One of the most difficult halachos to keep is not to walk behind a woman. If you walk down the street no matter how fast you walk there will always be a woman walking ahead of you [unless you live in New Square but then you miss out on mitzvas yishuv Eretz Yisrael]. Some poskim say that since it is impossible don't worry about it. I spoke to one gadol who said that if the gemara says it and it is paskened li-halacha then we have to keep it to the best of our ability- no questions asked. Here is a teshuva from Rav Munk of Chaifa on this question and on the question of SITTING behind a woman on the bus [which could be avoided if you get a seat up front].
