[JumpCrisscross]

> It's interesting how you still observe the doppler effect even though the road isn't moving

Relativity. Christian Doppler's equation for the change in frequency of a wave, for subsonic velocities, is approximated by three variables: the velocity of the receiver relative to the source, the velocity of waves in the medium and the emitted frequency [1]. The first term is symmetric between a source approaching the receiver and a receiver approaching a source.

(The actual equation measures the velocity of the source and the receiver relative to the medium. Here, too, the symmetry between a source moving towards a receiver, the latter un-moving relative to the medium, and a receiver moving towards a source, the latter un-moving relative to the medium, is preserved.)

[1] https://en.wikipedia.org/wiki/Doppler_effect