Answered by Sidi Mostafa Azzam
What is the distance that one may walk past a person who is praying? I’ve heard that it is the length of two safs (rows). Is this true?
If someone is praying towards a valid barrier (Ar. sutrah), then it is forbidden (haram) to walk between him and the barrier. Amongst the conditions of a valid barrier is that it be within three cubits (approximately 1.5 m) of the person praying.
If the person praying lacks a valid barrier, then it is not forbidden to walk in front of him, though it is better not to walk in his place of prostration (Anwar al-Masalik Sharh `Umdat al-Salik, 83-84 (Dar al-Tabbaa); Bushra al-Karim, 1:227-229 (Dar al-Fikr)). So whether the person praying has a valid barrier or not, it will not be a problem to walk in front of him by more than about 1.5 m.
And Allah knows best.