynamic Warning Sign WA1 WB 4 -1.5 0.3 -4.97 0.016
NCHRP 3-74 Appendix H Selection of Speed Reduction Treatments at High-Speed Intersections Secondary Analysis Page H- 2 Kittelson & Associates, Inc. ANALYSIS BY TREATMENT TYPEE The estimated effectiveness of each of the three treatments across all intersection approaches on 85th-pwercentile speeds is presented in Exhibit H-2. The results are based on an ANOVA where the dependent variable is, as before, the paired difference between the before and after 85th-percentile speeds at each site and location. The âMain Effect or Interactionâ column refers to either the overall treatment effect (Treatment), the interaction effect between treatment and data collection location (abbreviated as âTreatment x Locationâ), or the treatment effect at the individual locations (A, B, C, or D). The statistical significance of an effect is based on the p-value in the last column. Rows with p-values highlighted in yellow and bold indicate statistical significance based on a p-value cut-off of 0.05 (i.e., 5-percent significance level). The two footnotes pertain to the type of test used to test for significance: an F-test to test for significant treatment or treatment-by-location interaction; a t-test to test for significant treatment effect at a particular data collection location. The treatment effect is the estimated 85th-percentile speed reduction across all intersection approaches. The treatment-by-location interaction tests whether the treatment effect is the same across data collection locations (note that in this analysis, differences between intersection sites are controlled for in the model). The location effects correspond to the estimated 85th-percentile speed reduction across sites for each data collection location (i.e., A, B, C, or D).
NCHRP 3-74 Appendix H Selection of Speed Reduction Treatments at High-Speed Intersections Secondary Analysis Kittelson & Associates, Inc. Page H-3 Exhibit H-2. Overall Treatment Effect on 85th-Percentile Speed Across All Intersection Approaches Treatment Type Main Effect or Interaction Estimated Speed Difference in 85th Percentiles (mph) Standard Error (mph) Test Statistic Degrees of Freedom P-Value Treatmenta -0.9 0.2 17.23 (1,24) 0.0004 Treatment x Locationa 0.17 (3,24) 0.9130 Location Ab -0.9 0.5 -1.91 24 0.0675 Location Bb -1.0 0.4 -2.32 24 0.0289 Location Cb -0.6 0.4 -1.64 24 0.1150 Transverse Pavement Markings Location Db -1.0 0.4 -2.42 24 0.0234 Treatmenta -0.7 0.4 2.42 (1,8) 0.1584 Treatment x Locationa 0.54 (3,8) 0.6697 Location Ab -1.1 0.8 -1.26 8 0.2445 Location Bb -1.2 0.8 -1.44 8 0.1886 Location Cb -0.5 0.8 -0.60 8 0.5673 Rumble Strips Location Db 0.2 0.8 0.18 8 0.8623 Treatmenta -1.4 0.1 167.05 (1,5) <.0001 Treatment x Locationa 15.95 (3,5) 0.0054 Location Ab -1.4 0.3 -4.70 5 0.0053 Location Bb -2.3 0.2 -13.13 5 <.0001 Location Cb -1.6 0.2 -9.42 5 0.0002 Dynamic Warning Sign Location Db -0.4 0.2 -1.84 5 0.1254 a F-test b T-test
