A side street or byway — a small road branching off from a main street.
駅前の横道を曲がると古い喫茶店がある。
If you turn down the side street in front of the station, there's an old coffee shop.
住宅街には自転車で走れる細い横道が多い。
Residential areas have many narrow side streets you can ride a bike through.
大通りを避けて横道を通ったほうが早く着いた。
Avoiding the main road and taking a side street got me there faster.
A wrong way or digression — departing from the main line, either literally or figuratively (e.g., a topic digressing).
議論が横道にそれて、最初の問題が忘れられてしまった。
The discussion went off on a tangent and the original issue was forgotten.
目標から横道に逸れないように気をつけて進めよう。
Let's be careful not to stray from the goal and proceed.
彼は研究中に別の興味に気を取られて横道にそれた。
He got distracted by another interest during his research and went off track.