Compliance with local and international guidelines
While not all countries have legally enforceable standards for tunnel visibility monitors, most provide values for normal operation for the layout of tunnel ventilation according to the World Road Association (PIARC) international guidelines.
These lie typically at a value of Klim = 0.005 m-1 or 5mE/m (milli-extinction per metre). At a visibility of 12mE/m, the tunnel has to be closed for safety reasons.
||Extinction coefficient K
||Length of light beam L with I/lo=exp(-K L) =20%
Table 1: Correlation between visibility conditions, extinction coefficient and light bean length for an acceptable light level of 20% of the light source intensity.
|Free flowing peak traffic 50 – 100 km/h
||Normal operation *
||0.003 – 0.007 m1
|Daily congested traffic, stopped on all lanes
||Planned maintenance work in a tunnel under traffic**
|Exceptional congested traffic, stopped on all lanes
||Threshold value for tunnel closure
* Intermediate set points and thresholds may be set at levels below the design values
**National workplace levels to be considered
Table 2: Design values for various traffic situations and the limits for tunnel operation according to PIARC.