<P> In Japan, tram signals are under the regular vehicle signal; however the colour of the signal intended for trams is orange . </P> <P> In Hong Kong Island, Hong Kong, a yellow T - signal is used for trams, in place of the green signal . Addition to that, at any tramway junction, another set of signals is available to indicate the direction of the tracks . </P> <P> In Australia a white "B" or "T" sometimes replaces the green light indicating that buses or trams (respectively) have right of way . </P> <P> Some regions have signals that are interruptible, giving priority to special traffic usually emergency vehicles such as fire apparatus, ambulances, and police squad cars . Most of the systems operate with small transmitters that send radio waves, infrared signals, or strobe light signals that are received by a sensor on or near the traffic lights . Some systems use audio detection, where a certain type of siren must be used and detected by a receiver on the traffic light structure . </P>

When might the green light be missing from the traffic light sequence