How are big scenes such as fights and car accidents filmed in movies?

I'm preparing a car accident scene recently, so I'll just answer it casually. But I still think the specific situation needs specific analysis.

Let me first talk about how to shoot shooting and car accident scenes; Then I'll simply answer the zombie part. Let me introduce the main premise first. Generally speaking, if you are in North America, you need to apply for a shooting license. If it is an ordinary student film, as long as there is a contract between the school and the government, there is no need to pay for the simple broadcast equipment. If you set lights, you need a generator. In this case, the government will start charging. Because of traffic control, you need police, but the number of police is determined by the government. The police department also has a film class to manage this part of the affairs. New york can directly ask that department for help in management, and even let them play the police in the play. So in many new york movies, most of the time, the police are really the police in new york. If you are shooting a car chase scene, the scope of traffic control will be wider and more police will be needed. Being a policeman for an hour is expensive, which is why making a movie is so expensive. Traffic control needs money, ask the police for money, and apply for money. Regarding shooting scenes, special guns are generally used, which are close to real guns in structure and material, and will emit sparks, but there are no bullets; Nevertheless, the chemicals that cause sparks are also very dangerous, so it is necessary to invite the police uncle, because it may cause panic among real pedestrians on the street. If it is a simple shooting, it is usually taken directly on the street. It depends. If time is tight, sometimes two cameras shoot at the same time. Generally, the slow motion of bullet flight or the simple special effects such as air fluctuation caused by bullet flight are all synthesized in the later stage; This relatively simple special effects scene generally does not need a virtual scene (such as extending a street view or changing a window into a door with special effects), so a green screen will not be used. It is troublesome to have large-scale war special effects, and the photographer's team will be replaced by special effects units. (If you look at the credit behind the scenes, you will find that photography has a first camera unit and a second camera unit, that is, the staff in different seats, sometimes called Camra A unit and Camera B unit; ; The special effects unit is VFX unit, and there is a photography team specialized in shooting special effects shots. For example, reunion requires a green screen, or even shooting directly in the shed, and then the street view is completely modeled.