it seems having 2 tasks is inappropriate (much less the use of an OS). there's really just one task that manages the stop light sequence with two stimuli that determine which sequence is "requested".
for this application, a request should only be started after a previous request is completed (unless an input needs to take priority)
shouldn't a light sequencer run continuously? why the need for 2 buttons? wouldn't just one that extends the period of one light be sufficient?