Question regarding comma vs. colon

Were I editing the first sentence, it's the word 'as' which would go.

Changing the colon to a comma makes the sentence imply that the study was somehow the cause of the decline it found. It's a very muddy construction.

With the colon, but without the 'as', we get two independent statements linked by the colon indicating the second as amplification of the first.

Not only should there not be a colon, but even using a comma is unnecessary. Subordinate clauses that follow a main clause don't require commas.

A subordinate clause can go at the beginning of a sentence or later in a sentence. The only difference is that if it goes at the beginning, you need a comma after the subordinate clause, and if goes later, you don’t need a comma.

https://www.quickanddirtytips.com/education/grammar/what-is-a-subordinate-clause

Your sentence has two other problems, though:

1) The beginning of your sentence should be written as follows:

"In 2013, Tallinn, Estonia instituted fare-free..."

There should be a comma after the introductory phrase "in 2013," but there should be no comma after "Estonia."

2) It seems that you're using "as" to mean "because," but that study isn't why car use in Talinn has only slightly declined. I think what you're trying to say is:

"...according to a 2014 study by KTH Royal Institute of Technology in Sweden, which found that car traffic in Tallinn was down less than 3%."

LeahHG answered the same question correctly on Reddit 2 years ago

The answer to the first one is declined: a 2014 study... In other words, the "as" comes out too, as it is underlined. [...] Remember that the entire underlined section comes out.