Skip to contentUnited States Department of Transportation - Federal Highway Administration FHWA Home
Research Home
Report
This report is an archived publication and may contain dated technical, contact, and link information
Publication Number: FHWA-RD-02-089
Date: July 2002

Safety Effectiveness of Intersection Left- and Right-Turn Lanes

Yoked Comparison Evaluations

The results of the YC evaluations performed in this study are presented in tables C-1 through C-10. These tables include the results for four specific dependent variables, two different target areas, and two different types of analysis approaches.

The YC approach to before-after evaluation has been described conceptually in section 5 of this report. The procedures presented in that section were used to obtain the analysis results in tables C-1 through C-10. These analyses involved one-to-one matching of treatment and similar unimproved comparison sites. Two analyses presented below involved only treatment site data; in this approach, the project-related accidents for the treatment site were evaluated using the non-project-related accidents for that same site as the "comparison site" data. These were referred to as auto-matching analyses.

Description of Results Tables

Each of the tables of YC results presents the results of a set of similar YC evaluation for a specific dependent variable, target area, and analysis type. Each row in a table presents the results of the evaluation performed for a specific type of improvement project. The following discussion guides the interpretation of individual columns in these tables.

Area type. The first column for each row of the table identifies the area type for the intersections evaluated. The codes used in this column are:

R = rural

U = urban

Traffic control: The second column identifies the type of traffic control at the intersections evaluated. The codes used in this column are:

N = Newly signalized intersection

S = Signalized intersection

U = Unsignalized intersection

Table C-1. Yoked Comparison Evaluation Results for Total Intersection Accidents.
Area type Traffic control Project type No. of legs No. of improved sites Percent change in accident frequency Treatment effect Test for homogeneity
Mean Mean per added lane Standard error Lower 95% confidence limit Upper 95% confidence limit Ratioa Calculated Z Significant at 5% level? Calculated H probability Homogeneous at 5% level?
R N L 4 2 -70.31 -35.15 7.60 -44.56 -9.52 4.63 -2.37 SIG 0.39 Yes
R N LR 4 1 8.58 4.29 65.60 -44.92 529.83 0.07 0.07 n/a   n/a
R N R 4 1 -10.38 -5.19 20.07 -31.38 57.82 0.26 -0.24 n/a   n/a
R S XL 3 2 -27.76 -27.76 54.20 -83.40 214.40 0.51 -0.43 NS 0.41 Yes
R S XL 4 3 -30.64 -30.64 27.62 -68.21 51.37 1.11 -0.92 NS 0.87 Yes
R S XLR 4 1 -44.32 -11.08 7.85 -20.39 17.07 1.41 -1.04 n/a   n/a
R U L 3 31 -63.68 -63.68 7.21 -75.39 -46.41 8.83 -5.10 SIG 0.20 Yes
R U L 4 21 -58.08 -32.10 4.70 -39.70 -20.78 6.83 -4.28 SIG 0.04 No
R U LR 3 11 -56.64 -44.50 15.08 -64.27 2.57 2.95 -1.89 NS 0.76 Yes
R U LR 4 12 1.96 0.98 13.39 -19.53 35.31 0.07 0.07 NS 0.74 Yes
R U R 3 11 88.66 88.66 77.59 -15.75 322.45 1.14 1.54 NS 0.86 Yes
R U R 4 27 -23.91 -15.37 10.99 -32.80 11.69 1.40 -1.22 NS 0.86 Yes
R U XL 4 1 82.06 82.06 222.08 -83.33 1,888.76 0.37 0.49 n/a   n/a
U N L 3 4 -30.29 -20.19 14.09 -41.01 17.52 1.43 -1.19 NS 0.02 No
U N L 4 24 -43.77 -22.35 3.18 -27.95 -15.39 7.03 -5.20 SIG 0.03 No
U S L 3 3 -32.62 -32.62 30.80 -72.49 65.08 1.06 -0.86 NS 0.18 Yes
U S L 4 33 -42.00 -12.95 1.16 -15.09 -10.53 11.17 -8.40 SIG 0.00 No
U S LR 4 9 -21.25 -5.46 2.20 -9.35 -0.65 2.48 -2.20 SIG 0.01 No
U S R 3 1 -36.52 -18.26 15.54 -37.84 32.85 1.18 -0.93 n/a   n/a
U S R 4 14 -13.65 -6.82 4.23 -14.37 2.32 1.61 -1.50 NS 0.00 No
U S XL 3 1 -25.11 -25.11 43.24 -75.85 132.21 0.58 -0.50 n/a   n/a
U S XL 4 2 14.95 9.97 15.24 -14.77 46.50 0.65 0.70 NS 0.80 Yes
U U L 3 10 -36.55 -36.55 17.52 -63.07 9.00 2.09 -1.65 NS 0.28 Yes
U U L 4 8 -70.46 -35.23 2.92 -39.97 -28.24 12.07 -6.17 SIG 0.63 Yes
U U LR 4 1 -0.85 -0.42 37.06 -38.55 164.58 -0.01 0.01 n/a   n/a
U U R 4 1 22.76 22.76 228.29 -96.79 4,599.26 0.10 0.11 n/a   n/a

a Ratio of mean percent change in accident frequency to standard error of percent change in accident frequency per turn lane installed.

Table C-2. Yoked Comparison Evaluation Results for Fatal and Injury Intersection Accidents.
Area type Traffic control Project type No. of legs No. of improved sites Percent change in accident frequency Treatment effect Test for homogeneity
Mean Mean per added lane Standard error Lower 95% confidence limit Upper 95% confidence limit Ratioa Calculated Z Significant at 5% level? Calculated H probability Homogeneous at 5% level?
R N L 4 2 -82.61 -41.31 6.58 -48.03 -11.64 6.27 -2.31 SIG 0.69 Yes
R N LR 4 1 -3.95 -1.97 61.88 -46.16 550.18 0.03 -0.03 n/a   n/a
R N R 4 1 -2.86 -1.43 32.03 -36.67 126.92 0.04 -0.04 n/a   n/a
R S XL 3 2 -35.44 -35.44 63.34 -90.56 341.70 0.56 -0.45 NS 0.72 Yes
R S XL 4 3 63.45 63.45 85.33 -41.25 354.74 0.74 0.94 NS 0.98 Yes
R S XLR 4 1 18.67 4.67 24.66 -19.18 126.30 0.19 0.21 n/a   n/a
R U L 3 34 -58.55 -58.55 11.78 -76.26 -27.63 4.97 -3.10 SIG 0.78 Yes
R U L 4 22 -70.41 -39.72 4.85 -46.97 -26.90 8.18 -4.19 SIG 0.88 Yes
R U LR 3 11 -51.67 -40.60 22.18 -66.48 40.71 1.83 -1.25 NS 0.99 Yes
R U LR 4 14 -7.72 -3.86 16.19 -26.80 41.77 0.24 -0.23 NS 0.98 Yes
R U R 3 11 18.83 18.83 62.88 -57.88 235.23 0.30 -0.33 NS 1.00 Yes
R U R 4 28 -22.28 -14.51 15.09 -36.91 25.69 0.96 -0.85 NS 0.94 Yes
R U XL 4 2 156.22 156.22 335.09 -80.26 3,225.51 0.47 0.72 NS 0.27 Yes
U N L 3 4 -10.57 -7.04 28.32 -43.17 84.60 0.25 -0.24 NS 0.11 Yes
U N L 4 23 -42.68 -21.81 5.40 -30.70 -9.05 4.04 -3.02 SIG 0.67 Yes
U S L 3 3 -41.97 -41.97 40.03 -84.99 124.28 1.05 -0.79 NS 0.25 Yes
U S L 4 35 -39.35 -12.41 2.04 -16.01 -7.96 6.08 -4.69 SIG 0.00 No
U S LR 4 8 -27.75 -7.16 3.75 -13.23 1.84 1.91 -1.62 NS 0.36 Yes
U S R 3 1 -74.25 -37.13 12.09 -47.96 31.17 3.07 -1.44 n/a   n/a
U S R 4 12 -20.22 -9.71 7.22 -21.54 7.41 1.34 -1.20 NS 0.38 Yes
U S XL 3 1 -0.14 -0.14 99.86 -85.93 608.91 0.00 0.00 n/a   n/a
U S XL 4 3 -7.71 -4.62 19.19 -31.92 49.21 0.24 -0.23 NS 0.14 Yes
U U L 3 10 -41.59 -41.59 24.13 -74.01 31.27 1.72 -1.30 NS 0.67 Yes
U U L 4 8 -79.48 -39.74 3.04 -44.26 -31.65 13.06 -5.34 SIG 0.93 Yes
U U LR 4 1 -73.75 -36.88 18.17 -49.13 147.92 2.03 -0.97 n/a   n/a
U U R 4 1 84.14 84.14 379.61 -96.76 10,370.69 0.22 0.30 n/a   n/a

a Ratio of mean percent change in accident frequency to standard error of percent change in accident frequency per turn lane installed.

Table C-3. Yoked Comparison Evaluation Results for Project-Related Intersection Accidents.
Area type Traffic control Project type No. of legs No. of improved sites Percent change in accident frequency Treatment effect Test for homogeneity
Mean Mean per added lane Standard error Lower 95% confidence limit Upper 95% confidence limit Ratioa Calculated Z Significant at 5% level? Calculated H probability Homogeneous at 5% level?
R N L 4 2 -33.84 -16.92 44.21 -47.59 404.14 0.38 -0.31 NS 1.00 Yes
R N LR 4 1 -47.97 -23.99 68.83 -49.85 4,598.55 0.35 -0.25 n/a   n/a
R N R 4 1 0.01 0.00 141.41 -49.80 12,730.59 0.00 0.00 n/a   n/a
R S XL 3 2 -7.57 -7.57 184.85 -98.17 4,558.33 0.04 -0.04 NS 0.98 Yes
R S XL 4 4 23.06 23.06 119.45 -81.64 724.77 0.19 0.21 NS 0.99 Yes
R S XLR 4 1 -2.90 -0.73 47.01 -24.45 1,055.24 0.02 -0.02 n/a   n/a
R U L 3 34 -1.04 -1.04 47.90 -61.68 155.56 0.02 -0.02 NS 1.00 Yes
R U L 4 23 -38.43 -21.56 16.71 -42.72 33.06 1.29 -1.00 NS 1.00 Yes
R U LR 3 12 -15.23 -12.18 53.44 -65.53 237.78 0.23 -0.21 NS 1.00 Yes
R U LR 4 15 68.38 34.19 44.25 -19.95 185.88 0.77 0.99 NS 0.95 Yes
R U R 3 11 4.90 4.90 89.46 -80.28 458.06 0.05 0.06 NS 1.00 Yes
R U R 4 28 -7.41 -4.83 28.83 -41.50 88.81 0.17 -0.16 NS 1.00 Yes
R U XL 4 2 50.26 50.26 242.29 -93.63 3,443.23 0.21 0.25 NS 0.57 Yes
U N L 3 4 -32.95 -21.97 59.60 -63.39 543.20 0.37 -0.30 NS 0.99 Yes
U N L 4 26 22.55 11.73 23.61 -21.17 79.74 0.50 0.55 NS 0.76 Yes
U S L 3 3 0.06 0.06 155.01 -95.20 1,984.29 0.00 0.00 NS 1.00 Yes
U S L 4 35 -39.09 -12.55 3.17 -17.87 -5.25 3.96 -3.06 SIG 0.53 Yes
U S LR 3 2 222.83 148.55 351.45 -57.90 5,216.77 0.42 0.72 NS 0.96 Yes
U S LR 4 7 -59.77 -15.50 2.24 -19.09 -10.02 6.90 -4.23 SIG 0.84 Yes
U S R 3 1 -46.90 -23.45 35.12 -48.01 304.93 0.67 -0.48 n/a   n/a
U S R 4 13 9.10 5.38 19.84 -23.82 58.74 0.27 0.28 NS 0.00 No
U S XL 3 1 299.43 299.43 1,018.34 -97.30 59,004.76 0.29 0.54 n/a   n/a
U S XL 4 2 5.15 3.44 118.54 -64.12 1861.12 0.03 0.03 NS 0.78 Yes
U U L 3 10 12.61 12.61 100.01 -80.25 542.02 0.13 0.13 NS 1.00 Yes
U U L 4 8 -78.97 -39.49 4.87 -45.76 23.93 8.11 -3.37 SIG 0.57 Yes
U U LR 4 1 162.46 81.23 215.63 -44.76 3236.40 0.38 0.59 n/a   n/a
U U R 4 1 -7.93 -7.93 260.41 -99.64 23,435.42 0.03 -0.03 n/a   n/a

a Ratio of mean percent change in accident frequency to standard error of percent change in accident frequency per turn lane installed.

Table C-4. Yoked Comparison Evaluation for Project-Related Fatal and Injury Intersection Accidents.
Area type Traffic control Project type No. of legs No. of improved sites Percent change in accident frequency Treatment effect Test for homogeneity
Mean Mean per added lane Standard error Lower 95% confidence limit Upper 95% confidence limit Ratioa Calculated Z Significant at 5% level? Calculated H probability Homogeneous at 5% level?
R N L 4 2 -28.78 -14.39 57.63 -48.51 799.48 0.25 -0.21 NS 0.83 Yes
R N LR 4 1 -47.97 -23.99 68.83 -49.85 4,598.55 0.35 -0.25 n/a   n/a
R N R 4 1 -0.01 0.00 141.41 -49.80 12,730.59 0.00 0.00 n/a   n/a
R S XL 3 2 -7.57 -7.57 184.85 -98.17 4,558.33 0.04 -0.04 NS 0.98 Yes
R S XL 4 4 177.73 177.73 311.15 -69.10 2,396.19 0.57 0.91 NS 0.86 Yes
R S XLR 4 1 45.65 11.41 80.05 -24.51 2,682.84 0.14 0.17 n/a   n/a
R U L 3 34 1.31 1.31 49.14 -60.85 162.14 0.03 0.03 NS 1.00 Yes
R U L 4 23 -36.33 -20.38 18.87 -43.42 44.50 1.08 -0.85 NS 1.00 Yes
R U LR 3 12 -17.33 -13.87 53.37 -66.40 241.59 0.26 -0.24 NS 1.00 Yes
R U LR 4 15 -4.91 2.46 30.40 -36.42 116.47 0.08 -0.08 NS 1.00 Yes
R U R 3 11 4.90 4.90 89.46 -80.28 458.06 0.05 0.06 NS 1.00 Yes
R U R 4 28 -4.40 -2.87 31.87 -42.30 104.69 0.09 -0.09 NS 1.00 Yes
R U XL 4 2 143.69 143.69 451.22 -93.53 9,082.86 0.32 0.48 NS 0.73 Yes
U N L 3 4 -3.68 -2.46 90.81 -62.65 959.95 0.03 -0.03 NS 1.00 Yes
U N L 4 26 -8.78 -4.57 22.33 -33.14 67.33 0.20 -0.20 NS 1.00 Yes
U S L 3 3 1.06 1.06 165.02 -95.88 2,380.82 0.01 0.01 NS 1.00 Yes
U S L 4 35 -32.67 -10.49 6.31 -19.91 6.21 1.66 -1.35 NS 1.00 Yes
U S LR 3 2 -25.28 -16.86 96.24 -65.54 2,131.34 0.18 -0.15 NS 0.89 Yes
U S LR 4 8 -49.63 -12.81 4.96 -19.65 1.64 2.58 -1.80 NS 0.65 Yes
U S R 3 1 -73.45 -36.72 33.85 -49.91 1,914.45 1.09 -0.52 n/a   n/a
U S R 4 14 40.32 20.16 30.02 -19.67 112.29 0.67 0.79 NS 0.59 Yes
U S XL 3 1 99.71 99.71 528.39 -98.88 35,587.40 0.19 0.26 n/a   n/a
U S XL 4 3 34.97 20.98 109.53 -54.29 1,087.46 0.19 0.22 NS 0.79 Yes
U U L 3 10 3.97 3.97 92.99 -81.99 500.14 0.04 0.04 NS 1.00 Yes
U U L 4 8 -81.50 -40.75 5.62 -47.19 -19.58 7.25 -2.78 SIG 0.95 Yes
U U LR 4 1 -47.51 -23.75 69.44 -49.85 4,639.94 0.34 -0.24 n/a   n/a
U U R 4 1 -7.93 -7.93 260.41 -99.64 23,435.42 0.03 -0.03 n/a   n/a

a Ratio of mean percent change in accident frequency to standard error of percent change in accident frequency per turn lane installed.

Table C-5. Yoked Comparison Evaluation Results for Total Accidents on Individual Intersection Approaches.
Area type Traffic control Project type No. of legs No. of improved sites Percent change in accident frequency Treatment effect Test for homogeneity
Mean Mean per added lane Standard error Lower 95% confidence limit Upper 95% confidence limit Ratioa Calculated Z Significant at 5% level? Calculated H probability Homogeneous at 5% level?
R N L 4 4 -67.45 -67.45 9.33 -44.71 0.07 3.61 -1.96 SIG 0.65 Yes
R N LR 4 2 -11.77 -5.88 47.18 -44.58 308.79 0.12 -0.12 NS 0.78 Yes
R N R 4 2 -7.11 -7.11 23.17 -32.53 73.49 0.15 -0.15 NS 0.49 Yes
R S XL 3 2 61.29 61.29 189.96 -83.96 1,522.35 0.32 0.41 NS 0.62 Yes
R S XL 4 4 -50.86 -50.86 26.61 -83.00 42.03 1.91 -1.31 NS 0.69 Yes
R S XLR 4 2 -39.42 -19.71 7.49 -14.31 26.55 0.88 -0.68 NS 0.41 Yes
R U L 3 34 -47.46 -47.46 16.56 -71.67 -2.55 2.87 -2.04 SIG 0.61 Yes
R U L 4 40 -47.32 -47.32 6.35 -35.29 -9.58 3.97 -2.84 SIG 0.70 Yes
R U LR 3 23 -51.34 -25.67 18.04 -63.87 17.14 2.26 -1.54 NS 0.99 Yes
R U LR 4 27 5.26 2.63 14.21 -19.00 39.36 0.19 0.19 NS 0.99 Yes
R U R 3 11 79.25 79.25 107.95 -44.94 483.59 0.73 0.97 NS 1.00 Yes
R U R 4 43 -18.72 -18.72 11.48 -28.98 17.69 0.96 -0.86 NS 1.00 Yes
R U XL 4 1 0.00 0.00 193.65 -97.75 4,350.15 0.00 0.00 n/a   n/a
U N L 3 6 -25.86 -25.86 19.80 -41.41 46.43 0.78 -0.67 NS 0.24 Yes
U N L 4 47 -55.50 -55.50 3.36 -34.08 -20.70 8.43 -5.47 SIG 0.32 Yes
U S L 3 3 70.74 70.74 121.99 -57.92 592.68 0.58 0.75 NS 0.98 Yes
U S L 4 106 -42.02 -42.02 1.19 -13.98 -9.32 9.97 -7.50 SIG 0.00 No
U S LR 3 3 -8.02 -4.01 31.23 -41.79 107.29 -0.15 0.15 NS 0.46 Yes
U S LR 4 32 -30.94 -15.47 1.56 -8.74 -2.55 3.84 -3.17 SIG 0.65 Yes
U S R 3 2 -62.59 -62.59 15.45 -46.30 44.46 2.02 -1.19 NS 0.04 No
U S R 4 28 -25.74 -25.74 3.86 -16.67 -1.36 2.59 -2.23 SIG 0.04 No
U S XL 3 1 33.33 33.33 238.82 -96.02 4,362.97 0.14 0.16 n/a   n/a
U S XL 4 4 -4.97 -4.97 15.07 -25.62 36.40 0.19 -0.18 NS 0.08 Yes
U U L 3 10 -55.44 -55.44 17.53 -79.39 -3.65 3.16 -2.05 SIG 0.61 Yes
U U L 4 16 -69.37 -69.37 3.26 -39.91 -26.76 10.64 -5.56 SIG 0.59 Yes
U U LR 4 2 53.64 26.82 80.23 -40.08 545.03 0.33 0.41 NS 0.18 Yes
U U R 4 1 -69.49 -69.49 71.55 -99.69 2,924.77 0.97 -0.51 n/a   n/a

a Ratio of mean percent change in accident frequency to standard error of percent change in accident frequency per turn lane installed.

Table C-6. Yoked Comparison Evaluation Results for Fatal and Injury Accidents on Individual Intersection Approaches.
Area type Traffic control Project type No. of legs No. of improved sites Percent change in accident frequency Treatment effect Test for homogeneity
Mean Mean per added lane Standard error Lower 95% confidence limit Upper 95% confidence limit Ratioa Calculated Z Significant at 5% level? Calculated H probability Homogeneous at 5% level?
R N L 4 4 -78.75 -78.75 8.67 -47.85 2.62 4.54 -1.90 NS 0.65 Yes
R N LR 4 2 -24.25 -12.12 44.62 -46.24 331.17 0.27 -0.24 NS 0.50 Yes
R N R 4 2 32.74 32.74 48.91 -34.35 231.36 0.33 0.38 NS 0.80 Yes
R S XL 3 2 64.93 64.93 203.08 -85.24 1,742.63 0.32 0.41 NS 0.86 Yes
R S XL 4 4 21.37 21.37 78.62 -65.90 332.02 0.27 0.30 NS 1.00 Yes
R S XLR 4 2 21.91 10.96 24.06 -14.67 190.29 0.15 0.17 NS 0.51 Yes
R U L 3 34 -40.80 -40.80 22.53 -71.92 24.82 1.81 -1.38 NS 0.99 Yes
R U L 4 41 -54.97 -54.97 6.89 -39.60 -11.12 4.25 -2.77 SIG 1.00 Yes
R U LR 3 24 -28.23 -14.11 28.77 -58.50 73.32 0.78 -0.66 NS 1.00 Yes
R U LR 4 29 -6.43 -3.22 15.69 -25.76 40.29 0.20 -0.20 NS 1.00 Yes
R U R 3 11 26.71 26.71 86.25 -66.62 381.08 0.31 0.35 NS 0.99 Yes
R U R 4 43 -10.90 -10.90 15.76 -29.77 35.65 0.41 -0.38 NS 1.00 Yes
R U XL 4 2 121.96 121.96 319.21 -86.75 3,619.01 0.38 0.55 NS 0.30 Yes
U N L 3 6 -38.23 -38.23 24.38 -49.79 74.56 0.94 -0.73 NS 0.80 Yes
U N L 4 49 -58.14 -58.14 4.99 -37.53 -17.26 5.94 -3.73 SIG 1.00 Yes
U S L 3 3 210.12 210.12 284.05 -48.49 1,767.16 0.74 1.24 NS 0.94 Yes
U S L 4 114 -39.99 -39.99 1.93 -14.58 -6.97 5.81 -4.46 SIG 0.81 Yes
U S LR 3 4 55.16 27.58 74.90 -41.65 360.97 0.49 0.61 NS 0.88 Yes
U S LR 4 34 -34.35 -17.17 2.62 -10.95 -0.39 2.56 -2.06 SIG 0.92 Yes
U S R 3 2 -86.69 -86.69 9.25 -49.56 51.49 4.69 -1.45 NS 0.78 Yes
U S R 4 30 -29.85 -29.85 6.36 -21.30 4.57 1.78 -1.48 NS 0.98 Yes
U S XL 3 1 -28.57 -28.57 148.85 -98.80 4,143.93 0.19 -0.16 n/a   n/a
U S XL 4 4 -53.37 -53.37 14.86 -48.21 22.37 2.05 -1.37 NS 0.30 Yes
U U L 3 10 -52.96 -52.96 26.42 -84.35 41.44 2.00 -1.34 NS 0.84 Yes
U U L 4 16 -77.79 -77.79 3.70 -44.22 -28.66 10.51 -4.52 SIG 0.90 Yes
U U LR 4 2 -62.87 -31.43 29.45 -49.17 365.89 1.07 -0.62 NS 0.58 Yes
U U R 4 1 -38.98 -38.98 149.46 -99.50 7,321.45 0.26 -0.20 n/a   n/a

a Ratio of mean percent change in accident frequency to standard error of percent change in accident frequency per turn lane installed.

Table C-7. Yoked Comparison Evaluation Results for Project-Related Accidents on Individual Intersection Approaches.
Area type Traffic control Project type No. of legs No. of improved sites Percent change in accident frequency Treatment effect Test for homogeneity
Mean Mean per added lane Standard error Lower 95% confidence limit Upper 95% confidence limit Ratioa Calculated Z Significant at 5% level? Calculated H probability Homogeneous at 5% level?
R N L 4 4 -29.35 -29.35 40.58 -46.28 285.61 0.36 -0.30 NS 0.85 Yes
R N LR 4 2 -28.14 -14.07 69.42 -49.19 1,535.50 0.20 -0.17 NS 0.87 Yes
R N R 4 2 0.01 0.01 100.01 -49.01 2,470.19 0.00 0.00 NS 1.00 Yes
R S XL 3 2 -12.07 -12.07 175.87 -98.26 4,331.89 0.07 -0.06 NS 0.96 Yes
R S XL 4 4 15.21 15.21 111.82 -82.81 672.11 0.14 0.15 NS 0.98 Yes
R S XLR 4 2 14.57 7.29 32.12 -15.96 499.57 0.08 0.08 NS 0.67 Yes
R U L 3 34 2.03 2.03 49.49 -60.57 164.00 0.04 0.04 NS 1.00 Yes
R U L 4 41 -25.66 -25.66 15.50 -34.87 32.03 0.88 -0.76 NS 1.00 Yes
R U LR 3 24 -10.95 -5.48 40.46 -56.60 136.84 0.22 -0.20 NS 1.00 Yes
R U LR 4 30 19.17 9.58 26.34 -24.94 91.70 0.36 0.40 NS 1.00 Yes
R U R 3 11 5.84 5.84 90.26 -80.11 463.06 0.06 0.07 NS 1.00 Yes
R U R 4 43 -9.68 9.68 21.27 -34.60 57.55 0.27 -0.25 NS 1.00 Yes
R U XL 4 2 30.05 30.05 209.71 -94.48 2,966.70 0.14 0.16 NS 0.57 Yes
U N L 3 6 -21.71 -21.71 51.73 -54.57 346.63 0.25 -0.22 NS 1.00 Yes
U N L 4 50 -8.21 -8.21 15.32 -26.36 37.90 0.27 -0.26 NS 1.00 Yes
U S L 3 3 -0.44 -0.44 162.58 -95.94 2,344.12 0.00 -0.00 NS 0.99 Yes
U S L 4 115 -33.88 33.88 2.87 -14.34 2.90 3.30 -2.66 SIG 1.00 Yes
U S LR 3 5 95.11 47.55 140.81 -49.82 1,109.84 0.42 0.58 NS 0.99 Yes
U S LR 4 32 -59.67 -29.83 1.77 -14.68 7.51 6.69 -4.10 SIG 0.79 Yes
U S R 3 2 -41.79 -41.79 47.53 -48.81 664.46 0.44 -0.33 NS 0.88 Yes
U S R 4 34 66.04 66.04 15.90 1.30 66.00 1.64 2.09 SIG 0.01 No
U S XL 3 1 0.00 0.00 282.84 -99.61 25,462.64 0.00 0.00 n/a   n/a
U S XL 4 5 108.84 108.84 113.24 -38.42 730.24 0.53 0.75 NS 0.71 Yes
U U L 3 10 14.06 14.06 101.30 -79.99 550.30 0.14 0.15 NS 1.00 Yes
U U L 4 16 -78.11 -78.11 5.15 -45.65 22.50 7.59 -3.23 SIG 0.97 Yes
U U LR 4 2 141.99 70.99 169.28 -42.21 1,828.13 0.42 0.63 NS 0.65 Yes
U U R 4 1 -38.98 -38.98 172.58 -99.76 15,497.21 0.23 -0.17 n/a   n/a

a Ratio of mean percent change in accident frequency to standard error of percent change in accident frequency per turn lane installed.

 

Table C-8. Yoked Comparison Evaluation Results for Project-Related Fatal and Injury Accidents on Individual Intersection Approaches.
Area type Traffic control Project type No. of legs No. of improved sites Percent change in accident frequency Treatment effect Test for homogeneity
Mean Mean per added lane Standard error Lower 95% confidence limit Upper 95% confidence limit Ratioa Calculated Z Significant at 5% level? Calculated H probability Homogeneous at 5% level?
R N L 4 4 -15.07 -15.07 52.96 -46.32 439.40 0.14 -0.13 NS 1.00 Yes
R N LR 4 2 -28.14 -14.07 69.42 -49.19 1,535.50 0.20 -0.17 NS 0.87 Yes
R N R 4 2 0.01 0.01 100.01 -49.01 2,470.19 0.00 0.00 NS 1.00 Yes
R S XL 3 2 -12.07 -12.07 175.87 -98.26 4,331.89 0.07 -0.06 NS 0.96 Yes
R S XL 4 4 163.58 163.58 295.30 -70.67 2,268.98 0.55 0.87 NS 0.86 Yes
R S XLR 4 2 29.41 14.70 37.44 -15.95 631.00 0.13 0.15 NS 0.90 Yes
R U L 3 34 2.03 2.03 49.49 -60.57 164.00 0.04 0.04 NS 1.00 Yes
R U L 4 41 -23.73 -23.73 16.79 -35.19 38.08 0.75 -0.66 NS 1.00 Yes
R U LR 3 24 -11.66 -5.83 40.56 -57.05 137.67 0.23 -0.22 NS 1.00 Yes
R U LR 4 30 -2.49 -1.25 23.43 -30.99 75.04 0.05 -0.05 NS 1.00 Yes
R U R 3 11 5.84 5.84 90.26 -80.11 463.06 0.06 0.07 NS 1.00 Yes
R U R 4 43 -5.70 -5.70 23.30 -34.49 67.48 0.14 -0.14 NS 1.00 Yes
R U XL 4 2 110.86 110.86 390.43 -94.40 7,845.84 0.28 0.40 NS 0.73 Yes
U N L 3 6 2.78 2.78 71.21 -53.59 532.89 0.02 0.02 NS 1.00 Yes
U N L 4 50 -16.09 -16.09 15.63 -30.09 36.55 0.53 -0.48 NS 1.00 Yes
U S L 3 3 -0.44 -0.44 162.58 -95.94 2,344.12 0.00 0.00 NS 0.99 Yes
U S L 4 116 -2.23 -2.23 5.62 -9.66 13.00 0.11 -0.11 NS 1.00 Yes
U S LR 3 5 -3.01 -1.51 75.60 -57.24 636.13 0.02 -0.02 NS 1.00 Yes
U S LR 4 33 -50.96 -25.48 3.37 -14.93 0.69 3.01 -2.06 SIG 1.00 Yes
U S R 3 2 -10.24 -10.24 89.76 -49.11 2,211.92 0.06 -0.05 NS 0.97 Yes
U S R 4 35 62.30 62.30 21.67 -6.69 84.83 1.12 1.41 NS 0.96 Yes
U S XL 3 1 0.00 0.00 282.84 -99.61 25,462.64 0.00 0.00 n/a   n/a
U S XL 4 5 29.49 29.49 80.25 -47.48 584.96 0.20 0.23 NS 0.99 Yes
U U L 3 10 5.30 5.30 94.18 -81.76 507.82 0.06 0.06 NS 1.00 Yes
U U L 4 16 -68.60 -68.60 8.60 -44.63 4.08 3.99 -2.12 SIG 1.00 Yes
U U LR 4 2 -27.38 -13.69 70.16 -49.18 1,552.27 0.20 -0.17 NS 0.86 Yes
U U R 4 1 -38.98 -38.98 172.58 -99.76 15,497.21 0.23 -0.17 n/a   n/a

a Ratio of mean percent change in accident frequency to standard error of percent change in accident frequency per turn lane installed.

ss
Table C-9. Yoked Comparison Evaluation for Project-Related Intersection Accidents—Auto Matched.
Area type Traffic control Project type No. of legs No. of improved sites Percent change in accident frequency Treatment effect Test for homogeneity
Mean Mean per added lane Standard error Lower 95% confidence limit Upper 95% confidence limit Ratioa Calculated Z Significant at 5% level? Calculated H probability Homogeneous at 5% level?
R N L 4 2 109.08 54.54 104.11 -35.16 686.24 0.52 0.74 NS 0.99 Yes
R N LR 4 1 283.33 141.67 393.27 -46.56 10,643.71 0.36 0.65 n/a   n/a
R N R 4 1 -38.64 -19.32 61.82 -49.41 1,542.16 0.31 -0.24 n/a   n/a
R S XL 3 2 350.52 350.52 705.43 -79.07 9,595.30 0.50 0.96 NS 0.94 Yes
R S XL 4 4 7.61 7.61 82.50 -76.06 383.60 0.09 0.10 NS 0.91 Yes
R S XLR 4 1 -9.38 -2.34 36.95 -24.07 528.79 0.06 -0.06 n/a   n/a
R U L 3 34 123.68 123.68 83.83 7.30 366.29 1.48 2.15 SIG 1.00 Yes
R U L 4 22 3.70 2.09 20.20 -26.68 58.70 0.10 0.11 NS 0.90 Yes
R U LR 3 12 120.86 96.69 118.15 -32.36 575.28 0.82 1.18 NS 0.98 Yes
R U LR 4 15 66.35 33.17 30.01 -9.00 118.72 1.11 1.41 NS 1.00 Yes
R U R 3 11 7.02 7.02 75.92 -73.36 329.86 0.09 0.10 NS 1.00 Yes
R U R 4 28 27.10 17.64 30.75 -25.16 106.30 0.57 0.65 NS 1.00 Yes
R U XL 4 2 89.07 89.07 190.91 -73.87 1,268.04 0.47 0.63 NS 0.22 Yes
U N L 3 4 33.86 22.57 89.80 -54.25 574.73 0.25 0.29 NS 0.81 Yes
U N L 4 24 92.22 48.12 23.77 10.85 107.42 2.02 2.76 SIG 0.41 Yes
U S L 3 3 72.16 72.16 220.51 -86.02 2,019.53 0.33 0.42 NS 0.64 Yes
U S L 4 37 -0.32 -0.10 3.88 -6.86 8.50 0.03 -0.03 NS 0.05 Yes
U S LR 3 2 21.81 14.54 107.34 -60.58 1,016.60 0.14 0.15 NS 0.64 Yes
U S LR 4 9 -44.51 -11.45 2.30 -15.31 6.15 4.98 -3.66 SIG 0.28 Yes
U S R 3 1 237.50 118.75 181.91 -29.60 1,345.85 0.65 1.13 n/a   n/a
U S R 4 12 2.36 1.23 8.42 -12.97 20.58 0.15 0.15 NS 0.02 No
U S XL 3 1 100.00 100.00 406.20 -96.27 10,611.75 0.25 0.34 n/a   n/a
U S XL 4 3 -31.92 -19.15 21.37 -45.35 53.87 0.90 -0.73 NS 0.78 Yes
U U L 3 10 202.64 202.64 213.44 -24.04 1,105.75 0.95 1.57 NS 0.90 Yes
U U L 4 8 -38.17 -19.09 12.07 -35.62 16.45 1.58 -1.23 NS 0.56 Yes
U U LR 4 1 66.67 33.33 115.87 -44.54 1,221.71 0.29 0.37 n/a   n/a
U U R 4 1 500.00 500.00 1,509.97 -95.68 83,140.41 0.33 0.71 n/a   n/a

a Ratio of mean percent change in accident frequency to standard error of percent change in accident frequency per turn lane installed.

 

Table C-10. Yoked Comparison Evaluation Results for Project-Related Fatal and Injury Intersection Accidents—Auto Matched.
Area type Traffic control Project type No. of legs No. of improved sites Percent change in accident frequency Treatment effect Test for homogeneity
Mean Mean per added lane Standard error Lower 95% confidence limit Upper 95% confidence limit Ratioa Calculated Z Significant at 5% level? Calculated H probability Homogeneous at 5% level?
R N L 4 2 326.58 163.29 262.40 -30.87 2,327.76 0.62 1.18 NS 0.39 Yes
R N LR 4 1 225.00 112.50 338.02 -47.24 9,532.79 0.33 0.57 n/a   n/a
R N R 4 1 -11.76 -5.88 89.61 -49.18 2,313.44 0.07 -0.06 n/a   n/a
R S XL 3 2 311.85 311.85 700.77 -85.33 11,464.18 0.45 0.83 NS 0.84 Yes
R S XL 4 4 42.50 42.50 132.34 -76.91 779.64 0.32 0.38 NS 0.94 Yes
R S XLR 4 1 -35.00 -8.75 27.08 -24.38 400.92 0.32 -0.26 n/a   n/a
R U L 3 34 117.81 117.81 87.87 -1.22 380.28 1.34 1.93 NS 1.00 Yes
R U L 4 23 72.85 40.87 38.93 -11.95 156.90 1.05 1.36 NS 0.98 Yes
R U LR 3 12 86.17 68.94 103.92 -42.06 504.69 0.66 0.89 NS 1.00 Yes
R U LR 4 15 58.74 29.37 39.34 -19.96 159.68 0.75 0.93 NS 1.00 Yes
R U R 3 11 -0.17 -0.17 75.13 -77.17 336.41 0.00 0.00 NS 1.00 Yes
R U R 4 28 34.30 22.34 38.64 -28.33 142.81 0.58 0.67 NS 1.00 Yes
R U XL 4 2 137.27 137.27 332.77 -84.81 3,607.34 0.41 0.62 NS 0.89 Yes
U N L 3 4 -15.45 -10.30 62.03 -60.14 420.50 0.17 -0.15 NS 0.86 Yes
U N L 4 25 107.23 55.85 34.87 5.22 151.21 1.60 2.26 SIG 0.95 Yes
U S L 3 3 2.14 2.14 143.17 -93.45 1,493.64 0.01 0.02 NS 0.64 Yes
U S L 4 36 15.98 5.09 7.60 -7.17 23.44 0.67 0.72 NS 0.67 Yes
U S LR 3 2 19.14 12.76 106.15 -60.88 1,023.81 0.12 0.13 NS 0.55 Yes
U S LR 4 9 -44.71 -11.50 4.42 -17.98 0.42 2.60 -1.91 NS 0.40 Yes
U S R 3 1 450.00 225.00 589.23 -45.87 18,280.96 0.38 0.80 n/a   n/a
U S R 4 14 8.34 4.17 12.30 -15.28 34.52 0.34 0.35 NS 0.81 Yes
U S XL 3 1 100.00 100.00 412.31 -96.48 11,272.58 0.24 0.34 n/a   n/a
U S XL 4 3 60.80 36.48 77.90 -40.18 409.57 0.47 0.59 NS 0.45 Yes
U U L 3 10 149.46 149.46 185.68 -42.00 972.97 0.80 1.23 NS 0.99 Yes
U U L 4 8 -15.40 -7.70 21.44 -34.34 64.22 0.36 -0.33 NS 0.92 Yes
U U LR 4 1 300.00 150.00 458.26 -47.76 17,790.77 0.33 0.61 n/a   n/a
U U R 4 1 100.00 100.00 529.15 -98.88 35,638.72 0.19 0.26 n/a   n/a

a Ratio of mean percent change in accident frequency to standard error of percent change in accident frequency per turn lane installed.

All of the unsignalized intersections evaluated in the study had two-way stop control; i.e., there were stop signs on the minor-road approach(es) and no control on the major road approaches. A newly signalized intersection is one that was unsignalized before the improvement and signalized after the improvement; i.e., the intersection was signalized in conjunction with the turn lane improvement.

Project type: The third column identifies the type of improvement project that was evaluated. the codes used in this column are:

L = Added left-turn lane(s)

LR = Added left- and right-turn lanes

R = Added right-turn lane(s)

XL = Extended the length of existing left-turn lane(s)

XLR = Extended the length of existing left- and right-turn lanes

Number of intersection legs: The fourth column identifies the number of intersection legs for the intersections evaluated. Only three- and four-leg intersections were considered in the evaluation.

Number of improved sites: The fifth column presents the number of improved or treatment sites included in the evaluation in question. In tables C-1 through C-4, C-9, and C-10, this represents the number of intersections evaluated. In tables C-5 through C-8, this represents the number of intersection approaches evaluated. The number of improved sites includes only those that were actually evaluated. Some sites were excluded because they were found to be outliers (see discussion in section 6 of this report). In the CG and EB analyses, some sites were excluded because no satisfactory regression model was available. For any given type of intersection and project type, the total number of sites available before such exclusions can be determined from tables in section 4 of this report.

Percent change in accident frequency: The sixth through tenth columns present the mean percent change in accident frequency determined in the YC analysis, the mean percent change in accident frequency per turn lane installed, the standard error, and upper and lower confidence limits of the mean change in accident frequency per turn lane installed. A negative value of percent change in accident frequency represents a reduction in accidents, while a positive value represents an increase in accidents. The values in these five columns correspond to Emean for projects as a whole and Emean, Emean(se), Emean(upper), and Emean(lower) per turn lane added, as determined in Equations (12), (19), (20), and (21).

The value in the seventh column is the value in the sixth column divided by the mean number of turn lanes added in the projects evaluated. For example, the analysis in table C-1 addresses total intersection accidents. However, in some projects, turn lanes were added on one approach; in some projects, turn lanes were added on two approaches; and in some projects at signalized intersections, left-turn lanes were added on all four approaches. The value in this seventh column of the table expresses the effectiveness of the project on a per-added-lane basis to facilitate comparisons across project types. It represents the best overall measure of treatment effectiveness for the projects evaluated.

In results tables for analyses conducted for individual intersection approaches, the project effectiveness estimate, Emean, and the effectiveness estimate on a per-added-lane basis are usually the same because, in most cases, only one turn lane was added to each approach. However, for projects involving the addition of both left- and right-turn lanes, the two effectiveness measures may differ.

The standard error and confidence limits presented in the eighth through tenth columns of the table are measures of the precision of the mean accident frequency per turn lane added presented in the seventh column. The confidence interval for the analysis results, shown by the limits in the ninth and tenth columns of the tables, is not symmetrical above and below the mean because, as shown in section 5 of this report, the evaluation was performed on a logarithmic scale. The standard error shown in the eighth column of the tables is the best single estimate of the precision of the estimated mean project effectiveness per turn lane installed.

Treatment effect: The eleventh through thirteenth columns of the tables provide an assessment of the statistical significance of the treatment effect determined above. The ratio in the eleventh column is the mean percent change in accident frequency per turn lane installed divided by its standard error. While this ratio was not used directly in the YC and CG approaches, a similar ratio was used to determine statistical significance for the EB approach.

The twelfth and thirteen columns present the calculated Z-score for the treatment effect and the significance of that Z-score at the 5 percent significance level (i.e., the 95-percent confidence level). The codes for significance of the results are:

SIG = Treatment effect is statistically significant

NS = Treatment effect is not statistically significant

n/a = Significance of treatment effect cannot be determined (typically because only one site is available for evaluation)

Only those effectiveness measures found to be statistically significant should be relied upon.

Test for homogeneity: The fourteenth and fifteenth columns present the test for homogeneity discussed for the YC approach in section 5 of this report. The fourteenth column gives the value of PH determined as shown in table 23. The fifteenth column shows whether the treatment group is homogenous at the 5 percent significance level (i.e, 95-percent confidence level). The homogeneity result was noted, but no results were excluded based on this criterion.

Specific Evaluation Results

Table C-1 presents the results of the evaluation of treatment effectiveness for accidents of all severity levels and for a target area including the entire intersection (i.e., including all at-intersection and intersection-related accidents). Analyses of this type are referred to in this report as addressing total intersection accidents. Table C-2 presents comparable data to table C-1 for a dependent variable that includes only fatal and nonfatal injury accidents (i.e., property-damage-only accidents were excluded). Analyses of this type are referred to in this report as addressing fatal and injury intersection accidents.

Table C-3 is analogous to table C-1 in that it addresses all accident severity levels and includes the entire intersection as the target area, but it includes only project-related accidents. Project-related accidents have been defined in section 4 of this report as those accidents that involve a turning maneuver that was related to the added or extended turn lane(s) being evaluated. Analyses of this type are referred to in this report as addressing total project-related intersection accidents. Table C-4 is similar to table C-3, but includes only fatal and nonfatal injury accidents. Analyses of this type are referred to in this report as addressing fatal and injury project-related intersection accidents.

Tables C-5 through C-8 are analogous to tables C-1 through C-4 except that they present results for evaluations in which each observation represents a treated intersection approach rather than a treated intersection as a whole. Only those approaches at the treated intersections on which a turn lane was added or extended were included in the analyses. The comparison site for each treated approach was comparable to the unimproved comparison site matched to that particular treatment site. Table C-5 presents the results for total accidents for individual intersection approaches. Table C-6 presents the results for fatal and injury accidents for individual intersection approaches. The results for project-related accidents for individual intersection approaches are presented in table C-7 and the results for project-related fatal and injury accidents for individual intersection approaches are presented in table C-8.

The results of the auto-matching approach referred to above are presented in table C-9 and C-10.

Comparison Group Evaluations

The results of the CG evaluation performed in this study are presented in tables C-11 through C-16. These tables include results for three of the four specific dependent variables that were included in the YC evaluation and for the same two different target areas included in the YC evaluation. No CG analyses were performed for project-related fatal and injury accidents because the available sample size of such accidents was too small to develop satisfactory regression relationships for use in the traffic volume adjustments.

Table C-11. Comparison Group Evaluation Results for Total Intersection Accidents.
Area type Traffic control Project type No. of legs No. of improved sites Percent change in accident frequency Treatment effect Test for homogeneity
Mean Mean per added lane Standard error Lower 95% confidence limit Upper 95% confidence limit Ratioa Calculated Z Significant at 5% level? Calculated H probability Homogeneous at 5% level?
R N L 4 2 -12.66 -6.33 9.22 -21.13 16.07 0.69 -0.64 NS 0.55 Yes
R N LR 4 1 -64.48 -32.24 9.19 -43.56 -1.05 3.51 -2.00 n/a   n/a
R N R 4 1 18.80 9.40 14.97 -13.76 47.35 0.63 0.68 n/a   n/a
R U L 3 35 -53.53 -53.53 5.63 -63.34 -41.08 9.52 -6.33 SIG 0.35 Yes
R U L 4 25 -60.61 33.67 2.54 -38.14 -28.07 13.23 -8.01 SIG 0.00 No
R U LR 3 12 -37.67 -30.13 12.57 -49.57 1.72 2.40 -1.88 NS 0.20 Yes
R U LR 4 15 -25.20 -12.60 5.28 -21.64 -0.68 2.39 -2.06 SIG 0.31 Yes
R U R 3 12 4.98 4.98 30.96 -41.11 87.14 0.16 0.16 NS 1.00 Yes
R U R 4 29 -35.07 -22.60 5.16 -31.58 -11.17 4.38 -3.51 SIG 0.86 Yes
R U XL 4 2 -20.58 -20.58 27.89 -60.09 58.07 0.74 -0.66 NS 0.19 Yes
U N L 3 4 29.35 19.57 13.88 -3.77 51.55 1.41 1.60 NS 0.01 No
U N L 4 28 -46.43 -24.08 2.01 -27.75 -19.85 11.98 -8.63 SIG 0.00 No
U S L 3 3 21.24 21.24 50.03 -46.00 172.22 0.42 0.47 NS 0.04 No
U S L 4 37 -18.28 -5.78 1.05 -7.76 -3.64 5.50 -4.96 SIG 0.00 No
U S LR 3 2 5.44 3.63 15.27 -20.75 40.93 0.24 0.24 NS 0.45 Yes
U S LR 4 10 -26.59 -6.82 1.30 -9.20 -4.08 5.24 -4.47 SIG 0.11 Yes
U S R 3 1 -26.37 -13.19 15.84 -34.16 35.54 0.83 -0.71 n/a   n/a
U S R 4 17 8.82 -4.05 2.46 -8.61 1.07 1.64 -1.57 NS 0.00 No
U S XL 3 1 84.98 84.98 82.12 -22.51 341.59 1.03 1.39 n/a   n/a
U S XL 4 3 42.34 25.40 11.20 6.04 50.44 2.27 2.69 SIG 0.60 Yes
U U L 3 10 -34.95 -34.95 13.11 -56.18 -3.43 2.67 -2.13 SIG 0.12 Yes
U U L 4 9 -53.42 -26.71 2.95 -31.83 -20.15 9.06 -6.04 SIG 0.20 Yes
U U LR 4 1 -59.39 -29.69 12.06 -43.66 15.03 2.46 -1.52 n/a   n/a
U U R 3 1 129.38 129.38 403.35 -92.69 7,100.71 0.32 0.47 n/a   n/a
U U R 4 3 -35.85 -21.51 23.30 -48.25 66.11 0.92 -0.73 NS 0.46 Yes

a Ratio of mean percent change in accident frequency to standard error of percent change in accident frequency per turn lane installed.

Table C-12. Comparison Group Evaluation Results for Fatal and Injury Intersection Accidents.
Area type Traffic control Project type No. of legs No. of improved sites Percent change in accident frequency Treatment effect Test for homogeneity
Mean Mean per added lane Standard error Lower 95% confidence limit Upper 95% confidence limit Ratioa Calculated Z Significant at 5% level? Calculated H probability Homogeneous at 5% level?
R N L 4 2 -57.47 -28.74 6.30 -38.10 -12.01 4.56 -2.89 SIG 0.48 Yes
R N LR 4 1 -68.83 -34.42 9.97 -45.55 4.61 3.45 -1.82 n/a   n/a
R N R 4 1 -40.11 -20.05 10.97 -35.39 11.39 1.83 -1.40 n/a   n/a
R U L 3 35 -54.80 -54.80 8.30 -68.46 -35.23 6.60 -4.33 SIG 0.81 Yes
R U L 4 25 -73.90 -41.06 2.40 -45.08 -35.49 17.07 -8.10 SIG 0.41 Yes
R U LR 3 12 -32.38 -25.90 17.21 -51.00 20.92 1.51 -1.23 NS 0.80 Yes
R U LR 4 15 -44.67 -22.34 6.07 -32.01 -7.46 3.68 -2.70 SIG 0.77 Yes
R U R 3 12 5.33 5.33 40.07 -50.03 122.02 0.13 0.14 NS 0.99 Yes
R U R 4 29 -37.21 -23.98 6.59 -35.05 -8.76 3.64 -2.86 SIG 0.97 Yes
R U XL 4 2 24.09 24.09 64.90 -55.49 245.89 0.37 0.41 NS 0.05 Yes
U N L 3 4 32.94 21.96 24.56 -15.18 85.91 0.89 1.03 NS 0.07 Yes
U N L 4 28 -48.65 -25.23 3.12 -30.69 -18.35 8.08 -5.69 SIG 0.00 No
U S L 3 3 2.67 2.67 71.14 -73.60 299.23 0.04 0.04 NS 0.31 Yes
U S L 4 39 -17.95 -5.79 1.73 -8.97 -2.17 3.35 -3.03 SIG 0.00 No
U S LR 3 2 -0.94 -0.62 21.64 -31.92 58.84 0.03 -0.03 NS 0.83 Yes
U S LR 4 10 -45.93 -11.78 1.80 -14.89 -7.76 6.54 -4.73 SIG 0.54 Yes
U S R 3 1 -61.42 -30.71 17.17 -46.63 60.41 1.79 -1.07 n/a   n/a
U S R 4 17 -8.61 -3.85 3.89 -10.81 4.54 0.99 -0.95 NS 0.01 No
U S XL 3 1 80.37 80.37 120.37 -51.23 567.14 0.67 0.88 n/a   n/a
U S XL 4 3 8.75 5.25 15.70 -19.29 44.56 0.33 0.35 NS 0.36 Yes
U U L 3 10 -31.93 -31.93 20.81 -62.62 23.95 1.53 -1.26 NS 0.65 Yes
U U L 4 9 -58.81 -29.40 4.04 -35.98 -19.74 7.28 -4.52 SIG 0.39 Yes
U U LR 4 1 -73.31 -36.66 15.56 -48.64 81.13 2.36 -1.13 n/a   n/a
U U R 3 1 135.84 135.84 425.90 -93.15 8,024.99 0.32 0.48 n/a   n/a
U U R 4 3 -1.49 -0.89 47.52 -47.77 225.77 0.02 -0.02 NS 0.82 Yes

a Ratio of mean percent change in accident frequency to standard error of percent change in accident frequency per turn lane installed.

 

Area type Traffic control Project type No. of legs No. of improved sites Percent change in accident frequency Treatment effect Test for homogeneity
Mean Mean per added lane Standard error Lower 95% confidence limit Upper 95% confidence limit Ratioa Calculated Z Significant at 5% level? Calculated H probability Homogeneous at 5% level?
R N L 4 2 119.39 59.70 108.53 -34.22 712.68 0.55 0.79 NS 0.95 Yes
R N LR 4 1 105.35 52.67 206.39 -48.00 5,228.66 0.26 0.36 n/a   n/a
R N R 4 1 11.81 5.91 112.35 -48.91 2,821.20 0.05 0.06 n/a   n/a
R U L 3 35 -62.33 -62.33 14.53 -82.32 -19.76 4.29 -2.53 SIG 1.00 Yes
R U L 4 25 -38.23 -21.24 9.33 -35.41 2.90 2.28 -1.77 NS 0.99 Yes
R U LR 3 12 -50.64 -40.51 27.28 -69.80 72.91 1.49 -1.02 NS 1.00 Yes
R U LR 4 14 6.66 3.33 20.51 -24.90 63.32 0.16 0.17 NS 0.94 Yes
R U R 3 12 -46.82 -46.82 38.63 -87.19 120.84 1.21 -0.87 NS 1.00 Yes
R U R 4 29 0.88 0.57 20.49 -29.39 56.15 0.03 0.03 NS 1.00 Yes
R U XL 4 2 -30.26 -30.26 66.53 -89.25 352.44 0.45 -0.38 NS 0.39 Yes
R N L 3 4 119.85 79.90 149.53 -46.82 1,015.85 0.53 0.77 NS 0.99 Yes
U N L 4 27 38.91 20.21 15.26 -4.27 57.26 1.32 1.55 NS 0.49 Yes
U S L 3 3 44.59 44.59 185.80 -88.35 1,694.62 0.24 0.29 NS 0.82 Yes
U S L 4 39 -7.22 -2.33 3.16 -7.92 4.56 0.74   NS 0.20 Yes
U S LR 3 2 249.52 166.35 356.90 -55.09 4,623.02 0.47 0.82 NS 0.51 Yes
U S LR 4 9 -40.18 -10.33 2.19 -14.07 -5.39 4.73 -3.62 SIG 0.09 Yes
U S R 3 1 -8.81 -4.40 55.35 -45.78 442.35 0.08 -0.08 n/a   n/a
U S R 4 17 5.88 2.78 8.38 -11.22 22.21 0.33 0.34 NS 0.00 No
U S XL 3 1 683.87 683.87 1,960.49 -94.17 105,378.46 0.35 0.82 n/a   n/a
U S XL 4 3 25.14 15.08 38.48 -32.51 145.03 0.39 0.44 NS 0.88 Yes
U U L 3 10 59.28 59.28 113.90 -60.78 546.90 0.52 0.65 NS 1.00 Yes
U U L 4 9 -60.42 -30.21 7.58 -40.66 -8.07 3.99 -2.42 SIG 0.78 Yes
U U LR 4 1 -5.67 -2.83 61.43 -46.33 555.81 0.05 -0.04 n/a   n/a
U U R
3
1
48.58
48.58
401.88
-99.26
29,705.68
0.12
0.15
n/a   n/a
U U R
3
36.47
21.88
91.63
-50.87
674.17
0.24
0.28
NS 0.81 Yes

a Ratio of mean percent change in accident frequency to standard error of percent change in accident frequency per turn lane installed.

Table C-14. Comparison Group Evaluation Results for Total Accidents on Individual Intersection Approaches.
Area type Traffic control Project type No. of legs No. of improved sites Percent change in accident frequency Treatment effect Test for homogeneity
Mean Mean per added lane Standard error Lower 95% confidence limit Upper 95% confidence limit Ratioa Calculated Z Significant at 5% level? Calculated H probability Homogeneous at 5% level?
R N L 4 4 -44.09 -44.09 7.26 -33.19 -3.51 3.04 -2.24 SIG 0.60 Yes
R N LR 4 2 -68.84 -34.42 8.75 -44.82 -3.15 3.93 -2.08 SIG 0.90 Yes
R N R 4 2 -29.49 -29.49 10.30 -30.11 12.49 1.43 -1.20 NS 0.17 Yes
R U L 3 70 -51.91 -51.91 7.31 -64.29 -35.23 7.10 -4.82 SIG 0.99 Yes
R U L 4 50 -61.03 -61.03 2.74 -38.66 -27.81 12.38 -7.45 SIG 0.17 Yes
R U LR 3 24 -29.08 -14.54 16.75 -48.20 21.21 1.39 -1.16 NS 0.64 Yes
R U LR 4 30 -27.90 -13.95 5.70 -23.55 -0.85 2.45 -2.07 SIG 0.99 Yes
R U R 3 24 5.74 5.74 37.73 -47.46 112.78 0.15 0.16 NS 1.00 Yes
R U R 4 58 -31.55 -31.55 5.53 -29.95 -8.04 3.67 -3.02 SIG 1.00 Yes
R U XL 4 4 6.54 6.54 42.74 -51.47 133.90 0.15 0.16 NS 0.65 Yes
U N L 3 8 30.27 30.27 17.86 -8.64 63.30 1.13 1.29 NS 0.22 Yes
U N L 4 56 -45.73 -45.73 2.35 -27.96 -18.71 10.09 -7.32 SIG 0.02 No
U S L 3 9 34.12 34.12 58.51 -42.97 215.40 0.58 0.67 NS 0.57 Yes
U S L 4 147 -27.96 -27.96 1.03 -10.89 -6.85 8.70 -7.35 SIG 0.00 No
U S LR 3 6 -10.13 -5.06 14.65 -29.57 30.09 0.46 -0.44 NS 0.62 Yes
U S LR 4 39 -34.45 -17.23 1.24 -11.10 -6.23 7.14 -5.74 SIG 0.90 Yes
U S R 3 3 -23.41 -23.41 19.93 -36.19 56.22 0.59 -0.51 NS 0.25 Yes
U S R 4 66 -6.98 -6.98 2.59 -8.09 2.10 1.27 -1.22 NS 0.01 No
U S XL 3 1 -57.24 -57.24 66.62 -97.98 806.21 0.86 -0.55 n/a   n/a
U S XL 4 12 45.26 45.26 12.11 6.38 54.43 2.24 2.69 SIG 0.08 Yes
U U L 3 20 -49.26 -49.26 11.15 -67.02 -21.93 4.42 -3.09 SIG 0.83 Yes
U U L 4 18 -54.42 -54.42 3.11 -32.56 -20.21 8.74 -5.75 SIG 0.39 Yes
U U LR 4 2 -56.21 -28.11 15.45 -44.51 37.27 1.82 -1.17 NS 0.21 Yes
U U R 3 2 -2.67 -2.67 141.65 -94.39 1,587.02 0.02 -0.02 NS 1.00 Yes
U U R 4 6 -20.14 -20.14 27.03 -44.14 84.77 0.45 -0.40 NS 0.90 Yes

a Ratio of mean percent change in accident frequency to standard error of percent change in accident frequency per turn lane installed.

Table C-15. Comparison Group Evaluation Results for Fatal and Injury Accidents on Individual Intersection Approaches.
Area type Traffic control Project type No. of legs No. of improved sites Percent change in accident frequency Treatment effect Test for homogeneity
Mean Mean per added lane Standard error Lower 95% confidence limit Upper 95% confidence limit Ratioa Calculated Z Significant at 5% level? Calculated H probability Homogeneous at 5% level?
R N L 4 4 -76.35 -76.35 4.15 -44.06 26.46 9.19 -4.11 SIG 0.84 Yes
R N LR 4 2 -69.73 -34.86 9.97 -45.84 5.02 3.50 -1.81 NS 0.43 Yes
R N R 4 2 -65.60 -65.60 7.55 -42.73 9.32 4.34 -2.43 SIG 0.09 Yes
R U L 3 70 -43.57 -43.57 10.90 -61.35 17.61 4.00 -2.96 SIG 1.00 Yes
R U L 4 50 -70.83 -70.83 2.73 -43.91 33.01 14.41 -7.31 SIG 0.99 Yes
R U LR 3 24 -15.48 -7.74 23.51 -45.79 53.66 0.53 -0.48 NS 0.91 Yes
R U LR 4 30 -35.96 -17.98 7.57 -29.86 0.90 2.37 -1.88 NS 0.99 Yes
R U R 3 24 -20.87 -20.87 30.78 -63.09 69.62 0.68 -0.60 NS 1.00 Yes
R U R 4 58 -36.95 -36.95 6.33 -34.50 9.31 3.76 -2.96 SIG 1.00 Yes
R U XL 4 4 100.52 100.52 121.69 -38.96 558.79 0.83 1.15 NS 0.75 Yes
U N L 3 8 -11.37 -11.37 21.91 -38.10 55.55 0.35 -0.33 NS 0.50 Yes
U N L 4 55 -46.93 -46.93 3.67 -30.68 16.12 6.63 -4.75 SIG 0.92 Yes
U S L 3 9 18.52 18.52 73.44 -64.81 299.22 0.25 0.27 NS 0.95 Yes
U S L 4 154 -22.58 -22.58 1.71 -10.45 3.71 4.26 -3.74 SIG 0.94 Yes
U S LR 3 6 -5.03 -2.51 23.09 -35.68 62.72 0.15 -0.14 NS 0.94 Yes
U S LR 4 39 -49.70 -24.85 1.83 -15.88 -8.62 6.98 -4.85 SIG 0.97 Yes
U S R 3 3 -31.42 -31.42 30.06 -43.85 141.11 0.52 -0.43 NS 0.68 Yes
U S R 4 70 1.18 1.18 4.61 -7.68 10.52 0.12 0.12 NS 0.28 Yes
U S XL 3 2 5.75 5.75 124.45 -89.47 961.68 0.05 0.05 NS 0.99 Yes
U S XL 4 12 0.93 0.93 15.15 -22.91 38.89 0.04 0.04 NS 0.99 Yes
U U L 3 20 -37.99 -37.99 19.50 -66.53 14.86 1.95 -1.52 NS 0.96 Yes
U U L 4 18 -55.38 -55.38 4.78 -35.34 -16.06 5.80 -3.77 SIG 0.72 Yes
U U LR 4 2 -41.19 -20.60 31.78 -46.46 194.57 0.65 -0.49 NS 0.41 Yes
U U R 3 2 -19.16 -19.16 123.35 -95.94 1,508.78 0.16 -0.14 NS 1.00 Yes
U U R 4 6 46.78 46.78 60.66 -37.17 279.74 0.46 0.56 NS 0.99 Yes

a Ratio of mean percent change in accident frequency to standard error of percent change in accident frequency per turn lane installed.

Table C-16. Comparison Group Evaluation Results for Project-Related Accidents on Individual Intersection Approaches.
Area type Traffic control Project type No. of legs No. of improved sites Percent change in accident frequency Treatment effect Test for homogeneity
Mean Mean per added lane Standard error Lower 95% confidence limit Upper 95% confidence limit Ratioa Calculated Z Significant at 5% level? Calculated H probability Homogeneous at 5% level?
R N L 4 4 6.61 6.61 45.97 -40.17 238.93 0.07 0.07 NS 0.96 Yes
R N LR 4 2 83.23 41.61 130.98 -44.44 1,459.75 0.32 0.42 NS 1.00 Yes
R N R 4 2 -3.71 3.71 68.71 -47.06 739.57 0.03 -0.03 NS 1.00 Yes
R U L 3 70 -64.29 -64.29 10.51 -79.95 -36.41 6.11 -3.50 SIG 1.00 Yes
R U L 4 50 -23.25 -23.25 10.03 -28.67 12.07 1.29 -1.12 NS 1.00 Yes
R U LR 3 24 -50.45 -25.23 20.80 -65.83 30.86 1.94 -1.34 NS 1.00 Yes
R U LR 4 30 11.61 5.80 16.19 -18.40 48.55 0.36 0.38 NS 1.00 Yes
R U R 3 24 -47.37 -47.37 30.26 -82.94 62.40 1.57 -1.12 NS 1.00 Yes
R U R 4 58 20.39 20.39 19.36 -16.87 62.08 0.68 0.74 NS 1.00 Yes
R U XL 4 4 -25.90 -25.90 62.07 -85.65 282.72 0.42 -0.36 NS 0.84 Yes
U N L 3 8 84.44 84.44 94.58 -39.44 488.65 0.60 0.80 NS 1.00 Yes
U N L 4 56 44.56 44.56 14.47 -0.51 57.58 1.60 1.91 NS 1.00 Yes
U S L 3 9 -0.47 -0.47 80.28 -79.52 383.64 0.01 -0.01 NS 1.00 Yes
U S L 4 155 7.92 7.92 3.46 -3.60 10.04 0.74 0.77 NS 1.00 Yes
U S LR 3 6 132.10 66.05 147.45 -42.76 934.97 0.60 0.88 NS 0.99 Yes
U S LR 4 38 -39.11 -19.56 2.28 -13.92 -4.88 4.41 -3.41 SIG 0.99 Yes
U S R 3 3 -21.65 -21.65 44.34 -45.74 310.13 0.24 -0.22 NS 1.00 Yes
U S R 4 72 -73.15 -73.15 12.06 14.32 62.26 2.89 3.76 SIG 0.44 Yes
U S XL 3 3 156.21 156.21 385.19 -86.55 4,778.84 0.41 0.63 NS 1.00 Yes
U S XL 4 12 36.34 36.34 38.97 -27.85 148.12 0.56 0.65 NS 0.99 Yes
U U L 3 20 54.97 54.97 82.88 -45.67 342.06 0.66 0.82 NS 1.00 Yes
U U L 4 18 -60.46 -60.46 6.92 -40.04 -10.76 4.37 -2.65 SIG 0.99 Yes
U U LR 4 2 31.64 15.82 72.94 -42.50 527.63 0.22 0.25 NS 0.77 Yes
U U R 3 2 46.13 46.13 335.89 -98.39 13,121.00 0.14 0.17 NS 1.00 Yes
U U R 4 6 91.07 91.07 98.90 -38.87 561.85 0.55 0.75 NS 1.00 Yes

a Ratio of mean percent change in accident frequency to standard error of percent change in accident frequency per turn lane installed.

The CG approach to before-after evaluation has been described conceptually in section 5 of this report. The procedures presented in that section have been used to derive the analysis results in tables C-11 through C-16. These analyses involved the matching of individual treatment sites to an entire group of similar unimproved comparison sites.

Description of Results Tables

The results tables for the CG approach are identical in format to the tables for the YC approach.

Specific Evaluation Results

The results of the CG evaluation are presented in the following tables:

  • Table C-11—total intersection accidents.
  • Table C-12—fatal and injury intersection accidents.
  • Table C-13—project-related intersection accidents.
  • Table C-14—total accidents for individual intersection approaches.
  • Table C-15—fatal and injury accidents for individual intersection approaches.
  • Table C-16—project-related accidents for individual intersection approaches.

Empirical Bayes Evaluations

The results of the EB evaluation are presented in tables C-17 through C-22. The tables include results for the same three dependent variables and the same two target areas that were included in the CG evaluation. No EB analyses were performed for project-related fatal and injury accidents because the available sample size of such accidents was too small to develop satisfactory regression relationships to represent expected accident frequencies in the EB analysis.

The EB approach to before-after evaluation has been described conceptually in section 5 of this report. The procedures presented in that section have been used to derive the analysis results in tables C-17 through C-22. These analyses involved weighing of observed and expected accident frequencies to obtain the best estimate of accident frequency for the before study period, which is then compared to the observed accident frequency for the after period.

 

Table C-17. Empirical Bayes Evaluation Results for Total Intersection Accidents.
Area type Traffic control Project type No. of legs No. of improved sites Percent change in accident frequency Ratioa Significant?
Mean Mean per added lane Standard error
R N L 4 2 6.59 3.30 9.79 0.3 NS
R N LR 4 1 -55.96 -27.98 9.56 2.9 n/a
R N R 4 1 46.76 23.38 16.22 1.4 n/a
R U L 3 36 -43.67 -43.67 5.47 8.0 SIG
R U L 4 25 -49.61 -27.56 2.63 10.5 SIG
R U LR 3 12 -29.42 -23.54 10.96 2.1 SIG
R U LR 4 15 2.06 1.03 5.35 0.2 NS
R U R 3 11 20.54 20.54 26.28 0.8 NS
R U R 4 28 -21.97 -13.98 5.17 2.7 SIG
R U XL 4 2 -22.00 -22.00 22.51 1.0 NS
U N L 3 3 46.57 27.94 13.78 2.0 SIG
U N L 4 25 -19.96 -10.40 2.77 3.8 SIG
U S L 4 39 -29.53 -9.52 0.83 11.4 SIG
U S LR 4 10 -27.80 -7.13 1.21 5.9 SIG
U S R 4 18 -9.01 -4.05 1.96 2.1 SIG
U S XL 4 3 49.45 29.67 10.62 2.8 SIG
U U L 3 8 -33.15 -33.15 12.11 2.7 SIG
U U L 4 9 -0.33 -0.17 4.97 0.0 NS
U U LR 4 1 -57.63 -28.82 11.01 2.6 n/a
U U R 3 1 7.05 7.05 111.08 0.1 n/a
U U R 4 3 -67.11 -40.26 10.08 4.0 SIG

a Ratio of mean percent change in accident frequency to standard error of percent change in accident frequency per turn lane installed.

Table C-18. Empirical Bayes Evaluation Results for Fatal and Injury Intersection Accidents.
Area type Traffic control Project type No. of legs No. of improved sites Percent change in accident frequency Ratioa Significant?
Mean Mean per added lane Standard error
R N L 4 2 -18.46 -9.23 10.11 0.9 NS
R N LR 4 1 -48.10 -24.05 13.71 1.8 n/a
R N R 4 1 4.07 2.03 16.28 0.1 n/a
R U L 4 24 -63.41 -35.39 3.01 11.8 SIG
R U LR 4 15 -22.50 -11.25 6.25 1.8 NS
R U R 4 28 -15.86 -10.33 7.93 1.3 NS
R U XL 4 2 1.19 1.19 39.29 0.0 NS
U N L 3 3 100.06 60.04 38.69 1.6 NS
U N L 4 14 -54.19 -28.10 4.95 5.7 SIG
U S L 4 39 -28.40 -9.15 1.31 7.0 SIG
U S LR 4 10 -45.23 -11.60 1.68 6.9 SIG
U S R 4 17 -20.55 -9.19 2.99 3.1 SIG
U S XL 4 3 31.88 19.13 15.83 1.2 NS
U U L 3 8 -23.54 -23.54 19.75 1.2 NS
U U L 4 2 -7.87 -3.93 33.49 0.1 NS
U U R 3 1 149.31 149.31 250.55 0.6 n/a
U U R 4 2 -53.41 -35.61 31.50 1.1 NS

a Ratio of mean percent change in accident frequency to standard error of percent change in accident frequency per turn lane installed.

 

Table C-19. Empirical Bayes Evaluation Results for Project-Related Intersection Accidents.
Area type Traffic control Project type No. of legs No. of improved sites Percent change in accident frequency Ratioa Significant?
Mean Mean per added lane Standard error
R N L 4 2 51.84 25.92 40.88 0.6 NS
R N LR 4 1 -100.00 -50.00     n/a
R N R 4 1 -100.00 -50.00     n/a
R U L 4 23 -66.23 -37.16 7.42 5.0 SIG
R U LR 4 14 53.05 26.52 22.68 1.2 NS
R U R 4 29 33.90 21.85 26.57 0.8 NS
R U XL 4 2 62.88 62.88 108.20 0.6 NS
U N L 4 13 6.03 3.13 9.49 0.3 NS
U U L 4 7 -51.15 -25.58 7.24 3.5 SIG
U U LR 4 1 -45.44 -22.72 29.05 0.8 n/a
U U R 4 1 -100.00 -50.00     n/a

a Ratio of mean percent change in accident frequency to standard deviation of percent change in accident frequency per turn lane installed.

Table C-20. Empirical Bayes Analysis Results for Total Accidents on Individual Intersection Approaches.
Area type Traffic control Project type No. of legs No. of improved sites Percent change in accident frequency Ratioa Significant?
Mean Mean per added lane Standard error
R N L 4 4 -22.62 -22.62 7.26 1.6 NS
R N LR 4 2 -61.41 -30.71 8.18 3.8 SIG
R N R 4 2 6.00 6.00 11.64 0.3 NS
R U L 3 62 -45.20 -45.20 6.50 7.0 SIG
R U L 4 50 -54.63 -54.63 2.41 12.6 SIG
R U LR 3 16 -30.38 -15.19 17.12 1.8 NS
R U LR 4 30 -16.76 -8.38 4.39 1.9 NS
R U R 3 18 104.41 104.41 61.09 1.7 NS
R U R 4 57 -26.66 -26.66 5.26 3.3 SIG
R U XL 4 4 -43.02 -43.02 17.03 2.5 SIG
U N L 3 6 44.68 44.68 18.88 1.4 NS
U N L 4 49 -28.02 -28.02 2.92 5.0 SIG
U S L 3 9 -49.26 -49.26 13.87 3.6 SIG
U S L 4 148 -34.15 -34.15 0.79 13.9 SIG
U S LR 3 6 -19.32 -9.66 9.15 1.4 NS
U S LR 4 38 -32.49 -16.24 1.14 7.3 SIG
U S R 3 3 -44.48 -44.48 10.42 2.1 SIG
U S R 4 67 -17.62 -17.62 1.96 4.2 SIG
U S XL 3 2 7.17 7.17 40.41 0.2 NS
U S XL 4 11 57.80 57.80 11.72 3.0 SIG
U U L 3 16 -32.28 -32.28 13.14 2.5 SIG
U U L 4 17 -20.13 -20.13 4.40 2.3 SIG
U U LR 4 2 -66.27 -33.13 9.93 3.3 SIG
U U R 3 2 -100.00 -100.00     NS
U U R 4 6 -75.80 -75.80 8.48 5.4 SIG

a Ratio of mean percent change in accident frequency to standard error of percent change in accident frequency per turn lane installed.

 

Table C-21. Empirical Bayes Evaluation Results for Fatal and Injury Accidents on Individual Intersection Approaches.
Area type Traffic control Project type No. of legs No. of improved sites Percent change in accident frequency Ratioa Significant?
Mean Mean per added lane Standard error
R N L 4 4 -42.11 -42.11 7.57 2.8 SIG
R N LR 4 2 -55.36 -27.68 11.57 2.4 SIG
R N R 4 2 -22.53 -22.53 12.25 0.9 NS
R U L 4 49 -60.99 -60.99 3.15 10.8 SIG
R U LR 4 30 -10.73 -5.37 7.33 0.7 NS
R U R 4 55 -24.28 -24.28 7.94 2.0 SIG
R U XL 4 4 -80.35 -80.35 70.18 1.1 NS
U N L 4 48 -43.18 -43.18 3.99 5.6 SIG
U S L 3 9 -47.59 -47.59 23.93 2.0 SIG
U S L 4 122 -35.32 -35.32 1.25 8.9 SIG
U S LR 3 5 -8.05 -4.03 19.49 0.3 NS
U S LR 4 35 -53.41 -26.70 1.51 9.1 SIG
U S R 3 3 -38.69 -38.69 22.06 0.9 NS
U S R 4 64 -22.20 -22.20 3.07 3.4 SIG
U S XL 3 2 -43.84 -43.84 56.78 0.8 NS
U S XL 4 11 31.47 31.47 14.96 1.2 NS
U U L 4 17 -5.21 -5.21 7.80 0.3 NS
U U LR 4 2 -42.22 -21.11 29.10 0.7 NS
U U R 4 6 -41.08 -41.08 25.25 1.0 NS

a Ratio of mean percent change in accident frequency to standard error of percent change in accident frequency per turn lane installed.

 

Table C-22. Empirical Bayes Analysis Results for Project-Related Accidents on Individual Intersection Approaches.
Area type Traffic control Project type No. of legs No. of improved sites Percent change in accident frequency Ratioa Significant?
Mean Mean per added lane Standard error
R N L 4 4 29.19 -29.19 32.30 0.5 NS
R N LR 4 2 -100.00 -50.00     NS
R N R 4 2 -100.00 -100.00   NS
R U L 4 50 -22.11 -22.11 11.06 1.1 NS
R U LR 4 30 97.51 48.76 24.04 2.0 SIG
R U R 4 58 34.23 34.23 26.33 0.8 NS
R U XL 4 4 50.96 50.96 97.67 0.5 NS
U N L 4 26 6.24 6.24 9.10 0.4 NS
U S L 4 127 -40.40 -40.40 1.82 6.9 SIG
U S LR 4 34 -49.50 -24.75 1.80 7.1 SIG
U S R 4 67 1.98 1.98 5.22 0.2 NS
U S XL 4 12 108.85 108.85 50.09 1.3 NS
U U L 4 14 -50.53 -50.53 7.24 3.5 SIG
U U LR 4 2 -42.14 -21.07 30.13 0.7 NS
U U R 4 2 -100.00 -100.00   NS

a Ratio of mean percent change in accident frequency to standard error of percent change in accident frequency per turn lane installed.

Description of Results Tables

The first seven columns of the results tables for the EB analysis are identical to the first five columns of the results tables for the YC and CG analysis. In particular, the sixth column presents the mean treatment effectiveness, q* from Equation (44), expressed as a percentage change in accident frequency. The seventh column presents the mean percentage change in accident frequency per turn lane added, determined in the same manner that it was for the YC and CG analyses.

The eighth column of the results tables for the EB analysis presents the standard error of the mean treatment effectiveness per turn lane added. This standard error is the square root of VAR{q*} from Equation (47), expressed as a percentage change in accident frequency.

The ninth column is a ratio determined as the mean treatment effectiveness (in the seventh column) divided by the standard error of treatment effectiveness (in the eighth column).

The tenth column shows the significance of the mean treatment effectiveness. The treatment effectiveness is considered to be significant if the ratio in the ninth column is greater than or equal to 2.0. This significance criterion is not a formal test of statistical significance at a specified confidence interval but, rather, is a criterion recommended by Hauer(2) for judging the results of EB analyses. This criterion is, however, equivalent to the statistical significance criteria used for the YC and CG approaches.

Specific Evaluation Results

The results of the EB evaluation are presented in the following tables:

  • Table C-17—total intersection accidents.
  • Table C-18—fatal and injury intersection accidents.
  • Table C-19—project-related intersection accidents.
  • Table C-20—total accidents for individual intersection approaches.
  • Table C-21—fatal and injury accidents for individual intersection approaches.
  • Table C-22—project-related accidents for individual intersection approaches.

Previous | Table of Contents | Next

ResearchFHWA
FHWA
United States Department of Transportation - Federal Highway Administration