Along with multiple classes of vehicles, frequent signalized intersections on the urban roads and the platoons thus created add complexity to the estimation of Passenger Car Units (PCU). Under such scenarios of interrupted traffic, a new approach based on platoon movement of vehicles is introduced for the realistic estimation of PCU values for urban roads by incorporating the appropriate vehicle behavior and interactions. The PCU values were derived by finding the trade-off between the speed reduction caused by the flow of passenger cars and other vehicle classes, as per the given definition by Transport Research Laboratory (TRL). However, the speed-flow, as well as the speed-density relations, were analyzed and found that later is preferred for the realistic estimation of PCUs for urban roads. The speed reduction is modeled using multiple linear regression with field data collected from typical urban roads characterized by platoon flow in Mumbai city (India).