If you enjoyed the film Helvetica (or at least find it interesting that somebody produced a documentary about a typeface), you may enjoy The Road to Clarity in the Sunday, August 12 New York Times Magazine section (registration required). The article tells the story of the development and current deployment of Clearview, a new typeface designed specifically for highway signage.
Central to the story is how typographic details like counter shapes (the closed spaces inside lower-case a, e, and o) and x-height (the height of the lower-case x) can have a profound effect on readability under real-world highway conditions (e.g., distance and darkness).
In user tests, signs that were legible at 700 feet in the current standard signage typeface, Highway Gothic, were readable in Clearview at 900-1,000 feet.
Thus far, 20 U.S. states have adopted Clearview, and are deploying the new typeface as current signs are replaced.