Reference: Fataawa Noor ‘alad-Darb – Volume 1, Page 163
Question: What is the [permissible] distance in front of the one praying that someone is allowed to pass by?
Response: The distance at which it is allowed to pass by in front of the one performing his prayer, so long as he has a sutrah, is that area beyond the sutrah of the one praying. And if he did not have a sutrah, and was praying on a prayer mat, then the [entire] area of the prayer mat is not permissible for anyone to pass by. And if he did not have a prayer mat, then it is prohibited for anyone to pass by the area between his feet and his prostration area/spot.