Because it's only this road with 〜だけ

Takagi convinces Nishikata to spend his summer practicing riding his bike with her on the back.

One day, she asks Nishikata if he’s progressed on his summer homework, saying they should go to his house to work on it.

Since the road back is wide and without cars, Takagi convinces Nishikata to ride doubles to his house.