The מחבר (סימן תקנ”א סעיף י”ח) rules that starting from the seventeenth of Taamuz through Tishah B’av one should not walk alone. This applies only from the end of the fourth hour of the day, until the end of the ninth hour. The Rema says the reason is that during this time the קטב מרירי which is a שד rules. During the three weeks there is a present danger that the שד may harm someone if they are alone. The source for this halacha is in מדרש איכה. The Biur Halacha brings the Gemara in Pesachim (111b) that says the קטב מרירי is for sure around from the first of Taamuz through the sixteenth, after that time it is questionable if it is around. Therefore one should already be careful from Rosh Chodesh Taamuz and onwards.
If one sends his friend on an errand for himself, he has transgressed the prohibition of לפני עור לא תתן מכשול – do not place a stumbling block if front of a blind person. There is a machlokes if this halacha applies for someone going to do a mitzvah. The התעוררות תשובה says that here we apply the concept of שומר מצוה לא ידע דבר רע – no harm will befall someone on his way to do a mitzvah. Reb Chaim Kaneivsky shlit”a says that “perhaps” this halacha of not going alone applies even for a mitzvah.
The ערוך השלחן explains that this halacha only pertains when one is on the road outside of the city. There is no קפידא to be alone inside the city. This is also the opinion of the חזון אי”ש. The שו”ת התעוררות תשובה however holds there is no difference between being inside the city and outside the city. The Mishna in Avos already taught us that it is prohibited to be outside the city by yourself המהלך בדרך יחידי הרי זה מתחייב בנפשו. It must be the Midrash is teaching us that even inside the city it is prohibited.
Reb Shmuel Kamenetzky shlit”a explains that today people rely on the fact that the שדים are not found amongst us anymore. This idea is already mentioned in Tosfos in יומא עז: and חולין קז:. The ארצות החיים points out that the Rambam does not bring any halachos pertaining to רוח רעה ומזיקין at all, because they are not found amongst us. We also have a קבלה from the Gr”a, that all the רוחות became בטל when the Ger Tzedek Avrohom Ben Avrohom was killed.
The Gemara in Pesachim (112b) brings another halacha that one should not be out by himself at night because of מזיקין. Here too most people are not makpid on this. It is for the same reason that the שדים don’t live amongst us and the רוחות are בטל. Since the minhag is to be lenient, one does not have to be careful not to be out by himself during those hours. The אשל אברהם (בוטשאטש) writes המנהג מכריע במדינות אלו שאין חשש בזה.1
Prepared by R’ Avrohom Yehoshua Ziskind
1Sources:קובץ הלכות,נחמת ישראל,דרשו