I was watching CSI New York tonight and a cyclist was knocked off her bike. Anyway they re-opened the case 2 years after it happened and it mentioned ghostriders/ghost bikes anyway i googled it after and what it is if there is an accident with a push iron/bicycle and the cyclist dies or badly injured a group of people put a white bike and a notice of who and what happened with the date where the accident happened.
Ghost Bikes - The City's Viral Memorials for Cyclists -- New York Magazine
Ghost Bikes | ghost bikes
On that second link these ghostbikes are in a few countries nevermind just in New York. There are some in the UK - 1 in Manchester, 1 Fort William, 1 in Brighton, theres a few in London.