Of course there's a right answer to this dilemma. It's just completely dependant on the ethical framework...
From an act utilitarian standpoint, you pick up the hitchhiker. The fact that there's a sign reminding you that it's illegal is irrelevant, as the act of picking up a stranded hitchhiker is the ethical thing to do.
From a consequentialist perspective, it depends. If the hitchhiker stabs you in the face and takes your ride...Well, maybe you should've obeyed the sign.
The important factor, however, is your occupation. The way you identify yourself with your job in the Forces reflects a particular moral stance (and as such, a perspective on upholding the law). The dividing factor may be in your belief that disobeying a meaningless law is not "wrong". However, how does one gauge whether the "do not pick up hitchhikers" law is meaningless. There's no easy way to determine neutrally and, all things being equal, that law must be considered reasonable.
Therefore, given the available information to "solve" the dilemma, the correct answer would be to keep on going, leaving the hitchhiker at the side of the road.
...and yes, we could play the "what if" this or that forever. Remember, for moral dilemma questions DO NOT READ THINGS INTO THE SITUATION ;D