This Diploma Thesis deals with the assessment of motorway creation impact on road safety with the use of before-and-after methodologies. Two segments of the main Greek road axis Patras-Athens-Thessaloniki are examined and a control area of the segments without any intervention up to 1999 were selected. A series of before-and-after methodologies were applied, which led to the conclusion that there is a statistically significant decrease in the number of accidents, which can certainly be attributed to the motorway construction. This improvement is due to the significant decrease of certain road accident types, which varies according to the various vehicle types and the weather and lighting conditions. The results from this Diploma Thesis quantify the motorway usefulness and can be useful within the selection process of the road segments to be first upgraded into a motorway.