I've seen these on various stretches of motorway and wondered what they were?

This is one near Chapeltown, Sheffield and even though it's above a speed camera sign, there are also others on stand-alone posts.

My thoughts are that it's related to the smart motorway system but has anyone got a more informed suggestion?

 

Pretty certain it's a stopped vehicle detection radar...   Probably at about 500m intervals??

Nail on head.  As can be seen on google images.