Within the framework of the present paper an attempt has been made to develop a methodology for the calculation of a Passenger Car Equivalent (PCE) factor in order to express the number of commercial vehicles in the equivalent number of passenger cars. The methodology is based on the Highway Capacity Manual, the examination of the international literature and an extended field survey. The research area is the Municipality of Thessaloniki, Greece and specifically 27 at-grade, signalized intersections which were examined. The field survey was performed during two time-periods in order a comparison to be possible.