km plural or singular, "out of which 100 km is motorway"

The sentence is

The total length of the public road's network is 29151 km, out of which 1243 km are motorways, 6810 km of national roads and 21098 km of regional and local roads.
^{source: ASECAP}

However, I saw a sentence where “out of which” was used in a phrase, such as “out of which 1000 km is motorway”.
I want to ask whether “is” or “are” should follow after the word kilometre(s) or abbreviation km?

Solution 1:

I think it is a better idea to use 'is'. Here is the reason why:

Commonly, we say, '1000 km is a long distance.'

This is because, 1000 kilometers is considered as one whole unit/set. We are not referring to each kilometer of those 1000 kilometers separately. The idea of 1000 km is one whole, long distance, so we use 'is'. You can use 'are' if you have a discrete idea of 1000 km in mind(1000 km comprising 1000 individual units) but that usage is not seen very often. Hope that helped!

Solution 2:

The phrase Xkm is plural in every instance, unless the value of X is exactly 1 (assuming km is the head noun in the noun phrase). We can see this from the fact that in speech, the word kilometres (or kilometers for American readers) has the plural suffix, 'S', apart from when preceded by the numeral 1:

• 1 kilometre
• 0.5 kilometres
• 1.5 kilometres
• 1,000 kilomtres

However, when such phrases are the subjects of sentences, we see both singular and plural verb agreement (arguably singular is more frequent). This is because we can conceive of 2km as a single distance, or as two individual kilometres. As is the case, for example, with collective nouns, it is how we perceive the subject that matters, not its grammatical number or its plural or singular morphology.

This is nothing special about kilometre noun phrases. We see this type of singular agreement with all types of plural measure phrases:

• Twenty kilos is quite a lot to have to carry around all day.
• Two tonnes is the absolute limit.
• 100 decibels is far too loud.
• Ten up-votes is not nearly enough.

Here is an Ngram which might give us a vague idea of the relative frequency of kilometres is and kilometres are. The blue line is kilometres is, the red kilometres are:

Solution 3:

'km' is the short form for 'kilometre'. This makes it singular. However, the same symbol is used to denote 'kilometres', which is plural.

Understand that 'km' is not a word. Its a symbol. According to the SI system of units, there is no plural for these symbols and the same symbol denotes both 1 and plural quantities.

Therefore, distinction of singular and plural occurs only when using the full form and not the symbol.

Solution 4:

Both are correct. In my opinion it is most natural to interpret "1000 kilometres" as a distance, which makes it a singular. English has a tendency to somewhat illogically interpret such measurements as plurals as if we were interested in distinct individual kilometres. In my native German this tendency is even a strict rule in some grammatical contexts. But in this context you have a choice even in German and certainly in English.

Solution 5:

I would use "out of which 1,000 km are motorway".

My reasoning is that the 1,000 km almost certainly don't come together in one stretch and are an aggregate of separate, shorter sections of motorway.