yeah. about thr preferred route, needless to say, the shortest and fastest.
Also better le. Faster complete the fare, cabby can do more fares. if longer route by say 50cents and some more time, customer not happy.
if cabby complete fare faster, say maybe "sacrificed" 50cents for shorter route, then found another fare, ah! $2.40 already. 50c or $2.40 better?

if cabby as said, take more time on purpose, he may be slower and may lose to another taxi for a fare at say flagged down on the streets, not to mention, if on MRT stns for fares, this cabby may had arrived longer due to giving the previous customer a longer route, he may had gotten a fare which is gong near by, so again may lose out.
still, short and fast is better, dont go greedy over a few more cents and risk a complain.
