Often humans are braking before any braking occurs (negative reaction time).
You’re thinking 1.5 seconds is reaction time somehow (what is being discussed).
Fortunately the perception cycle which may last 1.5 seconds often starts well before anything happens, so in many situations this leads to negative reaction time.
In worst-case situations of sudden hazards the 1.5 seconds might happen.
Compare to FSD which seems to have extremely limited ability to perceive and just has reaction, which seems to be clocked currently at 0.35s.
Check
the possum video, for example (frame where timing began linked).
I did not time that but in my bleary eyed (I have night vision problems) and distracted state you can see my reaction time was similar to FSD. It took 1 second for me to react (watch the brake pedal) in my compromised vision-impaired state, which is coincidentally the same as FSD (observe regen bar - note the path planner did not fully reflect the braking of the car in this case). (I would beat it handily in the daytime,
assuming I saw the hazard.) And that is kind of a “sudden hazard” case.
My pedestrian case seems to have made the Dan O'Downer highlight reel. A dubious honor. I'm sad that he did not feature the Awesome Possum.