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APPENDIX C Testing Results Three speed reduction treatments were tested at 19 intersection approaches to determine the effectiveness of each treatment at reducing speeds on high-speed intersection approaches. The treatments included transverse pavement markings, rumble strips, and dynamic warning signs. Speed data were collected at four locations along each intersection approach before and after treatment installation. The speeds measured at these four locations established a speed profile that could be used to estimate the speed of a vehicle at any point on the intersection approach. Sensors were placed at the same locations before and after a treatment was installed, so that direct comparisons could be made between vehicle speeds on each intersection approach. The following three types of analyses were conducted: Analysis by location to determine the speed reduction observed at each data collection loca- tion (i.e., at each of the four locations along an intersection approach), Analysis by intersection approach to determine the overall speed reduction observed along an intersection approach, and Analysis by treatment type to quantify the effect of each treatment across all intersection approaches at which it was installed. The results of each analysis are summarized below. C.1 Analysis by Location An analysis by location was conducted to determine the speed reduction observed at each data collection location (Locations A, B, C, and D) on an intersection approach as a result of the treat- ment installed on that approach. Before-and-after speed comparisons were made for 51 individual locations. Key findings from this analysis include the following: At approximately 75% of the locations, reductions in speed of up to 3.2 mph were observed (with one anomaly speed reduction of 8.7 mph). At approximately 25% of the locations, increases in speed of up to 1.8 mph were observed. Of the 51 before-and-after speed comparisons, 34 were statistically significant. C.2 Analysis by Intersection Approach An analysis by intersection approach was conducted to determine the overall speed reduction observed along an intersection approach as a result of the treatment installed on that approach. Fourteen intersection approaches (eight with transverse pavement markings, three with rumble 95

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96 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections strips, and three with dynamic warning signs) were included in this analysis. Key findings include the following: At 11 of the 14 intersection approaches, the estimated speed difference from before to after treatment installation was negative (i.e., speeds decreased after treatment installation). The estimated speed reductions were statistically significant at the 5% significance level. At 3 of the 14 intersection approaches, the estimated mean speed difference was positive (i.e., speeds increased); however, only one of these speed differences was statistically significant. All three intersection approaches on which dynamic warning signs were installed experienced a statistically significant mean speed reduction. Speed reductions appeared to be greatest where dynamic warning signs were installed. Changes in 85th percentile speeds were comparable in magnitude to changes in mean speeds at all 14 intersection approaches. C.3 Analysis by Treatment Type An analysis by treatment type was conducted to quantify the effect of a specific speed reduc- tion treatment across all intersection approaches at which it was installed. Nineteen intersection approaches (10 with transverse pavement markings, 5 with rumble strips, and 4 with dynamic warning signs) were included in this analysis. Key findings include the following: Dynamic warning signs were the only treatment to have a statistically significant overall effect at the 5% significance level in reducing mean speeds from before to after installation, with a mean speed reduction of 1.7 mph. Transverse pavement markings had a statistically significant overall effect at the 10% signifi- cance level in reducing mean speeds from before to after installation, with a mean speed reduc- tion of 0.6 mph. Rumble strips did not have a statistically significant overall effect in reducing mean speeds from before to after installation at the 10% significance level. All three treatment types were effective in reducing mean speeds at Location C (intersection perception/response speed location) with mean speed reduction of 0.9 mph for transverse pavement markings; 1.3 mph for rumble strips; and 2.3 mph for dynamic warning signs. Dynamic warning signs were effective in reducing mean speeds at Location B (accident avoid- ance speed location) with a mean speed reduction of 2.8 mph. Dynamic warning signs resulted in the largest speed reduction when compared to all other treatments. The changes in 85th percentile speeds were comparable in magnitude to those in mean speeds for all three treatment types. The reduction was significant at the 5% level for transverse pave- ment markings (0.9 mph) and for dynamic warning signs (1.4 mph). The reduction of 0.7 mph due to rumble strips was not significant at the 10% level. It should be noted that a com- parison of 85th percentile speeds is not as statistically rigorous as a comparison of mean speeds. C.4 Key Findings Key findings from the testing results include the following: All three treatment types may reduce speeds on high-speed intersection approaches; however, that speed reduction is likely to be minimal (i.e., less than 3 mph). The three treatment types appear to be most effective at reducing speeds at Location C (intersection perception/response speed location), which is the point where the intersec-

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Testing Results 97 tion would first become visible to the driver or where the driver might first react to the intersection. Of the three treatment types, dynamic warning signs may be the most effective at reducing speeds. However, this conclusion is based on only three intersection approaches. Transverse pavement marking also appears to be potentially effective at reducing speeds. Based on a limited number of sites, rumble strips do not appear to be as effective at reducing speeds as dynamic warning signs or transverse pavement markings.