Should i comment the answer or add another answer referring to the original poster? See this case for example: Is the government a producer, a consumer, or both?
In principle, anyone can edit an answer by anybody else, to include additional material, but I haven't seen it happening on any site, probably out of respect for the original author of the answer. Usually, a comment below an answer suggests to the author some additional info/data/result that he could include in his answer, and then it is up to the author to do that or not.
Regarding whether to post a separate answer, mostly it boils down to the length of the additional contribution. In the example you provide, your answer is more of a comment below my answer, rather than a separate answer (and have in mind that comments, although officially looked-upon by the SE approach, are avidly read by users).
And by the way, since my answer mentions roads-infrastructure, why do you think it does not include the investing activities of the government?