If you want a simple way to map them, you can also use https://mapcomplete.org/maxspeed.html

To add to the other replies, you add it to the road. You can also add the speed signs if you want however

@MXC48 The documentation for the speed limit tag is here:
https://wiki.openstreetmap.org/wiki/Key:maxspeed

The "default" iD editor has a field for this if you apply a road preset to a way.

@openstreetmap

Are these new speed limits only due to the roadworks or are they permanent? If it's the former, I probably wouldn't bother mapping it, or only if the construction lasts for at least a few months.