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Road tunnel visibility measurements explained

Visibility is a critical measurement parameter for the safe and efficient operation of a road or rail tunnel. Acoem’s VICONOX, a multiparameter tunnel atmosphere monitor, employs the visible light transmission technique to directly measure the visibility levels experienced by tunnel users. The continuous measurement data gathered by the VICONOX can be used by tunnel operators as part of a ventilation control system. 

The level of visibility in a road tunnel needs to be controlled to provide safe driving conditions, the driver’s vision of the road ahead must not be restricted due to high levels of particulate (dust, smoke, water vapor etc) generated by road vehicles and prevailing weather conditions.

The VICONOX monitor can report a range of visibility measurement parameters as detailed in the table below, with the Extinction Coefficient (k) being the most frequently used parameter in road tunnel environments.

Measurement Parameter

Description

Units

Extinction Coefficient (k)

The number of particles in the atmosphere between the observer and the object, and how effectively they can absorb and scatter light. This value is defined by a term called the atmospheric extinction coefficient (k factor). As the k factor increases, visibility decreases. km-1

Meteorological Optical Range (MOR)

A formally defined distance, essentially equivalent to visibility: the path length in the atmosphere over which the light from a known source is reduced by 95%. Often used in the measurement of fog or mist and calculated as 3/k factor.

m

Opacity

The state or quality of being opaque. 0% opacity equals completely transparent, 100% opacity is completely opaque.

%

Particulate Density (Dust) A measurement of the average airborne particulate density, calculated using a density scale factor.

mg/m3

Examples
The Extinction Coefficient (the k value) is a measure of the light attenuation from scattering and absorption. The MOR is the path length in the atmosphere that will reduce the visible light by 95% of its original value. The k value and MOR are inversely proportional, as k increases, the MOR decreases.

  • Road tunnel with stationary traffic 
    • Extinction Coefficient = 9km-1
    • MOR value = 0.33km

With an extinction value of 9km-1 (9 per km), we can calculate the distance that visibility is reduced by dividing 1 by 9km-1, which equals 0.11km. Using the accepted attenuation factor of 63%, a k factor of 9km-1 means light is attenuated by 63% every 0.11km. This would be representative of a traffic jam in a very congested tunnel.

  • Road tunnel with traffic congestion
    • Extinction Coefficient = 7km-1
    • MOR value = 0.43km

With an extinction value of 7km-1 (7 per km), we can calculate the distance that visibility is reduced by dividing 1 by 7km-1, which equals 0.14km. Using the accepted attenuation factor of 63%, a k factor of 7km-1 means light is attenuated by 63% every 0.14km. This would indicate moderate congestion in a road tunnel. 

  • Road tunnel with free flowing traffic
    • Extinction Coefficient  = 3km-1
    • MOR value = 1km

With an extinction value of 3km-1 (3 per km), we can calculate the distance that visibility is reduced by dividing 1 by 3km-1, which equals 0.33km. Using the accepted attenuation factor of 63%, a k factor of 3km-1 means light is attenuated by 63% every 0.33km. This level is typical for free flowing traffic conditions within a tunnel.

Airborne particles in the tunnel atmosphere are generated by vehicle exhaust emissions, wear and tear of brakes and tyres, and water vapor (spray, mist and fog). These particles reduce visibility by scattering and absorbing visible light. The Acoem VICONOX can accurately measure total visibility levels experienced by drivers using direct, open path technology.  

Please contact us directly if you would like more information about the VICONOX tunnel air quality monitor or any of our other monitors and accessories for your next tunnel project.

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