To clarify here - A simile is a subcategory of a metaphor, it's just stated differently. So all similes are metaphors but not all metaphors are similes. It's arguable there are only two tropes - metaphor and symbol, with metonymy and synecdoche essentially being kinds of metaphors. But Traffic flowed like a river is a verbal way of referencing an underlying metaphor 'traffic is a river', just like 'we got off to a bumpy start' references an underlying metaphor 'a relationship is a journey', or 'they really swallowed that' the metaphor 'ideas are food'. In other words metaphors can be referenced indirectly.