Ofcourse every route does not need DDs but if a route has 65-70+
pax, putting a DD is worth it, if you want to provide a good travel
to people. Otherwise you really have to cram in.
Some statistics:
Apr 2012: 1054 DD, 1637 SD (39.08% of fleet
DD)
Sep 2013: 1215 DD, 1701 SD (41.67% of fleet
DD).
This is only counting perm deployed DDs/SDs - does not include
SP buses, special buses such as CT18.
If you observe, there is a remarkable increase in fleet of both
DD and SD, but DDs have far exceeded the # of SDs deployed on
services. SBST should aim for a 50-50 distribution in the next 2
years.
Services like 27 and 36? Buses packed to door, if put DD then
will crash at changi airport