New York City subway cars of a bygone era, where graffiti started and was once most prolific, or freight trains in the US or passenger trains in Italy and other European countries where graffiti is currently commonly seen, are not the only types of vehicles or moving objects that graffiti can been found on. Trucks and sometimes cars in cities are also quite common targets for graffiti artists to tag up. Generally these are commercial vehicles since the code (albeit not always a strict one) among graf artists is to exercise respect for private property - but to hell with businesses and city owned property, especially when you can get away with the illegal act.
Always fascinated with this aspect of graffiti done on moving vehicles - oft times really rushed tags since the truck or van is only parked temporarily for as short a stop as a traffic light - I have been snapping pictures of what I have named this "moving violations" part of graffiti. Taken over the last few years in various cities including San Francisco, Oakland, Amsterdam, Los Angeles, and New York City they include a broad spectrum of graffiti from some intricate pieces to some very basic and obviously rushed tag jobs - kinda like the one above on the truck with Santa Rosa plates parked in the Mission District of San Francisco.
One truck owner in Chinatown in New York told me that he had long stopped trying to erase the tags on his once white van that he used to transport garments all over the city in. Other vehicle owners said that they actually commissioned artists to paint their trucks because then they knew that most other graffiti artists out of respect would then leave the vehicle alone. This way at least they could pick the art themselves. There are also some shots (including immediately below) of a graffiti'ed barge on a canal in Amsterdam, a city rife with graffiti everywhere, even along its waterways.