Can PCU (passenger Car Unit) Be used as indicator of congestion in urban traffic? Is it even constant in urban traffic or very dependent on the geometry of the road?
Yes, if you have the number of PCUs (for example 1500 PCU/hour/lane), then you can have a good judgement about congestion. maximum volume which can pass from an urban road - in one hour and one lane- is a thing between 1800-2000 PCUs. It changes a little in different geometric roads. Some factors which affect flow rate (veh/hour/lane) are: Heavy Vehicles, Driver population (commuter or noncommuter), Lane Width and Lateral Clearance, Median and barriers, Access-Point Density (Access-point density is the total number of active intersections and driveways on the right side of the road divided by the length of the facility), Specific Grade or General Terrain, Base FFS -Free Flow Speed- and so on. For more information, I suggest you to read "Principles of Highway engineering and traffic analysis" book written by Fred L. Mannering and etal," or HCM 2000 or "Traffic Engineering" written by Roess, Prassas and McShane.
PCU for individual vehicles cannot describe congestion on the road. Although they are shown to depend on volume level, but the variation is so small that these can not be used to quantify congestion.
Passenger car unit describes the amount of capacity that one individual vehicle uses when compared to a passenger car. Usually the base value for passenger car (1,0) is the value for good rad and weather conditions in uniterrupted road conditions (meaning a freeway or a highwoay). This means that the PCU-value for a passenger car in other conditions (for example in intersections or in rainy conditions) is more than 1.0. Correspondingly the PCU-value for a truck or bus is more than 1,0, even in good traffic condotions. At intersections, the start-up times for heavy vehicles are longer than for passenger cars and therefore the PCU-values are higher than on uninterrupted conditions. At intersections the PCU-value depends also of the operation (straight through, right turn, left turn).
As such, PCU is not an indicator of congestion, but total volume of flow can be given in PCUs and you can make estimations of congestion based on that value. In your question you do not define what you mean with your expression "indicator of congestion", but I hope that my answer is of some help to you.
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