An underpass takes you beneath the traffic of life. It is read as a way around an obstacle that is less direct and less comfortable, and as a short passage through something gloomy.
Has this come up in your dreams before? What was going on in your life then?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.