Why is a yellow card for going to Hong Kong valuable?

Rare things are expensive. Shanghai A yellow license plate is also known as the most expensive piece of iron in history. In order to alleviate the long-standing urban traffic congestion problem, the Shanghai municipal government has been controlling the number of license plates, and only 1 10,000 license plates are put in every month, while there are hundreds of thousands of online auctions every month. The competition is fierce, and things are rare. Shanghai A license plate refers to motorcycles registered within the jurisdiction of Shanghai, which can only drive in urban areas.