Grumbler: than USUAL

Well, I know that 'usual' is most common in informal language, but *AFAIK*, formally it has to be "usual" if refers to a noun (the number of people was smaller than usual <number>") and "usually" if refers to a verb (are fewer people than usually <are> )

E.g.
https://english.stackexchange.com/questions/247512/more-than-usual-vs-more-than-usually
https://www.englishforums.com/English/ThanUsualVsThanUsually/zzqdc/post.htm

I agree that USUALLY is an option, but it can't stand alone. You could say "fewer customers than THERE usually ARE" for example. Otherwise it just doesn't sound idiomatic.

This page explains it best: https://www.englishforums.com/English/ThanUsualVsThanUsually/zzqdc/post.htm

The only better explanations I see are "English is full of exceptions" and "This is a fixed phrase. There is no logic involved." If so, I agree as English isn't my first language.

In their example usual/usually can be either "usual steak" or "usually buy". Hence, "than usual" is preferable.

However, in our case I don't see how it can be "usual people". The only possibility I see is "usually are".

In our case, there are two options: (1) the fixed phrase "than usual" referring to the entire scenario, or (2) "than there usually are" in which USUALLY is (necessarily) part of a verb phrase. The only variant that doesn't work here is the THAN USUALLY on its own and unlicensed. So it isn't a matter of formal vs informal language, it's about licensing. In this case, USUALLY must be licensed by a verb form.

Well, I can accept 'fixed phrase', but your logic isn't convincing.
Some more googling...
http://news.bbc.co.uk/1/hi/world/asia-pacific/7813267.stm
"...others left their jobs earlier than usually because work has slowed down as factory orders slump."
"than usually" is on it's own here, as there is no any verb that 'usually' addresses directly. Implicitly it's "than <they> usually <leave>" - and this is logical.

I'm not looking to convince you, I'm very sure of what I'm telling you :) Remember, English is my native language and my profession as a translator and editor. I'm merely trying to help you. Everything you see online is not necessarily correct – even the BBC can obviously make mistakes.