The insides of TriMet's MAX light rail vehicles are practically an ad-free zone; in the days I spent in Portland I only noticed one or two that weren't run by TriMet itself, in stark opposition to pretty much every other transit agency I've had experience with. On the outside, it's a different story - it was a rare train that didn't have at least one car wearing a wrap ad, and sometimes both of them would. I only saw this particular Oregon Zoo ad once, but you have to admit, it's pretty eye-catching.
I, the copyright holder of this work, hereby release it under the Creative Commons Attribution-Non Commercial-Share Alike 3.0 Unported license.
This means that you are free to Share (to copy, distribute and transmit the work) and to Remix (to adapt the work) under the following conditions: Attribution (you must attribute the work in the manner specified by the author or licensor, but not in any way that suggests that they endorse you or your use of the work), Noncommercial (you may not use this work for commercial purposes), and Share Alike (if you alter, transform, or build upon this work, you may distribute the resulting work only under the same or similar license to this one).