Answer: past the hearer
Step-by-step explanation:
Pitch is related to frequency. Higher the frequency, higher would be the pitch. From Doppler's effect, we understand that there is apparent change in the frequency heard by the listener from the one emitted by the source. The apparent frequency is higher when the source and listener approach and is lower when they are moving apart.
Thus, when the pitch of a vehicle sounds lower, it means the vehicle is moving away from the hearer. It must have moved past the hearer.