While I don't have an in-depth technical understanding of exactly why this happens in every instance, I have definitely observed this while playing Cities: Skylines, a SimCity-like game which emulates traffic flow.
However, note that the traffic flow simulation in C:S is rather simplistic for performance reasons: specifically the pathfinder doesn't take dynamic traffic conditions into account at all! A traveler never chooses an alternative route even if the primary one is congested. But yeah, adding routes usually means also adding choke points (that is, intersections) which naturally slows down traffic.