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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.