回复：thank you for your reply!

You are absolutely right on that reasonable is very subjective. But, the subjectiveness is not on the police officer. He will be judged by a third person, either a jury or a judge (in a bench trial). In that sense reasonable is based entirely on objective standard, not the police officer, but a prudent third person. This is true for all the substantive laws. A good lawyer might argue that a police officer searching a backseat with a gun on it is reasonable, but his searching of something inside the glove box is totally unreasonable, in this context.