What font is used in stop signs?

Photoshop

Pretty sure it's Helvetica.

If you have Photoshop or some program like that, use Bell Gothic Std and increase the vertical scale to %130.

The "S" in Bell Gothic Std doesn't look the same as the stop signs in St. Louis (and there are thousands of them). So far the closest font I've found is ITC Avant Garde Gothic (Medium Condensed).

Found it! Roadgeek 2005 Series C is a perfect match. Get it here: http://www.triskele.com/index.html These are freeware fonts, created by Mike Adams and based on all of FHWA Series B through F, plus additional fonts based on highway signage around the world.