Roadside Barrier design 1980's flyover

[Reading]

This happened in Reading a couple of weeks ago https://www.readingchronicle.co.uk/news ... n-reading/

The barrier is along the side of the IDR here https://goo.gl/maps/JWPBueJb7Mjcp8KE8, this road is now a 40mph but was I believe NSL when opened which would have been in the 80's I think (some on here likely had input into that road over the years) - what is the barrier actually meant to do design wise - is it just to mark the edge of the road or is it meant to keep cars from coming off in a glancing blow or is it (i doubt it) to stop commercial vehicles escaping