MetaphorBottleneck
A narrow point that slows everything down.
The narrow bridge is a bottleneck during rush hour.
Remember: Calling a narrow point a 'bottleneck' compares it to a bottle's thin neck, where water slows. No 'like' or 'as', so it's a metaphor.
Short lessonSimiles & metaphorsTwo ways to compare things so a picture forms in the reader's mind.