Funnily enough I can smell the filthy stuff - which is just as well, because I ride a lot of early-morning miles in the dark during the winter.
What I really want to know is.. who drops it? Buses? Lorries? Tractors? And how do they get away with it? You sometimes see cars and bikes being stopped by police cars, and checked out for some infringement or other, but how many people have seen an artic pulled over and the driver being ticketed for leaving his tank-cap open?
Dropping diesel, no matter what circumstances, is (I believe) an absolute offence against the RTA. Why do we hear so little about prosecutions?
Cleaning up a road surface is the responsibility of the local authority (or, in Scotland and if it is a trunk road, the Scottish Executive). I know it is shutting the stable door after the horse has bolted, but one can and ought to complain about a spill to the Highways Dept of one's LA, and ask for something to be done about it.
M-o-M