That's right. This means the same thing as "three-and-a-half years of travel". The apostrophe tells you that it's possessive, but since it is also plural, the apostrophe is put after the "s"

Hi Kyouko, We use "... years' travel “ to show that "years" is connected to or is a part of "travel". It shows some kind of possession and we can also call it the genitive case. Another example: Picasso's works (the art pieces are no longer possessions of Picasso. However the apostrophe s expresses works that originate from Picasso. From this perspective, this can be a genitive case).

Hi, Yes it is being used as a plural possessive because the apostrophe (') is after the "s" rather than before, which would make it a singular possessive.

Hi Angela, thank you for your answer. I did not know this structure is unusual phrase.

Yes. This structure is not common in American English.