Traffic Capacity Studies are an essential aspect of highway engineering that focuses on understanding the capacity of roads to accommodate traffic volume. To delve into this topic, let’s explore a few important terms.
Traffic Volume
Traffic volume refers to the number of vehicles passing a specific point on a highway lane in a given direction within a particular unit of time, typically per hour.
Traffic Capacity
Traffic capacity is a measure of a road’s ability to handle the traffic volume. It represents the maximum number of vehicles that can pass a given point on a lane or roadway within a unit of time, usually an hour (expressed as vehicles per hour per lane or roadway). Please note that traffic volume and traffic capacities have a slight difference in
Similarities and Differences between the Traffic Capacity and Traffic Volume
The only similarity between traffic capacity and traffic volume lies in their units, both being expressed as the number of vehicles within a specific time unit. However, the two terms differ in their definitions and implications.
Traffic volume represents the actual rate of traffic flow, fluctuating in response to variations in traffic demand. On the other hand, traffic capacity signifies the maximum rate of flow that a road can sustain while maintaining a certain level of service. Capacity depends on factors such as prevailing road conditions and traffic characteristics.
Basic capacity
Basic capacity refers to the maximum number of vehicles (often measured in Passenger Car Units or PCUs) that can pass a given point on a lane or roadway within one hour under nearly ideal road and traffic conditions. Roads with similar physical features will have the same basic capacity regardless of the traffic conditions.
Possible Capacity
Possible capacity denotes the maximum number of vehicles that can pass a given point on a lane or highway within one hour, considering the prevailing roadway and traffic conditions. In most cases, the possible capacity will be lower than the basic capacity unless the traffic conditions approach ideal circumstances. Thus, the possible capacity can vary between 0 and the maximum (i.e., the basic capacity).
Practical Capacity
Practical capacity represents the maximum number of vehicles that can pass a given point on a lane or roadway within one hour without causing excessive delays, hazards, or restrictions on driver maneuverability. It is of particular interest to designers who aim to provide adequate highway facilities, and hence, it is also referred to as design capacity.
In conclusion, traffic capacity studies play a crucial role in highway engineering by examining a road’s capability to handle traffic volume. Understanding terms like traffic volume, traffic capacity, basic capacity, possible capacity, and practical capacity helps professionals in designing efficient and safe highway systems.
Determination of Theoretical Maximum Capacity
Using the relation C = 1000 * V / S, one can easily determine the Theoretical Maximum Capacity.
Here, C represents the capacity of a single lane in terms of vehicles per hour. V denotes the speed in kilometers per hour, and S represents the average center-to-center spacing of vehicles when they follow one another as a queue or space headway.
The capacity of a lane depends on the speed and spacing. The spacing is determined by the safe stopping distance required by the rear vehicle in case the vehicle ahead comes to a sudden stop.
Numerically, the spacing is given by S = Sg + L, where Sg represents the space gap (head to rear) between the vehicles, and L is the average length of the vehicle. These combined values determine the center-to-center spacing of the vehicles.
Here, Sg = 0.278 * V * t, where ‘V’ is in kilometers per hour, Sg is in meters (m), and ‘t’ is the total reaction time of the driver, typically assumed to be between 0.70 to 0.75 seconds.
Assuming t = 0.70 seconds, S = (0.7V + L) = (0.2V + L) in meters (m).
Therefore, by knowing the design speed, one can calculate the spacing S and, consequently, determine the theoretical capacity of the lane.
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