Click for next page ( 24


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 23
23 3.4.5 Conduct of Data Collection were lost in transit from College Station, Texas, to Lincoln, Nebraska. Although reconstructions were possible for these The work on supplemental field data collection, quality 35 cases, much of the supplemental information such as road- control, and manual review and reconstruction of the sam- side topography, land use, highway classification, and highway pled cases was conducted over a period of approximately alignment could not be determined. 12 months. Of the 404 sampled cases, 15 were found to have major construction/reconstruction at the crash sites and thus were eliminated from the sample. One additional case was 3.5.2 Incorporation of Prior Data eliminated because it involved two vehicles. Thus, the final into Database sample size was reduced from 404 to 388. After the reconstructions and manual reviews were repeated for the TTI cases, the TTI and 17-22 data sets were subjected 3.5 Data from Previous Studies to a comprehensive evaluation to determine the appropriate- ness of combining them into a single data set. Each impor- NCHRP Project 17-11 and FHWA's Rollover Study incor- tant variable was tested to determine the significance of porated the same data collection procedures as used in the cur- differences between the two data sets. Whenever a variable rent study and included at total of 485 cases from NASS CDS was found to be significantly different at the p = 0.05 level, all for the years 1997 through 1999. These studies were conducted 877 cases were re-examined to identify the source of the by the Texas Transportation Institute (TTI) and therefore the error. In some cases, the errors were found to be related to data from the two studies will be referred to collectively as the way a specific parameter was measured. For example, "TTI data." Because the TTI data was collected and processed the heading angle at departure was measured from 180 to using the same protocol as the data collected in this (17-22) 180 degrees in the 17-22 data and from 0 to 360 degrees in study, it was believed to be appropriate to combine the two the TTI data. These errors were easily corrected. Other data data sets into a single file. Unfortunately, upon comparison of elements were found to have been poorly recorded on the basic crash data, such as departure velocity and angle, it became supplemental data forms. For example, in some cases, the apparent that the two data sets were not sufficiently similar to roadside slope was recorded as the highway grade. In this sit- be combined. The biggest differences were found in departure uation, the research team was forced to re-examine every case and impact angles. For example, the average departure angle to compare photographs at the scene with the recorded high- for the TTI data was found to be 19.9 degrees, compared to way grade. Whenever there was reasonable evidence of an 17.2 degrees for the 17-22 data. This 15% difference in average error, the entire file was examined for evidence of the highway departure angle was considered to be excessive. When a simple grade. In some cases, the highway grade was found in investi- T-test was applied to compare the two data sets, differences in gator notes on the supplemental data forms. In other situa- departure angle were found to be significant at the p = 0.001 tions, the elevation changes along the roadway were recorded level. These findings prompted a more careful examination of between the point of departure and at a point where the vehi- the differences between the TTI data set and the NCHRP 17-22 cle re-entered the roadway. These elevation changes were then data set. It was discovered that the TTI cases were recon- used to estimate highway grade at the crash site. Unfortunately, structed from scene diagrams downloaded from the NASS there were many cases where the highway grade could not be CDS website. These scene diagrams had been converted to identified and the variable had to be labeled as unknown. This PDF format before being posted on the website. Unfortu- type of examination was undertaken for a large number of nately, the process of converting the scene diagrams to PDF data elements that were found to be significantly different in changed the scaling of the drawings. The compression in the the two data sets. longitudinal direction was found to be greater than the com- As shown in Table 13, most variables with significant dif- pression in the lateral direction. As a result, all angle measure- ferences between the two data sets were corrected and the two ments were corrupted. data sets could be considered to be relatively similar. Unfor- tunately, significant differences remained for some variables, including speed limit, vehicle weight, height and width of 3.5.1 Manual Review and Crash object struck, rollover, and vehicle class. Differences in speed Reconstruction of Prior Cases limit and vehicle weight are believed to be appropriate. The In order to salvage the 485 cases included in the TTI data national speed limit law was repealed in late 1995 and was not set, it was necessary to obtain the original scene diagrams and implemented immediately in many states. In fact, 18 states repeat the reconstruction process for all of the cases. Unfor- had not implemented any change in speed limit before the end tunately, supplemental data forms for 35 of the TTI cases of 1997. Many of these states eventually raised speed limits.

OCR for page 23
24 Table 13. Comparison of 17-22 and TTI data. 17-22 Data TTI Data Std Std P Variable Units Mean SEM Mean SEM Dev. Dev. Value Dep. velocity km/h 80.00 26.00 1.32 78.70 25.30 1.15 0.48 Dep. angle deg. 17.20 11.90 0.60 16.90 10.20 0.47 0.70 IS value kJ 41.70 59.60 3.02 36.90 74.90 3.41 0.31 Degree of curvature deg. 2.27 7.50 2.65 2.65 6.72 0.32 0.45 Driver action 3.92 3.03 0.15 4.16 3.09 0.15 0.27 Month 6.68 3.45 0.17 6.39 3.00 0.14 0.20 Access control 2.27 0.92 0.05 2.28 0.94 0.04 0.82 Accident time 0.48 0.30 0.02 0.52 0.40 0.02 0.10 Alignment 1.53 0.79 0.04 1.61 0.80 0.04 0.17 Curb height mm 5.59 29.23 1.48 8.24 39.31 1.86 0.27 Curbs 0.09 0.40 0.02 0.08 0.36 0.02 0.64 Departure side 1.49 0.50 0.03 1.43 0.50 0.02 0.07 Divided/undivided 1.43 0.50 0.03 1.38 0.49 0.02 0.14 Grade % 1.50 1.67 0.08 1.39 1.49 0.07 0.10 Highway speed mph 57.45 9.24 0.47 55.68 9.44 0.43 0.006 limit Land use 1.76 0.43 0.02 1.70 0.47 0.02 0.06 Lane width m 3.69 0.55 0.03 3.64 0.52 0.02 0.18 Lat distance from m 0.07 13.45 0.68 1.24 13.22 0.60 0.20 departure to rest Lateral travel m 0.37 12.22 0.62 1.02 12.93 0.59 0.45 Heading angle at deg. 166.15 111.15 5.63 165.27 111.47 5.21 0.91 point of rest Long. distance from m 46.40 37.83 1.91 44.84 40.05 1.82 0.56 dep. to rest st Long. travel, 1 m 39.14 30.89 1.56 39.79 34.71 1.58 0.78 encroachment Material of 5.02 2.60 0.13 4.70 2.29 0.11 0.06 object struck No. of slopes 4.11 1.81 0.09 3.94 1.59 0.08 0.15 Object diameter cm 33.54 26.55 2.81 29.44 38.90 2.66 0.36 Object height cm 475.84 699.70 62.33 215.86 240.01 17.60 0.0001 Object length cm 2937 6326 922.8 1145 2229 388.1 0.12 Object width m 68.91 292.38 19.15 292.38 433.98 38.97 0.0003 Road class 2.77 2.86 1.29 2.86 1.43 0.07 0.33 Road condition 1.36 0.82 0.04 1.31 0.72 0.03 0.32 Road profile 0.52 0.82 0.04 0.53 0.89 0.04 0.90 Road surface 1.21 0.65 0.03 1.25 0.75 0.03 0.46 Rollover 0.59 0.49 0.02 0.50 0.50 0.02 0.008 Shoulder type 1.27 0.74 0.04 1.30 0.84 0.04 0.55 Shoulder width m 1.77 1.31 0.07 1.86 1.40 0.07 0.37 Sideslip angle deg. -1.02 38.61 1.38 0.63 38.59 1.76 0.46 Vehicle weight lb 3348.32 861.96 43.59 3154.16 738.30 33.52 0.0003 Weather 1.24 0.69 0.03 1.20 0.57 0.03 0.32 X-section at 5.43 2.66 0.13 5.45 2.67 0.13 0.89 departure