Depends on where you live. In Germany any CCTV recording of a public place is automatically illegal. There’s a handful of exceptions (for example, for the parliament), but generally, this holds true.
Opt-in is what’s legally required, and should be the default.
Unless there is a specific situation that there is a threat there, and a warrant for that situation, no.
For traffic situations there are specialized systems that open a physical filter in front of a camera lens only if someone is detected to be speeding, and the camera is then limited to that lane, for that short moment.
Opt-in is what’s legally required, and should be the default.