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Publication Number: FHWA-HRT-05-042
Date: October 2005

Safety Effects of Differential Speed Limits

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APPENDIX G: EXAMINATION OF THE EFFECTS OF ADT ON TOTAL CRASH RATES

Table 14 showed that while the removal of sites with extremely high and low ADT affects the results of the statistical tests, the relationship between ADT and crashes is still unclear. Researchers may speculate that various types of relationships exist. For example, an investigator could argue that a high ADT/low speed situation results in more vehicle interaction, which would therefore increase the likelihood of a property damage only crash, but lessen the likelihood of a serious crash.

Histograms of ADT Versus Total Crash Rate

The data employed in these histograms were the total crash rates and related ADTs from two States: Arizona and Virginia. The highest 5 percent and lowest 5 percent of ADTs were removed from the data set, because they were regarded as extreme conditions. Figures 51-53 relate ADT to crash rates for Arizona (where no speed limit change was instituted), the Virginia data when DSL was in place, and the Virginia data where USL was in place.

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Figure 51. Chart. Arizona total crash rate versus ADT.

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Figure 52. Chart. Virginia total crash rate versus ADT (DSL in place).

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Figure 53. Chart. Virginia total crash rate versus ADT (USL in place).

Figure 53 suggests that, in Arizona, as the ADT increases, the total crash rate decreases, which would in fact support a β2 ADT exponent less than 1.0. However, figures 52 and 53 do not show a comparable trend.

Two-Way Analysis of Variance

The effect of two independent variables (ADT and speed limit type) was assessed on the total crash rate for both Arizona and Virginia, as illustrated in table 34 and the ANOVA results in tables 35 and 36 for Arizona and Virginia, respectively. In table 35, no significant difference was found between the before and after period, noting the p values of 0.137 and 0.129 for Arizona and Virginia. However, ADT did have a significant influence on total crash rates in both States. Meanwhile, the interaction of these two variables was significant in Arizona but not in Virginia.

Table 34. ANOVA variable definitions.

Variable State Level Definition Note
Speed Limits
Virginia
1 1991-1993 Year
2 1995-1999 Year
Arizona
1 1991-1995 Year
2 1996-1999 Year
ADT
Virginia
1 0-14,999 ADT Value
2 15,000-27,499 ADT Value
3 27,500-39,999 ADT Value
Arizona
1 0-14,999 ADT Value
2 15,000-27,499 ADT Value
3 27,500-39,999 ADT Value
4 40,000-52,499 ADT Value
5 52,500-65,000 ADT Value

Table 35. ANOVA Arizona results.

Source Type III Sum of Squares df Mean Square F Significance
Corrected Model 692,674.348a 9 76,963.816 22.487 0.000
Intercept 365,693.055 1 365,693.055 106.848 0.000
Speed Limit Type (DSL or USL) 7,570.345 1 7,570.345 2.212 0.137
ADT 480,476.984 4 120,199.246 35.096 0.000
Speed Limit Type (ADT) 36,564.203 4 9,141.051 2.671 0.031
Error 8,529,027.298 2,492 3,422.563 - -
Total 18,507,548.500 2,502 - - -
Corrected Total 9,221,701.646 2,501 - - -

Table 36. ANOVA Virginia results.

Source Type III Sum of Squares df Mean Square F Significance
Corrected Model 59,074.774a 5 11,814.955 9.438 0.000
Intercept 1,034,578.071 1 1,034,578.071 826.446 0.000
Speed Limit Type (DSL or USL) 2,891.319 1 2,891.319 2.310 0.129
ADT 45,023.284 2 22,511.642 17.983 0.000
Speed Limit Type (ADT) 746.468 2 373.234 0.298 0.742
Error 2,121,868.836 1,695 1,251.840 - -
Total 6,480,839.204 1,701 - - -
Corrected Total 2,180,943.611 1,700 - - -

 

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