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31
Table 20. AMFs for median barrier ent values for a single or combination of design elements. The
presence, divided roadways. ratio of AMFs for two different conditions can be used to
establish the relative change in crashes anticipated from the
Category AMF
change in the values of the design element. The use of this
Single-vehicle 2.71
approach was noted as a method for estimating change in
Multi-vehicle 1.69
crashes by using
All crashes 2.18
AMF1
N = -1 (15)
AMF2
For all three models (single-vehicle, multi-vehicle, and all
crashes), the presence of median barrier had a negative effect where N is the change in crashes and AMFi are the AMFs for
(i.e., crashes increased). This trend is supported by the general the designs to be evaluated. This equation was modified from
observation that roadways with median barriers exhibit higher the form presented by Lord and Bonneson since no base
crash rates than do roads without them. The models developed models or base estimates are available in the method presented
in this research yielded coefficients of 0.999 (1-exp(0.999) = here (49). A positive value of N denotes an increase in crash
1.71) for single-vehicle; 0.523 (1-exp(0.523) = 0.69) for multi- frequency.
vehicle; and 0.781 (1-exp(0.781) = 1.18) for all crashes. The The following example demonstrates the use of the AMFs
analysis of the injury-only crashes included this variable only for estimating the safety implications from design choices:
in the single-vehicle and all-crashes models with similar trends
and magnitudes. The AMFs developed for each condition An agency is evaluating the effects of widening the shoulder of
from the models developed in this research are summarized a four-lane undivided highway from 4 ft to 8 ft. The AMFs for
in Table 20. divided roads obtained from Table 17 are 0.94 for 4-ft shoulders
and 0.71 for 8-ft. Using Equation 15, the expected crash change
will be
Applications
0.71
These AMFs can be used to estimate the relative impact of N = - 1 = -0.24
0.94
the choice of the value of a design element for a rural four-
lane roadway segment. The process described herein can also Therefore, increasing shoulder width from 4 ft to 8 ft will result
be applied to determine the safety implications using differ- in a 24% reduction in crashes per year per mile.
