Don't know about Singapore, but they do it the same way as taxis the world over - they either find some place that's out of the way of traffic, like a side street or driveway (especially if the stop will take a while)... or they just sit there disrupting traffic and make people deal with it. Usually it's either a multi-lane road, or a smaller low-speed road where traffic can slow down a little and squeeze by. Not strictly ideal, but people cope.