When designing pavement thickness for flexible and rigid pavements, PO one of many considerations that should be used whats mean

Po : initial serviceability index, Pt : terminal serviceability index ,PSI: present serviceability index ,PSI = Po - Pt

PSI is a mathmatical measurement for The PSR (Present serviceability rating) , it's related to the riding quality and Po value is taken immediately after the construction of the pavemnet, / usually a value of4.5 is taken for Po and (2-2.5) for Pt on major highways 

PO is Initial serviceability index.

PO is considered to be the a present serviceability index (PSI) immediately after the pavement is open.

 Its mean that the Initial serviceability index (Po) – Po is considered to be that PSI immediately after the pavement is open. AASHTO values are4.5 for rigid pavement and4.2 for flexible pavement.