But at one two-lane residential street
(right photo and in the video below), even when it was very quiet, experienced blind travelers with normal hearing were
unable to hear some of the approaching cars until the cars were only 4 seconds away!
Four seconds isn't long enough to cross the street, so in that situation
they could never be sure whether they had enough time to cross, even when it is very quiet.
Therefore
that was a situation of uncertainty for gap judgment.