Traffic Capacity Studies (Highway Engineering)

Traffic volume refers to the number of vehicles passing a specific point on a highway, typically on a particular lane and in a specific direction, over a defined time interval, usually measured in vehicles per hour.

Traffic capacity, on the other hand, represents the maximum number of vehicles that can traverse a given point on a lane or roadway per unit time (commonly per hour). It is expressed in the same units as traffic volume—vehicles per hour per lane or roadway.

Although both traffic volume and traffic capacity share the same units, they convey different meanings:

• Traffic volume indicates the actual flow rate of vehicles at a given time and reflects fluctuations in traffic demand.

• Traffic capacity denotes the maximum possible flow rate that a roadway can sustain under specific operating conditions while maintaining an acceptable level of service.

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Types of Traffic Capacity

Traffic capacity varies depending on roadway and traffic conditions. It can be classified into three major types:

1. Basic Capacity
   This is the maximum number of vehicles (in PCU - Passenger Car Units) that can pass a specific point on a lane or roadway in one hour under ideal traffic and roadway conditions.

   • Roads with identical physical characteristics will have the same basic capacity, regardless of current traffic behavior or conditions.

2. Possible Capacity
   This refers to the maximum number of vehicles that can traverse a point in one hour under actual (prevailing) traffic and roadway conditions.

   • It is always less than or equal to the basic capacity.

   • The value depends on how closely prevailing conditions resemble the ideal ones and can range from zero up to the basic capacity.

3. Practical Capacity (Design Capacity)
   This is the maximum number of vehicles that can pass a point on a lane or roadway per hour without causing unreasonable delays, hazards, or restrictions to drivers, under existing conditions.

   • As defined in "Highway Engineering" by S.K. Khanna, practical capacity is of primary importance to designers, as it directly influences the design of highway infrastructure.

   • For this reason, it is also known as the design capacity.

 

Center-to-center spacing of Vehicle (Traffic Capacity Studies)

• Vehicle Spacing and Traffic Capacity Studies

  Theoretical Maximum Capacity Calculation

  The theoretical maximum capacity (C) of a single lane can be determined using the formula:

  $$C = \frac{1000 \times V}{S}$$

  Where:

  • $C$ = Capacity of the lane, in vehicles per hour

  • $V$ = Speed of vehicles, in km/h

  • $S$ = Average center-to-center spacing between vehicles, in meters

  Capacity depends on both speed and spacing. Spacing is governed primarily by the safe stopping distance, ensuring that the rear vehicle has sufficient distance to stop if the vehicle ahead suddenly halts.

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  Determining Vehicle Spacing

  The center-to-center spacing (S) between vehicles is given by:

  $$S = S_g + L$$

  Where:

  • $S_g$ = Space gap (distance from the front of one vehicle to the rear of the vehicle ahead), in meters

  • $L$ = Average length of a vehicle, in meters

  The space gap $S_g$ can be estimated using:

  $$S_g = 0.278 \times V \times t$$

  Where:

  • $V$ = Speed in km/h

  • $t$ = Driver’s reaction time, usually between 0.70 to 0.75 seconds.

  • For calculation purposes, assume $t = 0.70$ seconds.

  Substituting in the formula:

  $$S_g = 0.278 \times V \times 0.70 = 0.2 \times V$$

  So, the total center-to-center spacing becomes:

  $$S = 0.2 \times V + L \quad \text{(in meters)}$$

  By substituting this value of $S$ back into the capacity formula, one can compute the theoretical maximum capacity of the lane for a given design speed and vehicle length.