noun
causeway; land bridge; path on an embankment
A raised path or road built on an embankment, often across water or low ground. Can refer to natural land bridges as well as man-made structures.
この土手道は、昔、田んぼの中を通るために作られた。
This causeway was built long ago to pass through the rice paddies.
湖の真ん中を土手道が走っている。
A causeway runs through the middle of the lake.
土手 refers to the embankment itself; 土手道 specifically means a path or road on that embankment.
Compound of 土手 (embankment) and 道 (path, road). The exact historical derivation is uncertain, but the word transparently describes a road on an embankment.