Hmmm.. I think that’s probably the way to go, some sort of visual indicator.
Maybe little HUD icons where nearby obstacles will intersect the edge of the screen would work. Similar to how nearby, but offscreen POI’s are displayed in a lot of games. They could change sizes depending on how soon they will show up on screen. That way you can adjust your speed to help navigate the upcoming encounter.
Not sure how helpful that would be, but maybe an idea to consider.