96 can have DDs.
The bridge between Engineering faculty and Central Library is
sufficiently high enough (4.5m) for a DD to pass. On the contrary,
it is the bridge between Sports Centre and Yusof Ishak House that
is too low for a DD to pass and which is why you do not see DD on
95 at all.
In fact, there was once a trial for DDs on both 96 and 151. I know
because there was a circular which was sent out to all NUS students
informing us of the trial. Why they didn't finally deploy DDs on
either service is something only SBST will know.