Why are HDB units all odd numbered?

Question

I just realised all the unit number of HDB units end in odd numbers. What's the rationale?
This is not so for HUDC and private apartments...

Answer 1

Development of 9-1-1:

Before the dial telephone came into widespread usage, a caller simply picked up the telephone receiver and waited for the operator to answer "number please?" The caller then said "connect me to the police," "I want to report a fire," or "I need an ambulance/doctor." It was usually not necessary to ask for any of these services by number, even in a large city. Furthermore, the operator instantly knew the calling party's number even if he couldn't stay on the line by simply looking at the number above the line jack of the calling party.
In implementing the 9-1-1 emergency calling system it was deemed prudent to have all odd house numbers to be on one side of the street and all even numbers to be on the other side. The simple solution to your puzzle is that the entire HDB development is on the odd side of the street.