This article discusses many of the milestones in the development of the Passenger Car Unit (PCU). An attempt has been made to cover the critical and most recent research work in the development of PCU in both homogeneous and non-homogeneous traffic conditions, although the assessment is not exhaustive. Passenger car units or passenger car comparisons (PCUs) play a crucial role in the study of the traffic flow of different types of vehicles in the traffic flow and are used to transform traffic flows of different vehicle types into traffic flows consisting solely of passenger cars and which have been developed to analyse the effect of different types of vehicles on the road. The current article discusses and compiles various methods that have been followed to estimate the value of PCUs. Each method has its own factors such as density, headway time, speed, delay, etc. which are taken into account for the calculation of PCU.