Perhaps everyone's impression of American police officers is like that of American TV characters: They are paragons of justice. On top of that, guns are legal in several places in the U.S., so the number of armed civilians needs no further discussion. To protect police officers, you would think that police cars would have bulletproof glass and other protective installments, but they don't. Do you know why?
In the manual of U.S. police tactics, police car engines or wheels are used as bunkers. Therefore, in a gunfight, U.S. police usually treat the front of the car as a danger zone, allowing enough engine space to resist danger. Then, what is the best bulletproof material to apply on a police vehicle? According to officer Christopher Hawk, "'bulletproof’ doesn't exist until you begin talking about thick steel or exotic Kevlar/ceramic/plastic blends. The best you can usually hope for is bullet resistant, and just for common handgun rounds."
Currently, the most common bulletproof material is Kevlar, but bullets from automatic rifles or other heavy-duty machine guns can still penetrate it easily. In this situation, all you can do is add a steel or ceramic plate, but that is an even bigger burden. As for the vehicle, you could try applying one or two sheets to the doors, but what about the windows? Bulletproof glass? It is too heavy and expensive for the design of police cars. Low-level bulletproof glass is more than an inch thick, so if installed, the entire car needs to be rebuilt. Even after installment, you would lose operability of the windows. Because bulletproof glass would be fixed onto the car doors, it would sacrifice the flexibility of the windows' basic daily functions, not to mention opening the windows to shoot.
You also need to consider costs. Several police departments struggle with their budgets, and some units even lack the necessary funds to purchase standard police cars, not to mention bulletproofing. So, what is the best bulletproof material to apply on a police vehicle? It is lightweight, cheap, highly effective against traditional handgun, rifle, and machine gun bullets and can be adapted to the body, car door, and car window. In other words, it doesn't exist.