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From page 69... ... 69 CHAPTER 4: DATABASE DEVELOPMENT This chapter describes a summary of the data assembled for safety prediction model development and calibration. The database is founded on the road inventory data obtained from the HSIS. Read the entire page →
From page 70... ... 70 Develop Enhanced Data Collection Process and Software Data enhancement consisted of using supplemental data sources to acquire additional data for each freeway segment, ramp segment, and crossroad ramp terminal in the state database. A key element of this process is the use of aerial photography to extract additional road inventory data for each segment or terminal. Read the entire page →
From page 71... ... 71 ramp terminal) Read the entire page →
From page 72... ... 72 associated with one or more crashes. This residual often exerts undue leverage on the regression model coefficients and increases the error variance beyond that explained by the negative binomial distribution. Read the entire page →
From page 73... ... 73 this segment-based file was assembled, the construction of the road inventory database for Maine proceeded in accordance with the steps outlined in the previous bullet list. A manual assembly process was necessary to assemble the crossroad ramp terminal database for each state. Read the entire page →
From page 74... ... 74 Database Attributes. Table 13 indicates the number of database variables in each of the nine databases that were assembled. Read the entire page →
From page 75... ... 75 Thus, a sampling technique was developed to select a cross section of freeway (and ramp) segments, such that a uniform distribution of values for several key variables was obtained. Read the entire page →
From page 76... ... 76 Geo-Location of Segments The third task of the database assembly activity was the geo-location of each segment and ramp terminal in the database. The location process was based on the use of Google Earth and Earth Tools software. Read the entire page →
From page 77... ... 77 The extraction and processing of supplemental data was the most time-consuming of all the data collection activities. Technicians were used for most of the extraction activities. Read the entire page →
From page 78... ... 78 Washington HSIS database includes a variable that indicates whether a crash is related to the intersection's activity, behavior, or control. This variable was evaluated using various crossvariable tabulations and found to be reasonably accurate in identifying crossroad-ramp-terminalrelated crashes. Read the entire page →
From page 79... ... 79 TABLE 15. Criteria for defining crossroad-ramp-terminal-related crashes State Leg Criteria Washington Crossroad 1. Read the entire page →
From page 80... ... 80 four states found that the percentage of PDO crashes varies from 60 to 77 percent. Slightly smaller variation was found for urban freeways. Read the entire page →
From page 81... ... 81 TABLE 17. Summary characteristics for freeway segments State Area Type Through Lanes Total Segments Total Length, mi Seg. Read the entire page →
From page 82... ... 82 lanes were added to the alignment without taking additional right-of-way. In these situations, the clear zone or the median width is reduced and barrier is needed to protect motorists from the resulting hazards. Read the entire page →
From page 83... ... 83 The proportion of the total segment length that is located in a weaving section varies widely among the states. Maine has the least mileage in the database with weaving. Read the entire page →
From page 84... ... 84 TABLE 19. Crash data summary for freeway segments State Area Type Through Lanes Exposure,1 mvm FI Crashes / 3 years 4 Crash Rate, cr/mvm MultipleVehicle 2 SingleVehicle Ramp Entrance 3 Ramp Exit 3 Total California Rural 4 1,927 108 165 33 20 326 0.17 6 3,271 223 229 22 25 499 0.15 8 4,004 222 269 21 15 527 0.13 Urban 4 1,819 193 128 48 8 377 0.21 6 4,560 641 428 141 54 1,264 0.28 8 4,057 782 274 125 44 1,225 0.30 10 3,873 888 235 112 39 1,274 0.33 Overall: 23,511 3,057 1,728 502 205 5,492 0.23 Maine Rural 4 2,506 154 229 10 8 401 0.16 6 446 25 16 2 0 43 0.10 8 Urban 4 1,050 88 80 14 11 193 0.18 6 31 5 3 3 2 13 0.42 8 17 6 4 0 1 11 0.63 10 Overall: 4,051 278 332 29 22 661 0.16 Washingt on Rural 4 1,954 64 179 4 2 249 0.13 6 3,332 184 189 15 6 394 0.12 8 Urban 4 1,358 175 108 17 9 309 0.23 6 2,259 301 136 15 10 462 0.20 8 2,301 594 115 83 22 814 0.35 10 Overall: 11,203 1,318 727 134 49 2,228 0.20 All states Rural 4 6,387 326 573 47 30 976 0.15 6 7,050 432 434 39 31 936 0.13 8 4,004 222 269 21 15 527 0.13 Urban 4 4,227 456 316 79 28 879 0.21 6 6,850 947 567 159 66 1,739 0.25 8 6,375 1,382 393 208 67 2,050 0.32 10 3,873 888 235 112 39 1,274 0.33 Overall: 38,765 4,653 2,787 665 276 8,381 0.22 All states, excluding speedchange lanerelated crashes Rural 4 6,387 326 573 899 0.14 6 7,050 432 434 866 0.12 8 4,004 222 269 491 0.12 Urban 4 4,227 456 316 772 0.18 6 6,850 947 567 1,514 0.22 8 6,375 1,382 393 1,775 0.28 10 3,873 888 235 1,123 0.29 Overall: 38,765 4,653 2,787 7,440 0.19 Notes: 1 - mvm: million vehicle-miles. Read the entire page →
From page 85... ... 85 Interchange Ramp This section describes the ramp segments represented in the safety database. Initially, the traffic and geometric characteristics of each segment are presented. Read the entire page →
From page 86... ... 86 TABLE 20. Summary characteristics for ramp segments State Ramp Type 1 Ramp Config. Read the entire page →
From page 87... ... 87 proportion of the segment with barrier for Maine ramps is about twice that of the Washington and California ramps. Read the entire page →
From page 88... ... 88 The sixth column of Table 21 indicates the average proportion of the ramp segment length that has curvature. This proportion is highest for the loop ramp configurations, which is logical given their design. Read the entire page →
From page 89... ... 89 respectively. The variation in these percentages is consistent with the variation among the states found in the freeway segment crash data. Read the entire page →
From page 90... ... 90 The data in Table 22 are summarized by ramp type and configuration in Table 23 to facilitate some preliminary examination of trend. The crash rates shown indicate that button hook exit ramps tend to have the highest crash rate, which is intuitive given their inherently short length and sharp curvature. Read the entire page →
From page 91... ... 91 Crossroad Ramp Type: D3ex Type: D3en Ramp Fr ee w ay Fr ee w ay Crossroad Ramp Type: D4 Type: D4 Ramp Fr ee w ay Fr ee w ay Crossroad Ramp Type: A4 Type: A4 RampRamp Ramp Fr ee w ay Fr ee w ay a. Three-leg ramp terminal with diagonal exit ramp or entrance ramp (D3ex and D3en) Read the entire page →
From page 92... ... 92 Crossroad Ramp Type: B4 Type: B4 Ramp RampRamp Fr ee w ay Fr ee w ay Crossroad Ramp Type: A2 Type: A2 Ramp Fr ee w ay Fr ee w ay Crossroad Type: B2 Type: B2 Ramp Ramp Fr ee w ay Fr ee w ay d. Ramp terminal at four-quadrant parclo B (B4) Read the entire page →
From page 93... ... 93 Ramp Terminal Characteristics All total, 569 ramp terminals are represented in the combined database. More specifically, the database includes 216, 62, and 291 ramp terminals in California, Maine, and Washington, respectively. Read the entire page →
From page 94... ... 94 TABLE 24. Summary characteristics for crossroad ramp terminals State Control Type Terminal Configuration Total Terminals Crossroad Volume, veh/day Ramp Volume, veh/day Minimum Maximum Minimum Maximum Californ ia Signal Parclo A2 11 7,700 48,700 2,900 15,100 Parclo A4 41 13,100 68,700 1,900 22,000 Parclo B2 6 7,300 45,700 2,000 11,700 Parclo B4 0 Diagonal 3 leg 4 12,100 30,000 1,000 10,500 Diagonal 4 leg 55 6,800 46,400 3,000 15,500 Unsignalized 1 Parclo A2 8 700 10,500 500 4,400 Parclo A4 16 4,000 17,300 340 10,200 Parclo B2 6 745 23,000 310 7,200 Parclo B4 1 13,600 13,600 4,300 4,300 Diagonal 3 leg 5 11,800 40,900 1,000 6,100 Diagonal 4 leg 63 1,900 18,900 320 4,700 Speed-change 0 Overall: 216 700 68,700 310 22,000 Maine Signal Parclo A2 0 Parclo A4 1 15,200 15,200 4,700 4,700 Parclo B2 2 14,700 15,600 3,200 3,400 Parclo B4 0 Diagonal 3 leg 1 34,000 34,000 10,800 10,800 Diagonal 4 leg 4 13,300 17,800 4,000 6,000 Unsignalized 1 Parclo A2 7 3,400 17,300 1,700 14,700 Parclo A4 0 Parclo B2 9 4,600 12,100 1,200 4,600 Parclo B4 1 18,300 18,300 3,100 3,100 Diagonal 3 leg 7 4,700 11,100 590 2,400 Diagonal 4 leg 24 1,900 12,700 510 4,400 Speed-change 6 8,270 20,530 373 7,300 Overall: 62 1,900 34,000 373 14,700 Washin gton Signal Parclo A2 11 8,100 42,000 1,800 13,100 Parclo A4 8 11,900 42,800 1,200 7,900 Parclo B2 6 10,400 25,300 3,600 8,500 Parclo B4 2 31,700 38,300 5,300 14,700 Diagonal 3 leg 6 2,900 33,400 4,700 21,600 Diagonal 4 leg 87 9,400 43,300 3,500 11,700 Unsignalized 1 Parclo A2 11 3,000 17,200 860 7,000 Parclo A4 2 12,900 22,700 590 5,000 Parclo B2 7 1,200 26,800 380 5,400 Parclo B4 4 8,800 24,800 940 9,900 Diagonal 3 leg 19 1,200 22,400 270 17,700 Diagonal 4 leg 127 370 10,400 180 4,200 Speed-change 1 24,200 24,200 4,700 4,700 Overall: 291 370 43,300 180 21,600 Note: 1 - Unsignalized intersections have an uncontrolled major street and a stop-controlled minor street. Read the entire page →
From page 95... ... 95 TABLE 25. Crash data summary for crossroad ramp terminals State Control Type Terminal Configuration Exposure,1 mev Crashes / 3 years Crash Rate, cr/mev PDO 2 FI 3 Total FI 3 Total Califo rnia Signal Parclo A2 442.1 111 41 152 0.09 0.34 Parclo A4 2,252.2 552 287 839 0.13 0.37 Parclo B2 253.6 47 19 66 0.07 0.26 Parclo B4 Diagonal 3 leg 125.3 16 14 30 0.11 0.24 Diagonal 4 leg 1,900.5 742 298 1,040 0.16 0.55 Unsignalized 1 Parclo A2 57.6 15 8 23 0.14 0.40 Parclo A4 208.4 89 43 132 0.21 0.63 Parclo B2 73.1 16 8 24 0.11 0.33 Parclo B4 19.5 0 1 1 0.05 0.05 Diagonal 3 leg 143.1 36 19 55 0.13 0.38 Diagonal 4 leg 648.0 144 72 216 0.11 0.33 Speed-change Overall: 6,123.4 1,768 810 2,578 0.13 0.42 Maine Signal Parclo A2 Parclo A4 21.8 8 4 12 0.18 0.55 Parclo B2 40.5 19 16 35 0.40 0.86 Parclo B4 Diagonal 3 leg 49.0 41 15 56 0.31 1.14 Diagonal 4 leg 90.2 87 45 132 0.50 1.46 Unsignalized 1 Parclo A2 114.7 56 26 82 0.23 0.71 Parclo A4 Parclo B2 105.6 29 10 39 0.09 0.37 Parclo B4 23.4 12 4 16 0.17 0.68 Diagonal 3 leg 73.3 10 9 19 0.12 0.26 Diagonal 4 leg 203.0 63 28 91 0.14 0.45 Speed-change 107.0 34 14 48 0.13 0.45 Overall: 828.5 359 171 530 0.21 0.64 Washi ngton Signal Parclo A2 320.9 125 82 207 0.26 0.65 Parclo A4 227.8 92 43 135 0.19 0.59 Parclo B2 169.0 78 46 124 0.27 0.73 Parclo B4 98.5 48 24 72 0.24 0.73 Diagonal 3 leg 231.1 73 36 109 0.16 0.47 Diagonal 4 leg 2,883.9 1,310 770 2,080 0.27 0.72 Unsignalized 1 Parclo A2 133.6 14 15 29 0.11 0.22 Parclo A4 45.1 2 3 5 0.07 0.11 Parclo B2 90.5 21 13 34 0.14 0.38 Parclo B4 88.1 22 14 36 0.16 0.41 Diagonal 3 leg 252.2 46 26 72 0.10 0.29 Diagonal 4 leg 870.5 211 121 332 0.14 0.38 Speed-change 31.7 3 3 6 0.09 0.19 Overall: 5,442.8 2,045 1,196 3,241 0.22 0.60 Notes: 1 - mev: million-entering-vehicles per year. Read the entire page →
From page 96... ... 96 The data in Table 25 are summarized by control type and terminal configuration in Table 26 to facilitate some preliminary examination of trend. The crash rates shown indicate that the signalized "parclo B4" configuration has the highest crash rate. Read the entire page →
From page 97... ... 97 TABLE 26. Crash data summary by ramp terminal configuration Control Type Terminal Configuration Exposure, mev Crashes / 3 years Crash Rate, cr/mev PDO FI Total FI Total Signal Parclo A2 763.0 236 123 359 0.16 0.47 Parclo A4 2,501.8 652 334 986 0.13 0.39 Parclo B2 463.1 144 81 225 0.17 0.49 Parclo B4 98.5 48 24 72 0.24 0.73 Diagonal 3 leg 405.4 130 65 195 0.16 0.48 Diagonal 4 leg 4,874.6 2,139 1,113 3,252 0.23 0.67 Unsignalized Parclo A2 305.9 85 49 134 0.16 0.44 Parclo A4 253.5 91 46 137 0.18 0.54 Parclo B2 269.2 66 31 97 0.12 0.36 Parclo B4 131.0 34 19 53 0.15 0.40 Diagonal 3 leg 468.6 92 54 146 0.12 0.31 Diagonal 4 leg 1,721.5 418 221 639 0.13 0.37 Speed-change 138.7 37 17 54 0.12 0.39 Overall: 12,394.8 4,172 2,177 6,349 0.18 0.51 TABLE 27. Read the entire page →
From page 98... ... 98 Freeway Speed-Change Lane This section describes the freeway speed-change lanes represented in the safety database. As described previously, all crashes located (by milepost) Read the entire page →
From page 99... ... 99 TABLE 28. Summary characteristics for freeway speed-change lane segments State Ramp Type No. Read the entire page →

From page 69...

... 69 CHAPTER 4: DATABASE DEVELOPMENT This chapter describes a summary of the data assembled for safety prediction model development and calibration. The database is founded on the road inventory data obtained from the HSIS.