The presence of those black boxes at ground level strongly suggests that they're battery boxes. i.e. the cameras are temporary installations only. As TWR suggests, that means that they're likely to be traffic flow cameras.
The company I contract for uses loads of them. They're generally only normal video cameras but they can also have ANPR facilities so that, for example, they can track how many vehicles leaving at a particular road exit are going to a nearby retail centre, how many are going into the local housing estate, how many are joining other major routes and so on.
Another possibility is 'radar ATCs' (automatic traffic counters), which are beginning to replace 'tube based ATCs' (with rubber tubes place across the road to measure traffic flows). That's good news for people like me as I hate trying to fix those tubes into place on roads with busy traffic flows. Laying tubes across the Southend Arterial Road, in busy traffic, is damned frightening. I know 'cos I've done it!