For a long time I have been puzzled about what a pollution control valve is. They are all along the M25 from Sevenoaks to the M4. Why do they need signposts? Can you really turn pollution off???... As you can see from the picture here, there is nothing to see apart from the sign! Well, I now have the answer:

The following solution is taken from:

 http://journals.iranscience.net:800/Default/www.newscientist.com/lastword/article.jsp@id=lw1015

"These valves are found beside most new major road developments and many older ones. Surface water from carriageways drains into a network of gullies leading to drainage systems and onto soakaways, ditches, holding ponds or adjacent streams and rivers. It is unfortunate, but nonetheless commonplace, for various contaminants, such as diesel or petrol, to end up spilt on the road after traffic accidents. These then run away with the surface water and can cause environmental damage. To protect against this, new drainage schemes have pollution control valves that, in their most basic form, consist of vertical sluice gates that are generally left open to allow the free flow of surface water. In the event of a spillage the emergency services can lower the sluice gates to contain the contaminants within a controlled area so that they can be pumped away into tankers for safe disposal." - Peter Elliott , Rochestown, Cork, Ireland

However, I am still unclear as to why these highly specialised pieces of highway engineering require large, expensive, signage... Surely the person who a) knows what a Pollution Control Valve is, and b) knows what to do with one, would also c) know where to find one if needed? Yet still we have the signs. "Oh Look! A Pollution Control Valve!" "Oh Look! Another one!" What a waste of money.