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29 C h a p t e R 5 Summary observations for each of the four report objectives follow. ⢠How do the selected sites in aggregate compare to the United States as a whole? ⢠How do the participants sampled compare to all drivers in the United States? ⢠How do the vehicles sampled compare to the entire U.S. fleet? ⢠How do police-reported crash rates observed in the study compare to crash rates in the United States as a whole? how Do the Selected Sites in aggregate Compare to the United States as a Whole? The six recruitment sites, when aggregated, are similar to the nation as a whole for summer and winter temperatures (Figure 2.12). The aggregated sites had more annual rainfall (Figure 2.13) and were more urban (Figure 2.10) than the nation. The sites include a wide range of north/south and east/west geographical locations, elevations, temperatures (especially in winter), and primary city population sizes. The population density comparison illustrates how the SHRP 2 data can be treated when they are analyzed. If pop- ulation density is irrelevant to the analysis topic, then the data can be used without adjustment or weighting. If popu- lation density is relevant, then the data can be adjusted in various ways. Table 2.1 and Figure 2.9 show that popula- tion density varied substantially across the six sites, from 112 persons per square mile in Pennsylvania to 959 in Florida. A study wishing to analyze a low-density traffic safety issue could restrict itself to data from the two low-density sites, Indiana and Pennsylvania. A study wishing to consider data from a population density similar to the nation as a whole could use the entire data set but weight data from these two low-density sites more heavily than data from the other four sites. how Do the participants Sampled Compare to all Drivers in the United States? The socioeconomic factors examined were age, gender, eth- nicity, race, income, employment, marital status, and level of education. SHRP 2 intentionally oversampled younger and older drivers so that SHRP 2 participants included more of the younger and older driver age groups than the U.S. driv- ing population; the proportion of males and females was about the same. Compared to the total U.S. population, SHRP 2 drivers have a lower proportion identifying them- selves as Hispanic (Figure 4.2 and Table 4.1), a higher pro- portion identifying as white (Figure 4.3), a similar proportion employed (Figure 4.6), a lower proportion employed full- time (Figure 4.7), a lower proportion married (Figure 4.9), and a higher proportion with a college degree (Figure 4.10). how Do the Vehicles Sampled Compare to the entire U.S. Fleet? The SHRP 2 fleet included a greater proportion of cars than other light vehicle types (0.71 and 0.29, respectively), a gap even more pronounced when considered against the national fleet as of January 1, 2012, which features an almost even split between cars and the other three light vehicle categories, namely trucks, vans, and SUVs/crossovers. The 26 model years and 19 makes included in the SHRP 2 study represent more than 95% and 98%, respectively, of the national fleet as of January 1, 2012. The SHRP 2 fleet substantially oversampled model years 2006 to 2011 to collect as much network data as feasible (Figure 4.12). Although the SHRP 2 vehicles include Summary and Conclusion
30 substantial numbers of the major vehicle makes, SHRP 2 had larger proportions of Hyundai/Kia, Nissan, Honda, and Toyota models and smaller proportions of Chrysler, Ford, and GM models than the national fleet (Figure 4.13). how Do police-Reported Crash Rates Observed in the Study Compare to Crash Rates in the United States as a Whole? Because some SHRP 2 crashes are known to have been reported to the police, but other crashes may or may not have been reported, two SHRP 2 crash rates were calculated: a lower âconfirmedâ estimate, using the crashes known to have been reported, and an upper âpossibleâ estimate, using crashes known to have been reported or possibly reported. The con- firmed SHRP 2 crash rate is slightly below the national PR crash rate, and the possible SHRP 2 crash rate is substantially above the national PR rate (Figure 4.15). When broken out by driver age (Figure 4.16) or by data collection site (Figure 4.17), the same observations generally hold except for middle-aged drivers. Conclusion SHRP 2 data are fairly inclusive of the nation in many respects. The counties from which participants were drawn, when aggregated, include a wide range of geographical features, roadways, and climates. Similarly, the SHRP 2 vehicle fleet included most of the national fleetâs light vehicle makes and fairly recent model years, but differed from the national fleet with respect to vehicle type. SHRP 2 drivers ranged in age from 16 to 95, with younger and older drivers oversampled. Two SHRP 2 crash rates were calculated: a lower confirmed estimate, using the crashes known to have been reported to the police, and an upper possible estimate, using crashes pos- sibly reported or known to have been reported. The con- firmed SHRP 2 crash rate is slightly below the national PR crash rate, and the possible SHRP 2 rate is substantially above the national PR rate. Analysts who wish to use the SHRP 2 data to make com- parisons with a national population of some characteristic, such as driver age, will need to weight the SHRP 2 data to match the national distribution of that characteristic. An example of weighting by driver age and gender is provided in Appendix A.
