What is the (permissible) distance in front of the one praying that someone is allowed to pass by?
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.
Shaykh Ibn ‘Uthaymeen
Fataawa Noor ‘alad-Darb – Volume 1, Page 163
Saudi Gazette – Monday, 16 March 2009 – 19 Rabi Al-Awaal 1430 H