@ossobuco,
Pros... You probably won't be killed yelling at the jerk who hit your car.
Most accident investigators can tell from the damage, how the accident happened. They can tell the speed, who hit who and so on. In other words, they don't have to witness it to figure out fault.
As for the other sign, regarding using breaks et al... they are worded differently in different places, provinces, states... They do not apply to passenger vehicles but to big trucks. Engine retarder breaks are very, very loud. You can often hear the loud rumbling very far away and so, it is common in populated areas to restrict truckers from doing so, everywhere in the n.america.