First of all, it should be clear that it is forbidden to cross in front of a praying person. In a big and open place or in a crowded place, it is good to put some shield or sutrah between oneself and one’s place of sujud, so that other people may pass by without inconvenience.
As regards putting a sutrah in front of the praying person, following is the fatwa issued by the Saudi House of Fatwa:
Sutrah is something a praying person puts in front of him in order to alert people that they shouldn’t pass in front of him while praying.
Putting sutrah in front of the praying person is a Sunnah (i.e. it is not obligatory). This applies to the person praying alone as well as the Imam in congregational prayer.
It stands to reason that sutrah maybe necessary when a person is praying in the desert or in an open place such as the case with the prayer of `Eids. It may also become necessary when one prays during traveling.
However, the issue of sutrah is no longer important in today’s mosques whose floor is covered with carpets which make it easier for a non-praying person to avoid passing in front of the praying one. In addition, carpets in mosques usually contain clearly defined lines that make it easy for a person praying in the mosque to avoid people passing in front of him.
Generally, sutrah is not obligatory, as we have stated. The Prophet (peace and blessings be upon him) is reported to have said: “When one of you takes a sutrah in prayer, let him inch closer to it.” (Reported by Ahmad and An-Nasa’i)”