Effect of varying geometric and traffic characteristics on Passenger Car Unit on two-lane roads

Traffic flow behavior is a complex phenomenon in heterogeneous traffic conditions, where vehicles with varying static features and characteristics share the same road. As a result, volume represented in vehicles per hour won't give a precise estimate. This problem of heterogeneous traffic could be solved by calculating the equivalent value of vehicles in the context of standard passenger car, known as the Passenger Car Unit (PCU). Although PCU is known to be influenced by roadway and traffic characteristics, most studies on its dynamic behavior have concentrated mainly on straight and level terrain. The primary objective of this study is to examine the impact of different geometric design elements and traffic characteristics on the PCUs of various vehicle types. Multiple Linear Regression (MLR) approach is used to develop models for predicting the PCUs of these vehicle types. The results showed that the PCU values for all vehicle categories increase linearly with the curve radius, but the rate of increase varies among vehicle classes. Heavier vehicles are more affected by changes in radius compared to lighter vehicles and standard passenger cars. On straight (tangent) sections, the PCU values for all vehicle types also increase linearly as the gradient rises, with multi-axle vehicles (MAV), buses, and tractors experiencing a higher rate of increase than other vehicle types. The models developed for curved and tangent sections were successfully validated against field data, demonstrating very low RMSE and MAPE values. This precise capacity estimation supports the optimization of road geometric design, ultimately improving road performance.