Effects on Driver Speeds of In-Lane Rumble Strips on Approaches to ...

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maximum time limit of 3 hours. after rumble strips were installed. Table 7 illustrates conditions at a typical test site before and Table 7. Before and After Comparison of Site 1 near Abilene. BEFORE Stop Ahead sign. AFTER No changes were made at this site other than the addition of two sets of rumble strips. Stop Ahead sign with rumble strips in view. Both sets are 200 ft (61 m) either side of Stop Ahead sign. No changes at the intersection. Rumble strips downstream of the Stop Ahead sign. Stagger starts with outside strip first. The distance between each strip is 4 ft, 6 inches (1.4 m). The total length of the set is 20 ft, 8 inches (6.3 m). Distance between inside and outside strip is approximately 10 inches (0.25 m). Both sets are similar in dimensions except for the leading strip. Rumble strips upstream of the Stop Ahead sign. Stagger starts with inside strip first in the direction of travel.
DISCUSSION OF RESULTS Site-by-Site Comparisons Analysis of the speed data produced site-by-site comparisons of the speed profiles before and after rumble strips were installed at each of the 11 study sites. These profiles consisted of speeds collected at regular intervals along each study site approach and allowed a direct assessment of the difference between speeds in the before and after periods. However, the magnitude of the difference between corresponding speeds in the two periods did not provide an indication of whether the difference is statistically significant. Therefore, a significance test was performed on the data from the 11 sites. While traditional analysis of speeds has focused on the 85 th percentile speed, the test for statistical significance uses the mean (average) speed. The test statistic is based on the following formula: t = X 1 s N 2 1 1 − X 2 s + N 2 2 2 where t = statistic of the t distribution X 1 = mean of first sample (before period speeds) X 2 = mean of second sample (after period speeds) s 1 = standard deviation of first sample s 2 = standard deviation of second sample N 1 = number of observations in first sample N 2 = number of observations in second sample The computed value of t is then compared with the critical value of t (t c ) for the sample size. The value of t c is selected in accordance with a specified level of significance, usually 0.05, which corresponds to 95 percent confidence. If the computed value of t is greater than t c , the difference between the two means is significant. For this test, the calculation of t was not performed when the difference between means was less than 1.0 mph (1.6 km/h), which was the error of the laser guns used in this study. Using mean speeds at 100 ft (30.5 m) intervals from the stop line, the significance test was performed at a 0.05 level of significance for each individual site. Table 8 contains a graph of the 85 th percentile speed profiles on 100 ft (30.5 m) intervals at a typical site, along with descriptions of noteworthy findings.
Page 1 and 2: Effects on Driver
Page 3 and 4: distance of the 85 th speed percent
Page 5 and 6: Deceleration Behavior The 1993 synt
Page 7 and 8: motorists to proceed through a Stop
Page 9: Before data were collected when the
Page 13 and 14: Distance from Rumble Strip (ft)* Nu
Page 15: Authors’ Information: Marcus A. B

Effects on Driver Speeds of In-Lane Rumble Strips on Approaches to ...

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