I think this fear of strangers, especially of men is hardwired into women's heads. Back before the civilized era it was almost a certainty that you'd get involuntary sex if you weren't protected by a strong alpha male or in a large group of other females. Whether we like it or not, most primates(which we are apart of) are rapists by nature. This doesn't mean ALL men go out there and rape at any given chance they get. There is no need for that anymore. Finding and impressing a partner is fairly easy for us nowadays. We have a good female to male ratio and designated places were you are guaranteed to find a partner(Clubs,pub etc). The need for rape doesn't exist anymore.
Rape is still an activity our species like to do though. It shows dominance over the victim, which can be a turn on for certain individuals. People in the non-civilized world still use rape on both men and women to show/claim dominance within a group. Why? Because it's an effective tool to make sure people fear and obey you. So, yes. Women have good reason to be cautious around men when walking alone at night. If you as a man want to help women out when meeting them at dark nights, walk to the other side of the street and be in clear view all the time. It will be most appreciated by all women.
Also, never EVER walk behind a women at a pace that is faster than her's during later hours. I learnt this the hard way! Just trust me on this one