I’ve been noticing this a lot lately, particularly on the M27, which I use frequently.
There are areas of the grassed embankment that are fenced off on three sides (open to the hard shoulder) with polythene to about a metre in height; the areas are perhaps 7 metres square or so. the polythene appears to have been buried in the soil at the base.
Is this some kind of controlled experiment into different methods of verge management, such as different herbicides or cutting regimes?
Anyone?
Can you see any small saplings planted within the area? Might be temporary protection for young trees, but admittedly this falls four-square into WAG territory.
No; in most cases, the enclosed area is just covered with the original grass; there might be some that enclose existing trees though; I’ll check.
WAG: There’s a manhole cover or other access panel in the ground there, which needs protection from drifting snow.
Open only on the roadway side you say? is there, by chance, any road construction taking place parallel to these fencings? As I understand it here in the States, when a barrier is erected in the manner you describe, it is to prevent run-off of potentially harmful construction solvents and other liquids to areas extending beyond the immediate vicinity.
Snow? This is the south coast.
You’ve never had snow? Anyway, I said it was a WAG - I’ve NO idea where the M27 runs 
It does snow here most years, but we haven’t had enough to form drifts for maybe twenty years.
Maybe it’s just the prep work and, when the chances of moring frost has completely passed, they’ll come back and plant the shrubs, or such like ?
morning.
btw, I have no garden, it’s a pure guess.
Resurrecting this thread with an update; my curiosity prompted me to email the highways agency, who promptly responded:
I have to admit the idea of “reptile barriers” strikes me as automatically amusing. Do the taller ones keep out Godzilla?
How the heck does a “barrier” that’s open on one side exclude reptiles from entering the area?
Well, seeing as the “open side” is a busy motorway … 
Yes; the open side adjoins the tarmac/concrete of the hard shoulder.
I’m assuming that the expert herpetologists consider this adequate to exclude most reptiles - I’m also assuming that they are cleared from the area immediately prior to the commencement of work - clearing them weeks or months in advance doesn’t sound like it would be so effective, as I’m sure a few creatures would venture around the fence by means of the road surface, particularly if they use the edge of the road for basking.
Reptiles aren’t terribly common in England - the email seemed to suggest that the workers might be at some risk, but this is quite unlikely - the reptiles in question probably include Slow Worms, Viviparous Lizards and Adders (only the latter of which is potentially harmful to humans) - I think the ‘difficulty to workers’ is more likely to mean inconvenience, if work has to be stopped to move them, although it is also notable that this work in particular includes the laying of subterranean conduits, the open ends of which might well seem attractive hiding places to lizards.