The hope is that statistical models fitted to historical data can be used to estimate the effect of road design elements on safety. Whether this can be done is not clear. A sign of trouble is that models based on diverse data sets tend not to yield similar results. Suggestions are made on how to increase the chance of success in this quest. Emphasis is on three questions: Which variables should serve in the model? What mathematical function should represent their influence? How does one check whether the representation of the influence of a variable is appropriate?
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