The objective of this Diploma Thesis is the investigation of the impact of weather conditions on the risk and the severity of road accidents in the Greek interurban road network. On the basis of large bibliographical research, the use of lognormal regression has been adopted and applied on the disaggregate road accident data for the period of 1992 – 1999 on the Patra – Athens – Thessaloniki – Evzoni road axis. The statistical analysis has led to the development of three mathematical models. The first model correlates the absolute number of accidents (concerning all vehicle categories) with the weather conditions (rain, temperature) and the traffic volume (vehicle-kilometres). The other two models correlate the weather conditions with the risk for 2-wheel vehicles and the severity of accidents. The application of the models revealed that low temperature leads to reduction in the number of accidents and the severity of accidents whilst it increases the risk for the 2-wheel vehicles. Furthermore, raining conditions lead to increase in the number of accidents and in the risk for 2-wheel vehicles whilst they reduce the severity of accidents.