The Passenger Car Unit (PCU) is an essential metric in the field of traffic engineering, playing a crucial role in the evaluation of road network capacity. PCU factors for various vehicle classes at roundabouts, however, have not been thoroughly studied. Taking into account the complexities of diverse traffic, the occupancy time (O.T) method was used to determine PCUs for different types of vehicles. The present work also introduces a precise linear regression model to evaluate the entry capacity of roundabout approaches within the specific context of traffic conditions in India. The linear model, which is based on the robust Theil-Sen regression analysis, serves to simplify the comprehension of this relationship, marking a significant departure from traditional exponential models. The application of Theil-Sen regression enhances the adaptability of the linear model. In roundabout planning and design, traffic flow at each approach and smooth transition of entry vehicles into the circulating flow must be carefully considered, as these factors are crucial in optimizing the efficiency and safety of roundabouts under mixed traffic situations. This study examines three geometrically distinct roundabouts located in the prominent urban centers of Chandigarh, Noida, and Jammu in India. The developed linear regression model correlates well with field entry flow data, unlike previous models. The findings of this study offer novel insights into the selection of appropriate PCUs and the efficacy of linear regression modeling. These insights challenge prevailing assumptions and underscore the significance of employing sophisticated modeling techniques in diverse traffic scenarios.