Sirens are designed to have an easily discernible doppler effect; a siren coming at you sounds different from a siren going away, which sounds different than a

Sirens are designed to have an easily discernible doppler effect; a siren coming at you sounds different from a siren going away, which sounds different than a siren keeping pace with you. And furthermore, a siren behind you sounds different from a siren in front of you. Human drivers generally manage all of this fine. If computers can't, then they aren't ready to drive.