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Publication Number: FHWARD98133
Date: October 1998 

Accident Models for TwoLane Rural Roads: Segment and Intersections6. Validation and Further AnalysisExplanatory Value of Final Models One way to assess the explanatory power of models is to examine the coefficient of determination R^{2} and see how it changes as one adds variables to the model. In Tables 46 and 47 and Figures 6
Table 46. Accident Variation by Groups of Covariates, Final Segment Model
and 7, this is done for three of the models S the combined segment model of Table 27, and the Minnesota threelegged and fourlegged models of Table 35. Because all of these models are of
Table 47. Accident Variation by Groups of Covariates, Final Intersection Models
negative binomial type, we use the LogLikelihood Rsquared proposed by Fridstrøm et al. (1995). With respect to this measure, negative binomial randomness is represented by 1  P^{2}_{ D}. The contribution of other factors is represented by R^{2}_{ D} for the first variable when a model with that variable present is used, and then the increment in R^{2}_{ D} for each additional variable as it is added to the model. Finally the unexplained portion of variation is P^{2}_{ D}  R^{2}_{ D}, where R^{2}_{ D} is the Rsquared value obtained when all variables are present. Although the LogLikelihood Rsquared is not the only way to compare explanatory values, it is a reasonable way to do so for negative binomial models (and we presume for their extended negative binomial counterparts). The tables and figures indicate that the portion of mean accident counts explained by variables other than exposure and ADT is small.

Topics: research, safety, rural roads, interchanges, intersections, twolane highway Keywords: research, safety, rural roads, interchanges, intersections, twolane highway, Minnesota, traffic accidents, crash data, mathematical models TRT Terms: Traffic accidentsMinnesota, Rural roadsMinnesota, RoadsMinnesotaInterchanges and intersections, Traffic accidentsWashington (State), Rural roadsWashington (State), RoadsWashington (State)Interchange and intersections, Two lane highways, Mathematical models, Accident data Updated: 04/12/2012
