Whilst it is contextual, in everyday use if one bothers to employ a word, it's because it is relevant.
You phone somebody and ask where they are?
"On the way"
"On the road"
"On Logpile Street."
All imply normal conditions, as soon as traffic is mentioned it means the traffic is relevant to the statement.
This will of course be changed by local context, in poorly managed traffic areas, merely mentioning what road one is on at a certain time of day will imply traffic and probably delays.
So in the end we have "In traffic" can always mean "in a traffic jam" and we could say the same of "In a jam." If the context is road travel either word can cover for the other without any problem.
Clearly both words have other applications, which of them is being made use of is made apparent by context, as is so often the case.