I work in the museum exhibit design industry. Accordingly, I work with ADA guidelines often. ADA is vague regarding specific fonts. Its more about size, placement and contrast. Essentially, use common sense. Consider the needs of your audience. Avoid decorative fonts and generally be a responsible, helpful graphic designer. If the project is related to type in an environment, the Smithsonian has developed a more comprehensive set of guidelines. They would be ADA compliant.

There are a few folks on Typophile who have a bit more experience than I, so I'm sure they'll add or correct as needed:
Although the ADA is vague about specific fonts, there are a couple of things to check when picking a font. See this part of the spec:
http://www.usdoj.gov/crt/ada/reg3a.html#Anchor-10133

The font should be "sans serif or simple serif type", and as far as proportions:
"Letters and numbers on signs shall have a width-to-height ratio between 3:5 and 1:1 and a stroke-width-to-height ratio between 1:5 and 1:10."

What falls into "sans serif or simple serif type" is not spelled out, but the proporitons should give you an idea. The basic idea is to steer you away from fonts that are too thin or too thick.

Jason C

Take a look at James Montalbano’s Clearview ADA.