it also takes about 2seconds to move 20feet.

for these reasons a 20' radius is a "retention" zone. anyone closer than 20' has a large probability of successfully rushing you and engaging for physical control of the weapon.

watch what an LEO does when this zone is entered, it is a good indicator of the frame of mind, do they say "stop right there", or mantle thier weapon?

if you were to stop at 20' just because, you may be told to "cmon over here" or LEO may become wary or softly hostile, as you may be aware of retention training as well, and intend to engage.