This paper investigates Passenger Car Equivalent (PCE) or Passenger Car Unit (PCU) values and traffic flow characteristics of Sri Lankan expressways: Southern (E01), Outer Circular (E02), and Katunayake (E03). The study highlights the importance of considering region-specific traffic parameters due to differences in road traffic systems, vehicle characteristics, and transport infrastructure across countries. PCE values vary based on vehicle dimensions, climatic conditions, and traffic flow. The calculated PCE values for passenger car - small category was found to range between 0.74-0.77, while heavy goods vehicles recorded significantly higher PCE values, ranging from 3.59-3.99. Research on PCE has been widely conducted globally, however limited studies exist in the context of Sri Lankan expressways. This study uses Chandra’s method to calculate PCE values for eight vehicle categories, and the accuracy of the results was validated using the R-squared method, with simulations run through PTV VISSIM software. Additionally, the research evaluates traffic flow characteristics such as speed, density, and volume to optimize traffic management and support sustainable transportation. Relationships between speed-density, volume-density, and speed-volume were analysed using the Greenshield model. The average traffic volumes were determined as 911.8, 833.2, and 1884.9 passenger car per hour (pcph) on the three selected expressways. Observed results with the design were compared and revealed lower observed density (36.4 pc/km – 76.8 pc/km) values than the design predictions (160 pc/km) in passenger cars per kilometres. This study provides insights into traffic dynamics on Sri Lankan expressways and contributes to sustainable transport planning by identifying gaps between design assumptions and actual traffic conditions.