Large, flat surfaces work best for graffiti artists. With this fact in mind, street buildings and facades often become targets of graffiti art. Taggers want large numbers of people to see their art as frequently as possible, and buildings are some of the most visible canvases possible. The upper stories of a building are preferred targets, mostly due to their inaccessibility. The higher up a tag, the more difficulty people will encounter removing or vandalizing it.
Graffiti on walls has existed since the times of ancient Rome, and at one point it served as advertisements. Walls are some of the most tagged surfaces due to a variety of reasons. They are easy to tag in a short amount of time (especially important since most graffiti is considered vandalism) and exist virtually everywhere. Underground subway stations and half-destroyed buildings act as canvases to a graffiti artist. Similarly, concrete bridges act in a similar manner: They are equally accessible and easy to tag quickly.
Despite being larger and effectively taking more time to tag, billboards remain one of the most tagged surfaces. Taggers target them largely in part because of their inaccessibility, as well as their large, open canvas size. Since billboards are designed for advertising in the first place, they essentially become free showrooms for graffiti artists.
As trains were used less and less frequently, they became prime targets for graffiti artists. Under the cover of night and lowered security, train cars have become commonly tagged surfaces. Despite their bumpy and irregular surface, train cars are typically tagged with elaborate name designs and phrases. Furthermore, their travel routes ensure a wide audience.