Road Tunnels Manual

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6.2. Theoretical and practical tunnel traffic capacity

The theoretical capacity of a road section is defined as the maximum through-flow of vehicles per hour. It is determined by measuring the maximum number of passenger cars in a fifteen-minute period and multiplying this by a peak hour factor. This is not an absolute maximum, but rather refers to reasonable repeatability. Expressed in this way, the capacity only depends on the number and width of lanes and off-carriageways, and the slope of the section. It does not depend on the percentage of heavy vehicles, since it is clear that this intensity will be a maximum when traffic is formed exclusively by light vehicles and regular drivers. If there is no element that limits it, this theoretical capacity is approximately 2.200 vehicles per hour per lane (v/h/l). More information is available in Chapter 4 "Capacity and speed in relation to the geometry of roads and roads tunnels" of Report 05.11.B and in Chapter 3 "Traffic speed and densities" of Report 05.12.B.

The practical capacity of a section is calculated based on the theoretical capacity without the previously mentioned restrictions (2.200 v/h/l). Limiting factors are applied based on the actual characteristics of the roadway. These main factors are:

  • Fw : Lane width factor, which reduces the capacity depending on the width of the lanes and the off-carriageways. It is considered that a lane does not limit the practical capacity if the width is equal to or greater than 3.60 m.
  • Fhv : Heavy vehicle factor, which adjusts the theoretical capacity depending on the percentage of heavy vehicles and the inclination and length of the ramp or slope of the roadway.
  • Fc : Correction factor due to the predominant type of driver. This factor adjusts the capacity based on whether the drivers are regular drivers along the route and if the type of traffic is that of a working day.

The practical capacity of a carriageway in one direction, Cp , is calculated then by:

Cp = 2200 . N . Fw . Fhv . Fc in which N is the number of lanes.

The factors can further be calculated and adapted according to formulas and tables given in Chapter 4 "Capacity and speed in relation to the geometry of roads and roads tunnels" of Report 05.11.B and in Chapter 3 "Traffic speed and densities" of Report 05.12.B.

More information can also be found in the HCM (Highway Capacity Manual) issued by the Transportation Research Board (USA).

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