Limits Vary, Limit Varies

Question

Let's say each city has its own city-wide speed limit.

1. The speed limit varies between cities.
2. The speed limits vary between cities.

Definition 1 for "vary" of this dictionary reads:

to be different in different situations

which implies that only one single characteristic, without it belonging to a specific thing, is being talked about, so sentence 1 should be the correct one.

But, definition 1a of the same dictionary reads:

if two or more things vary, they are different from each other

which implies that two or more characteristics, each belonging to a separate thing, are being talked about, so sentence 2 seems to be the correct one. What do native speakers think?

Answer 1

Nice question. In Br.E I'm leaning towards the single form: "The speed limit varies between cities". Why? Because I took your point about single characteristics. When I broke it down, a speed limit is only really one single thing/one concept; we're just referring to different instances of speed limit. However, I don't think the plural form would sound out of place at all, and in fact, could be used pretty much equally (given that my previous point is a bit flimsy when you consider that each city likely has many different speed limits). Google searches of the exact two phrases confirm that both are possible. Some tips I thought of:

If you're referring to one country for example, the singular option might be better.

The speed limit varies from town to town.

Yet, a global reference might read better with a plural:

Speed limits vary dramatically from continent to continent.

You might also consider using other words to see if you can resolve this type of doubt in future. For example, "temperature vs temperatures". "The average temperature in x country varies" vs "Temperatures in North America vary depending on..."

Answer 2

Hmm, very interesting question.

The easy way out is to say that a city could have multiple speed limits, like one speed limit for back roads and a higher speed limit for the main roads. So a collection of speed limits in one city is different from the collection of speed limits in another city, so, "The speed limits vary."

But suppose each city has only one speed limit, but the speed limit in, say, Detroit is 40 mph while the speed limit in Boston is 30 mph. Should you use the singular because each has only one speed limit, or the plural because, between the two, there are two speed limits.

I've heard people say it both ways, so I think the practical answer is, "either way is acceptable". But is one way arguably right and the other wrong? I don't know.