The snow piled up on the road was blocking traffic.

Literal

Road [on-に] piled-up snow [topic-は] traffic [possessive-の] obstruction [to-に] was-becoming.

積もった雪 is a relative clause: 'snow that had piled up.' 交通の邪魔になっていた ('was becoming an obstruction to traffic') uses 邪魔になる ('to become a hindrance'). A practical observation about winter weather.