Why are Gypsies or travellers known contemptuously as "pikeys?"

A Google on this subject turned up some very unpleasant sites. This was the only real attampt at definition that I could find.
"This term is of course offensive, and is a derogatory word for travelers, or gypsies. The term is supposed to originate from their nomadic existence on the roads. The main roads in England during the early 19th and 18th centuries were known as Pikes, or Turnpikes. A Pikie was someone who made a home of these. In Hertfordshire they are also known as Diddycoys."