Fingernails, along with toenails, are curious ongoing growing elements on a person's body. Unlike obvious growths that have to be trimmed (like hair), fingernails consist of solid, extremely palpable material that wouldn't appear at first glance to be something that needed to be cut off. But most intriguing about fingernails is the doctrine of fingernail etiquette, collectively followed by most civilized societies.
Fingernail etiquette largely involves the 'right time' and the 'wrong time' to cut, bite or pick off one's nails. If you're driving and you are alone in the vehicle, you can throw your trimmings anywhere you please. Even passengers in neighbouring vehicles will not think it out of line, if they see you throw a fingernail onto your floor. However, if you are driving and you have a passenger, he or she will have to be a relative or a really close friend not to find your behaviour offensive1. If you are the passenger, it is usually bad manners to throw fingernails around no matter whose car it is; if it is your car and someone else is driving, you are okay provided the driver is, again, a relative or a really close friend. (Hopefully it will be one or the other, if you're letting them drive).