End of heavy traffic zone, perhaps?

Build up area, more developed area, city and developed roadside business area with more traffic lights, leaving for less built-up, more rural areas.
search “left the build-up area” on wiki.openstreetmap

@anartist Has it correct.

Interesting – French road signs are almost identical to UK ones, although we don’t have an “end of build up” sign we have one that means a similar thing – end of speed restriction which basically means that national speed limits apply instead of local ones. It nearly always occurs as you leave a built-up area.

In the Netherlands we would refer to it as leaving the residential zone of a municipality. On the other side would be a sign welcoming you to – for instance – the municipality of Amsterdam or residential zone of Hoogland. All in all similar to @anartist‘s explanation.