Answered by ZUPAN | Dec. 16, 2019 14:58
Maria, the most important detail is that your itinerary id definitely eligible for 24 hour visa free transit and you don't need any kind of visa.
You will print out your flight tickets and have those printouts with you. Also, you will declare in Singapore that you will travel to US via Shanghai using 24 hour visa free transit. After a short inspection of your tickets, you will be allowed to board the plane.
Upon arrival in Shanghai, you will approach 24 hour visa free transit counter where you will apply for a temporary entry permit to be allowed to exit the airport for a short period. Since your layover is short, I advise you to tell to the immigration authorities one or two specific attractions that you intentionally made your layover exactly via Shanghai just to see them. I hope that they will understand you and grant you a permit.
By the way, it is true that you will need an hour, maybe a few minutes more to reach the center of the city by metro, so you will have to plan your layover trip very carefully to be safe to catch your onward flight. In my opinion, you will have about 2 hours for a very short and focused sightseeing if you want to be on the safe side. Also, use metro and not anything else and especially not road vehicles because metro cannot stuck in the traffic.