Bottleneck

A bottleneck is literally the neck of a glass or pottery bottle. An hourglass has a bottleneck at its mid-point whose diameter governs the time that granular contents of a given mass will take to pass through.

Metaphorically a bottleneck is a section of a route with a carrying capacity substantially below that characterising other sections of the same route. This is often a narrow part of a road, perhaps also with a smaller number of lanes, or a reduction of the number of tracks of a railway line. It may be due to a narrow bridge or tunnel, a deep cutting or narrow embankment, or work in progress on part of the road or railway.