Two cars \( P \) and \( Q \) are moving on a road in the same direction. Acceleration of car \( P \) increases linearly with time whereas car \( Q \) moves with a constant acceleration. Both cars cross each other at time \( t = 0 \), for the first time. The maximum possible number of crossing(s) (including the crossing at \( t = 0 \)) is _______.
