The present Diploma Thesis aims at studying the relative risk impact of median in straight and flat segments of the interurban road network. On the basis of large bibliographical research the use of lognormal regression has been adopted and applied in the disagregate road accident data of 1996 – 1999 on the Patra – Athens – Thessaloniki road axis. The statistical analysis has led to the development of three mathematical models. The introduction of the concept of the “relative road accident risk” allows the comparison of the road safety between segments with or without median. The first model correlates the median with the annual average daily traffic (AADT), the lighting and the weather conditions. The other two models correlate the above factors with passenger car accidents head on and at angle collisions. The application of the models showed a negative impact when the road environment characteristics are unfavorable (e.g no median, rain) and the ADT is greater than 5300 – 6000 vehicles per day.