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OCR for page 31

170
APPENDIX H
SPF Results for TOT, FI, and SSOD Crashes
on Selected Roadways Without
Centerline Rumble Strips
This appendix presents the SPF results developed for two- An empty cell in a row where an SPF was estimated indi-
lane roads based on all nontreatment sites (i.e., sites without cates the corresponding regression coefficient was not statis-
centerline rumble strips: BA-No RS). The same SPFs apply to tically significant at the 0.15 level or the coefficient's sign was
roads classified as urban and rural and to tangent and curved not of the expected direction.
roadways. Each line in each table provides the regression coef- For some analyses, the sample size was not sufficient to
ficients and their precision estimates for a given SPF where estimate an SPF directly from data. An approximation was
one was estimated. For example, using Table H-1 for two-lane employed in which a factor was applied to the SPF for total
roads in Pennsylvania: crashes in Table H-1. This factor is the ratio of the sum of all
relevant crashes at the NT sites to the sum of all (total)
Expected total crashes/mi/yr = exp ( -5.59 + 0.58 ln
n ADT crashes. The dispersion parameter is not adjusted, i.e., the one
+ 0.10 RHR + 0.30 SPD + 0.02 WIDTH ) for total crashes applies for all crash type subsets for which
SPFs are so approximated.
= e 5.59 × ADT 0.55 × e0.10 RHR × e0.30 SPD × e0.02 WIDTH (1)
where:
SPD = 0 if the posted speed limit is 55; otherwise 1
WIDTH = roadway width (ft)

OCR for page 31

Table H-1. SPF results for TOT crashes on two-lane rural roads based on all nontreatment sites.
Number Intercept lnADT RHR SPD WIDTH Dispersion
of p
State sites Estimate SEa Estimate SE pvalueb Estimate SE pvalue Estimate SE pvalue Estimate SE value Estimate SE R2LR
MN 244 -7.31 0.88 0.81 0.09 <.0001 0.22 0.09 0.022 0.74 0.16 30.25%
WA 228 -7.46 1.20 0.95 0.13 <.0001 0.57 0.12 46.37%
PA 603 -5.59 0.42 0.58 0.04 <.0001 0.10 0.04 0.005 0.30 0.06 <.0001 0.02 0.005 0.0007 0.29 0.03 91.31%
a
SE: standard error of estimate.
b
pvalue: significance level.
Table H-2. SPF results for FI crashes on two-lane rural roads based on all nontreatment sites.
Number Intercept lnADT RHR SPD WIDTH Dispersion
of p
State sites Estimate SEa Estimate SE pvalueb Estimate SE pvalue Estimate SE pvalue Estimate SE value Estimate SE R2LR
MN 244 A factor of 0.41 is applied to the SPF for total crashes in Table H -1
WA 228 A factor of 0.44 is applied to the SPF for total crashes in Table H -1
PA 603 -5.55 0.47 0.55 0.06 <.0001 0.31 0.07 <.0001 0.02 0.006 0.0005 0.36 0.04 86.91%
a
SE: standard error of estimate.
b
pvalue: significance level.
Table H-3. SPF results for SSOD crashes based on all nontreatment sites.
Number Intercept lnADT RHR SPD WIDTH Dispersion
of p
State sites Estimate SEa Estimate SE pvalueb Estimate SE pvalue Estimate SE pvalue Estimate SE value Estimate SE R2LR
MN 244 A factor of 0.12 is applied to the SPF for total crashes in Table H -1
WA 228 A factor of 0.05 is applied to the SPF for total crashes in Table H-1
PA 603 -7.78 0.95 0.73 0.11 <.0001 0.26 0.13 0.044 -0.02 0.01 0.10 0.60 0.03 39.36%
a
SE: standard error of estimate.
b
pvalue: significance level.