Two contraction joints and one expansion joints are usually adopted for drainage channels. Why?

In the life cycle of a concrete structure (not prestressed concrete), it will generally undergo the following process of contraction and expansion:

Contraction:
(a) Early thermal movement
(b) Seasonal contraction owing to drop in temperature
(c) Shrinkage

Expansion:
(a) Seasonal expansion owing to drop in temperature

The order of magnitude for items (a) to (c) is more or less the same. Hence, qualitatively speaking, for a given length of concrete structure, the number of contraction joints should be more than the number of expansion joints and they are roughly in the order of 3:1 to 2:1 based on the number of expansion and contraction process above. Of course, the actual spacing and number of contraction joints and expansion joints should be determined case by case.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.