Most of us are aware of the tiger escape and subsequent attack that occurred last Christmas at the San Francisco Zoo. The attack has spurned much controversy from the public (look at the comments on the YouTube videos below), and also within the zoo community regarding the "guidelines" the AZA puts forth. [youtube=http://www.youtube.com/watch?v=kh6finppjLI&hl=en]
The most recent published guidelines recommended 16' height on moats and with 25' width, and 20' height on walls. San Francisco's exhibit had 15' height moat. I'm unsure of the width. However, they've subsequently gone back and built additional height to the public side of the exhibit.
The topic demonstrates the fact that we can never be too safe when it comes to both animal safety and human safety (keeper and visitor). However, at some point, we're overdesigning to the point that the experience of visiting the zoo becomes more prison-like with 20 foot walls everywhere.
In this case, some evidence has been provided that the victims of the attack could have been taunting the tiger. These instances are such that any animal could find a way to escape any enclosure. Should we design for those instances? Or should we design for day to day safety?
This is a similar question to how many people should we be designing for? Peak day, where the park is cram packed, or a more average day. In this example, we generally design for the average day (aka design day), and accept a certain degree of discomfort on peak days. Ironically, if we take this same tactic to enclosure safety, the level of discomfort in those extreme situations is much more "uncomfortable" (death isn't exactly a "let it slide" circumstance).
For now, the AZA is redeveloping its recommendations for tiger barriers. Bare in mind these are simply recommendations, and zoos have every right not to follow the minimums, making the walls and moats larger or smaller, if they so choose.
Ultimately, we want to avoid design flaws that make guests able to directly access an animal or an animal to access the guest (unintentionally), while still allowing the guests to see the critters. As you can see, the guests as San Francisco Zoo still love their tigers.