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21
20 ft). These differences among states can affect the model Data Analysis
development because they may influence the presence or
As noted above, predictive models were developed to
absence of a variable as well as the magnitude of its coefficients.
evaluate trade-offs among selected design elements. The unit
In addition to this evaluation, a preliminary analysis was
of analysis is a roadway segment with its associated crash
also made to estimate the crash rates for the variables of concern
history. The database records are based on roadway segments
(see Table 11). The data show that, in general, divided high- that have consistent geometric features for their corresponding
ways have lower crash rates, the segments with a median length. Each record included the total number of crashes and
barrier have higher crash rates than do segments without, and total number of injury crashes. A distinction was made with
there is a difference between single- and multi-vehicle crashes respect to the number of vehicles involved in the crash,
depending on whether the roadway is divided. The median with crashes classified as single-vehicle or multi-vehicle for
width has a positive effect (i.e., lower crash rates) up to 40 feet; both total and injury crashes. The goal of the analysis was to
the crash rates increase above that width. The same could be isolate the effect of a single parameter. For example, all road
observed for shoulder width, where the crash rate decreases segments in four-lane undivided arterials would be used in
up to 6 ft and then varies as the shoulder becomes wider. developing a model to determine the potential effect of the var-
These trends are simple observations, and statistical tests were ious features on total number of crashes or other crash types
not conducted to determine their statistical significance. (i.e., single-vehicle, multi-vehicle or injury crashes).
Table 11. Crash rates for selected variables.
Variable Categories Divided Undivided
Yes 48.97 77.15
Principal arterial
No 51.63 77.83
Median barrier Yes 98.95 NA
No 46.67 NA
Single 29.21 36.68
Vehicles
Multi 20.15 39.44
Yes 74.21 128.84
Paved right shoulder
No 60.40 79.25
0 89.45 155.51
02 82.26 87.04
24 60.15 75.89
Average shoulder width (ft)
46 53.15 64.51
68 45.47 65.08
8+ 38.92 52.56
<5 72.78 92.90
510 49.88 75.94
ADT 1015 40.32 68.28
(vehicles/day; 000s)
1520 45.55 58.10
2025 38.86 89.10
>25 63.32 93.53
<10 74.75 NA
1020 55.65 NA
2030 47.99 NA
Median width (ft) 3040 38.85 NA
4050 42.56 NA
5060 43.90 NA
>60 46.98 NA
