Standard Sign Typefaces
The standard typefaces used for highway signs in the US are defined in the "Standard Alphabets for Traffic Control Devices", published by the Federal Highway Administration.
Series A: Discontinued
Series E Modified
The official designations for these typefaces are "FHWA Series B", "FHWA Series C", etc. In recent years, a practice has developed of referring to these standard typefaces as "Highway Gothic". This has caught on to the point where even FHWA occasionally refers to their own typefaces in this manner. However, for clarity, it's probably better to refer to these typefaces with the more correct 'series' name.
Series B through F are the standard typefaces used for most signs. They are variations on a standard style, where B has the narrowest letters (for example, the word "PARKING" on a