Don't think of the nouns as a compound subject, as they would be if joined by 'and'. Think of them as alternate subjects, so that in your sentence:

Neither the roadway nor the sidewalk WAS empty.

But note:
As alternate subjects, one might be singular and one might be plural. In that case, the verb agrees with the closest noun. If writing the sentence yourself, place the plural subject last:

Neither the student nor his teachers WERE able to...

Both "was" and "were" are fine.

"[Neither the roadway nor the sidewalk] was/were empty".

The bracketed element is a coordination of two noun phrases forming a larger noun phrase as subject.

The coordination can be conceptualised in one of two ways:

(1) "It isn't the case that either the roadway or the sidewalk was empty."
(2) "Both the roadway and the sidewalk were empty."


In other words, both singular and plural agreement are correct - it's a free choice.


Note that if either of the coordinates were plural, irrespective of its linear position, only a plural verb would be possible:

"Neither the roadway nor the sidewalks] is were empty".
"Neither the roadways nor the sidewalk] is were empty".


PaulM

I dug these up online:

https://www.grammarbook.com/grammar/subjectVerbAgree.asp
Rule 2. Two singular subjects connected by or, either/or, or neither/nor require a singular verb.

Examples:
My aunt or my uncle is arriving by train today.
Neither Juan nor Carmen is available.
Either Kiana or Casey is helping today with stage decorations.

Rule 3. The verb in an or, either/or, or neither/nor sentence agrees with the noun or pronoun closest to it.

Examples:
Neither the plates nor the serving bowl goes on that shelf.
Neither the serving bowl nor the plates go on that shelf.

https://www.englishgrammar.org/subjectverb-agreement-

2/Subject-verb agreement: either, neither, nor, or

Two singular nouns connected by or or nor take a singular verb.
• Neither Charles nor Benjamin was present there.
• Neither he nor his dad is good at driving.

When one of the nouns connected by or or nor is plural, the verb must be plural, and the plural subject must be placed next to the verb.
• Neither Peter nor his parents were aware of this. (More natural than ‘Neither his parents nor Peter was aware of this.)

When the subjects connected by or or nor are of different persons, the verb agrees with the noun that comes closer to it.
• Neither you nor he is responsible for this. (Here the verb is agrees with the third person pronoun he.)
• Either he or you are to clean up the mess. (Here the verb are agrees with the second person pronoun you.)
• Either you or John has to pay for the drinks.

I described very carefully in my previous answer the two possible conceptualisations of the OP's example, explaining why both singular and plural agreement are possible.


The agreement patterns in a neither nor coordination are:

1. neither sg nor sg = sg
2. neither sg nor pl = pl
3. neither pl nor sg = pl
4. neither pl nor pl = pl

And the patterns in an either or coordination are:

5. either sg or sg = sg
6. either sg or pl = sg/pl (many speakers would avoid this coordination altogether)
7. either pl or sg = sg/pl (many speakers would avoid this coordination altogether)
8. either pl or pl = pl





PaulM