time, in seconds, along the tunnel to the reference speed. t is
measured from the start of the transition zone.
The transition zone ends when luminance is equal to internal
luminance Li.
According to the UNI 11248:2007 and the UNI EN 13201-
2:2004 standards, the inner average zone luminance Li is the luminance
needed to ensure regularity and safety during traffic flow at
design speed. Li should be as in Eq. (5):