There was a study done awhile back (I don't remember the source.) that in the instance of being in a situation where a rape may occur. The victim was better off yelling "Fire!" then screaming or yelling rape. More people come to the rescue of people in the dangerous situation of being in a fire before they would help someone being raped.
Sad but true.
Also, as stated, self defense techniques and learning to fend off an attacker is a great way to protect and learn how to handle yourself if this horrible predicament is ever brought before you.
It probably wouldn't be a bad idea to carry a weapon of some sort either (mace, handgun, etc.)
"Do you feel lucky punk? Do ya?" - Dirty Harry