Road risk depends on several characteristics of the road environment. A methodology for the estimation of the relative impact of discrete road related parameters, for which the measure of elasticity cannot be defined, is presented in this paper. Many parameters, such as the number of directions and lanes of the road, the type of the area and the weather conditions, are often defined as discrete parameters, and therefore this approach has many practical applications. The methodology is based on the estimation of generalized linear models using the Poisson and quasi-Poisson statistical distributions. Useful conclusions are extracted from the application of these models, and related practical recommendations are formulated for the improvement of the road characteristics and related road safety in the Greek highway network.