we have driven by stretches of rivers where, it seemed, about every third bridge we passed was a covered bridge. why is one covered when the next two are topless? ① the most obvious advantage to a covered bridge is that it can block the elements, particularly snow. ② accumulated snow can render a bridge impassable, and it is true that covered bridges are found most often in cold climates. ④ of course, one could argue that engineers should design covers for all roadways. ⑤ but bridges remain frozen long after adjacent road surfaces, primarily because bridge surfaces are exposed to the elements from all sides, the bottom as well as the top.

위의 빨간색 부분 해석이 해석지 그대로 했을경우 무슨 의미인지 잘 와닿질 않네요.
모든 도로에 커버를 설치해야 한다고 주장할 수도 있다. 하지만 어차피 다리는 위 아래 모두 노출되기 때문에
모든 도로에 설치한다고 해서크게 효과가 없다.. 이런 뜻인가요? 특히 bridges remain frozen long after adjacent road surfaces,가 무슨 뜻인지..(사전적 해석말고요)고수님들 도와주세요.