Review of Methods for Estimation of Passenger Car Unit Values of Vehicles Pooja Raj 1 ; Kalaanidhi Sivagnanasundaram 2 ; Gowri Asaithambi 3 ; and Ayyalasomayajula Udaya Ravi Shankar 4 Abstract: Estimation of passenger car unit (PCU) values is very important for traffic capacity analysis and other relevant applications such as level of service (LOS) measures, determination of saturation flow rate, signal design and coordination, and development of traffic flow models. Because of such wide applications, the accuracy of PCU values is highly significant in traffic flow analysis. Many research works have been carried out on estimation of PCUs in the past few decades. In developed countries, various methods were developed for estimating PCU values for different types of facilities which carry trucks and buses of lesser composition. But these methods are not completely analo- gous for mixed traffic due to the presence of wide variety of vehicle types, nonlane discipline, intraclass variability of vehicles and their maneuverability. With this consideration, various methods used for estimating PCUs for different facility types under homogeneous and mixed traffic conditions are reviewed and the drawbacks of the existing methods are identified in this paper. The challenges in estimating PCU values and future directions for improving the PCU estimation methods are also presented. DOI: 10.1061/JTEPBS.0000234. © 2019 American Society of Civil Engineers. Author keywords: Passenger car unit; Passenger car equivalent; Passenger car unit (PCU) estimation methods; Facility type; Homogeneous traffic; Mixed traffic. Introduction The traffic flow characteristics and roadway system in developing and developed countries are basically different. The traffic in most of the developed countries, commonly referred as homogeneous traffic, follows lane discipline and is composed of large proportion of vehicles with not same similar dimensions—predominantly cars and a small proportion of trucks and other vehicles. In contrast, mixed traffic comprises of a wide variety of vehicles with different static and dynamic characteristics occupying the same road space without any confinements for maneuvering. These differences between homogeneous and mixed traffic show the complications in implementing traffic operations and designing roads. To overcome this, a uniform measure of vehicles called passenger car unit (PCU), also referred as passenger car equivalent (PCE) is used for converting traffic stream composed of two or more vehicle types into an equivalent traffic stream composed of exclusively passenger cars. The PCU values can be used for capacity analysis, signal design, traffic management, determination of saturation flow rate and developing traffic flow models. The Highway Capacity Manual (Highway Research Board 1965) first defined PCU as “the number of passenger cars displaced in the traffic flow by a truck or a bus, under the prevailing roadway and traffic conditions. ” Later, it is redefined by Transportation Research Board: National Research Council (2010) as “the number of passenger cars which will result in the same operational condi- tion as a single heavy vehicle of a particular type under specified roadway, traffic and control conditions. ” However, the TRRL (1965) in London defined PCU as “on any particular section of road under particular traffic condition, if the addition of one vehicle of a particular type per hour will reduce the average speed of the remaining vehicles by the same amount as the addition of, say, x cars of average size per hour, then one vehicle of this type is equivalent to x PCU. ” For mixed traffic, the Indonesian Highway Capacity Manual (I-HCM) (Directorate General of Highways 1997) defined PCU as “conversion factor for different vehicle types with regard to their impact on capacity as compared to a passenger car (i.e., for passenger cars and other light vehicle PCU ¼ 1.0). ” Indian Roads Congress (IRC 1990) suggests static PCUs for differ- ent vehicle types in India based on traffic composition. Because of the distinct nature of homogeneous and mixed traffic behaviors, different methods have been used for estimating PCU values for different facility types such as midblock section, signalized intersection, and uncontrolled intersection. Continuous research works have been carried out to overcome the complexities involved in accurate estimation of PCU. The review article by Metkari et al. (2012); Shalini and Kumar (2014); Kiran and Verma (2016); Rana and Bajaj (2016) have failed to examine all the existing methods of PCU estimation under homogeneous and mixed traffic and, failed to investigate them based on road facility types. The parameters influencing the PCU values are not same for all the road facilities and nature of traffic (homogeneous and mixed traffic). Hence, it becomes necessary to review PCU methods for each 1 Research Scholar, Dept. of Civil Engineering, National Institute of Technology Karnataka, Surathkal, Mangalore 575025, India. Email: poojarajhere@gmail.com 2 Research Scholar, Faculty of Civil and Environmental Engineering, Technion Israel Institute of Technology, Haifa 3200003, Israel. Email: kalaanidhis.tpe@gmail.com 3 Assistant Professor, Dept. of Civil Engineering, Indian Institute of Technology Tirupati, Andhra Pradesh 517506, India (corresponding author). Email: gowri@iittp.ac.in 4 Professor, Dept. of Civil Engineering, National Institute of Technology Karnataka, Surathkal, Mangalore 575025, India. Email: aurshankar@gmail .com Note. This manuscript was submitted on March 9, 2018; approved on October 15, 2018; published online on March 19, 2019. Discussion period open until August 19, 2019; separate discussions must be submitted for individual papers. This paper is part of the Journal of Transportation Engineering, Part A: Systems, © ASCE, ISSN 2473-2907. © ASCE 04019019-1 J. Transp. Eng., Part A: Syst. J. Transp. 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