Because double yellow lines have very important functions. They ban parking where it poses a road safety threat. And they ban parking where it causes congestion and delays to traffic.
That's CERTAINLY not the case any more in Lewes, as far as regular visitors to the town are concerned. Most of us now know where to park and go straight there - no longer driving around hopefully looking out for somewhere free and clogging up the streets as we do so. The few short-term spaces...
http://www.telegraph.co.uk/motoring/news/10207364/Motorists-allowed-to-park-on-double-yellow-lines-to-help-save-high-streets.html
Have they gone MAD? Are they so STUPID that they don't realise that a 15 minute "grace period" for one car will be followed by another car taking its 15 minutes...