| Comments: | This CMF is for total non-PDO crashes from Table 4.9 (p. 53). Therefore, this CMF applies to following crash severities: fatal, incapacitating injury, non-incapacitating injury, and possible injury. The Clearinghouse does not include “possible injury” within its list of crash severities so it cannot be selected. The CMF standard error was computed using the confidence interval given in Table 4.9 (p. 53): (CMF upper limit-CMF lower limit)/(2*1.96). The crash data consisted of records from 1998 to 2007 but excluded records from 2002 (p. 24). The study method was before-after with comparison group. Since this is not available on the list for study methodology, simple before-after was selected. The before and after sample sizes were computed from Table 4-1 as the sum of the before and after crash counts for the first four sites (i.e., the sites with comparison groups) (p. 41). The average ADT for the before and after period for each treatment site is given in Table 4-7 (p. 49); however, the overall average ADT for the treatment group is not provided. To compute the overall average ADT for the treatment group, a weighted average was taken using the treatment site average ADTs and the treatment site before and after period lengths listed in Table 3-4 (p. 36): 44*2200+60*1800+43*1800+60*2200+36*4600+60*5500+36*2600+60*3200/(44+60+43+60+36+60+36+60)=2996 |
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