Which police code signifies a crime in progress?

The code that signifies a crime in progress is indeed 10-31. This notation is commonly used in law enforcement communications to indicate that officers are responding to or are aware of a situation where a crime is actively being committed. It alerts responding units to the urgency of the situation, emphasizing the need for a swift and coordinated response to prevent further harm or loss.

Other codes in your question represent different situations: 10-50 typically refers to a traffic collision, 10-28 is used to request vehicle registration information, and 999 is often reserved for emergencies, such as needing immediate assistance or signaling a distress call. Each code serves a specific purpose within police operations, providing a clear and concise way for officers to communicate critical information.