There are two UI modes that are to be used when diagnosing traffic issues. Find the Root Cause of the Traffic Issues That’s what you should do to resolve the issue. To fix traffic Cities Skylines provides you with several tools. So your city became slower, commercial and industrial zones don’t get enough goods and workers, service vehicles can get to residential areas in time and you see a lot of traffic jams.
For traffic and road layout tips please visit this article. This is a guide on how to detect and resolve traffic issues in Cities Skylines.