On 7 Sep, 15:46, wrote:

Seeing as its either (as I recall) a round green light or a
round red light, is there also a colour-blindness defence?
And do the readers not sometimes beep (but maybe twice?) also
on a failed read? *Hmm. Since I don't seem to remember very
clearly, perhaps I better hope there's a "hopelessly confused"
defence as well.

Don't know about buses, but at Tube barriers it seems common to get a
successful read followed immediately by a failed "passback" read.
Thus people ignore the second one so long as the barrier doesn't
close.

Neil