I was tagging freeway ramps without lanes and encountering traffic signals on on-ramps that facilitate metered access to the main freeway:
Is there a special tag for this?
There is a tag traffic_signals=ramp_meter used 2.088 times if that is what you are looking for.
For those unfamiliar with the term, here’s the wikipedia page, some useful context on a UK roads site (where ramps are slip roads).