orh. tat one is DDU, part of the AVMS setup.
loud beep means u're early, driving too fast, ahead of schedule;
soft beep means u're late, driving too slow, drive faster pls, u're
behind schedule.
last time by right it'll beep if u deviate by 3 mins, but nowadays
2 mins also start to beep liao. 
when they've perfected this, they can roll out the estimated time
of arrival panels at bus stop lor. 
