Ron:"twice as many", "one-quarter as many", etc. are not only acceptable as idiomatic expressions but also extremely common in the english language.
since this idiom is present in the correct answer, which was written by gmac, IT IS CORRECT.

In general, you won't need the second half of that construction if you have already mentioned the data elsewhere in the sentence.
the earlier part of this sentence already mentions the # of drive-ins in the u.s. in the 1950's, so it is not stated again. since it is not stated as part of an actual parallel construction, you don't need the second "as".
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examples:
there were once 20 shirts on this shelf, but, now, barely half as many are left. --> i already mentioned the data (i.e., 20 shirts) earlier in the sentence, so it's not mentioned again.

there are barely half as many shirts on this shelf [i]as there were last week.[/i] --> i didn't mention this comparison point earlier, so i'm mentioning it now.