Traffic Capacity Studies (Highway Engineering)

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As we know, traffic volume is the numbers of vehicles that pass a point on a highway on a particular lane on particular direction in unit time, generally in per unit hour, there is a term which represents the capacity of a road to accommodate the traffic volume.
Traffic capacity is expressed as the maximum number of vehicle in a lane or a road that can pass a given point in unit time, usually an hour, i.e., vehicles per hour per lane or roadway.

Traffic capacity and traffic volume has same units, difference between the two is that traffic volume represents the actual rate of flow of the traffic and responds to the variation in the traffic demand, while capacity indicates a capability or maximum rate of flow with a certain level of service characteristics that can be carried by the road.

Traffic capacity of a roadway depends upon a number of prevailing roadway and traffic conditions.

  • Basic capacity is the maximum number of vehicles(PCU) that can pass a given point on a lane or roadway during one hour under the most nearly ideal roadway and traffic conditions which can possibly be attained. Two roads have same physical features will have same basic capacities irrespective of the traffic conditions.
  • Possible Capacity is the maximum number of vehicles which can pass a given point on a lane or highway during one hour under the prevailing roadway and traffic conditions. This means that the possible capacity of a highway will always be lower than the basic capacity unless the prevailing conditions of the traffic, approach the ideal conditions. Therefore the possible capacity may vary from 0 to the maximum, i.e., Basic capacity.
  • Practical Capacity is the maximum number of vehicle that can pass a given point on a lane or roadway during one hour, without traffic density being so great as to cause unreasonable delay, hazard or restriction to the driver's freedom to man-oeuvre under the prevailing roadway and traffic conditions.According to the book "Highway Engineering" by S.K.Khanna;  
 It(Practical Capacity) is the practical capacity which is of primary interest to the designers who strive to provide adequate highway facilities and hence this is also called the design capacity.
Center to Center spacing of Vehicle (Traffic Capacity Studies)

  • Determination of Theoretical Maximum Capacity
                        Using the relation:      C = 1000.V/S 
one can easily determine the theoretical Maximum Capacity; Here,
                      C = Capacity of a single lane, vehicle per hour
                      V = Speed, kmph
                     S = Average center to center spacing of vehicles, when they follow one behind the other as a queue or space headway, m.
Thus capacity depends upon the Speed and Spacing. Spacing is governed by the safe stopping distance required be the rear vehicle in case the vehicle ahead stops suddenly.

Numerically spacing is given by,
                         S = Sg + L
Where Sg is the space gap(Head to rear) between the vehicles and L is the average length of the vehicle, both combined makes the center to center spacing of the vehicles.
   Here,                     Sg = 0.278 V.t  , where V is in Kmph and Sg in m.
                          t is the total reaction time of the driver, generally assumed to be equal to 0.70 to 0.75 sec.
Assume t = 0.70;
  S = (0.7v + L) = (0.2V + L), m
Thus knowing the design speed, the spacing S can be found and thus the theoretical capacity of the lane can be found.

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REFERENCE: Highway Engineering by S.K.Khanna and C.E.G.Justo (NemChand and Bros.)

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