To be incredibly simplistic, the answer to your first question is yes, there are laws and regulations set into place in order to manage people moving at night. Now what does that say to night crime? I have no idea. I think that might be too big a question for me to answer.

Now, the situation is a little bit more complicated than just locking the streets. From what I know about the Tang dynasty (and I don't know how well this translates to Han), the neighborhoods in the capital were separated into small blocks of community. And within each community there are all the necessities of life. When the curfew hits, they would close off the door to the main streets of Changan, and so people would not be able to move from one community block to another. However, it is much more relaxed within the community block. For example, the block where the brothels and hotels are? Much more relaxed than you would imagine. I didn't want to tread too much on what I'm not sure about, so the information might not be completely relevant, but I thought it was worth sharing.