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The Three Components of The Traffic System And Their Effects on Capacity

The capacity of a highway facility will be affected by the components of the traffic system. The three-components are the driver, vehicles, and the highway. Driver and vehicle characteristics with the roadway infrastructure will affect the highway capacity. This article will describe the effects of traffic system component on the capacity.
Figure 1

Highway capacity is a function of speed, time headway, and spacing. These factors will be affected by the vehicle's characteristics. The following points illustrating vehicle characteristics that may affect the capacity of a highway facility.

  • Weight to horsepower ratio (Wt/hp): the Wt/hp will affect the maximum speed and the acceleration of vehicles, which will impact the road capacity. Heavy vehicles or vehicles with more Wt/hp will operate at low acceleration rates. Low speeding moving vehicles will significantly affect the capacity, especially when there is a minimal passing opportunity for other vehicles in the traffic stream.

  • Braking and declaration capabilities: The deceleration capability of a vehicle decreases with increasing size and weight.

  • Frontal area cross-section: The aerodynamic drag affects the acceleration of the vehicle.
  • Width, length, and trailer coupling: the width of a vehicle may affect the traffic operations. Wide vehicles will force vehicle on adjacent lanes to slow down while passing. Width, length, and trailer coupling affect the off-tracking characteristics of a vehicle and the required width lane. Particularly, the moving of long vehicles at a horizontal curve will impact the usability of adjacent lanes, which may affect the capacity.

Drivers' capabilities, personal preferences, and experience will affect the road capacity. The driver's characteristic will affect road capacity. The driver characteristics that affect the capacity of a facility are:

  • Perception and reaction times: this will affect car-following characteristics. In other words, it will affect the acceleration and deceleration of vehicles following another vehicle. Also, it will affect other drivers’ actions, such as changing lane and gap acceptance characteristics.
  • Selection of desired speeds: selecting the comfortable speed by drivers will affect the operation and entire traffic stream of the facility. Driving at low speed will have a detrimental effect on the highway facility, especially when high traffic demands are present.
  • Familiarity with the facility: Commuter traffic is typically more efficient in using a facility than are drivers unfamiliar with the facility, or recreational drivers.

The last element of a traffic system is roadway infrastructure, the following points will summarize how a roadway infrastructure can affect the roadway capacity.

  • Horizontal alignment and horizontal curves: sharp horizontal curves will result in reducing the speed of vehicles. Reducing the speed will have a detrimental effect on highway capacity.
  • Vertical curves: steep grades will result in reducing the speed for heavy trucks with low-performance characteristics. On the other hand, steep grades can block the sight distance and may act as a local bottleneck.
  • Cross-section: the number and width of lanes, as well as the width of the shoulder has been shown to affect the speed, which affects the roadway facility capacity.
  • Traffic control devices: clarity and appropriate traffic control devices will enhance roadway facility capacity.


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