This sign threw me into a Strunk and White funk. I just hate the use of "an" before an aspirated "h".
(Yeah, I know it's common on the other side of the pond, but those crazy Brits also drive on the wrong side of the road.)
When the "h" is aspirated, or breathed out as you pronounce the word, I think it should be treated as a consonant.
A historic event happened here.
When the "h" is silent, and the word begins with a vowel sound, then the use of "an" makes sense.
Meet me in an hour. After I've calmed down.