The Map is to small to show the start of the sprint.
My feeling is that the start is not that important, it is a timed segment so you need to go thru the segment (± 200m) so you need to be at top speed when you go over the start line. So even with a super big flashing banner you will still need to start way in advance if you want to set a good time.
One other thing that indicate the start of the Sprint is the Timer that pop up at the top.
And there is not that many sprint points.
London has 2 at the same place (one forward and one reverse) (Red road)
Watopia 2 at the same place (one forward and one reverse) (at the Bridge) And one in the Dessert.
and Richmond 2 these are at different spots.
INNSBRUCK has 2 at the same place (one forward and one reverse)
Look at this post, some good info.