Twip said:
Some of us have found out by that stuff actually happening and us discovering that our reaction is on the level.
Or discovering that one's reaction was quick enough for that situation/those situations, and mistakenly inferring that it was just as good as normal. That's a bit like saying "I got stabbed and I survived, so I'm immune to knives!"
Assume:
Reaction time sober: 300 ms
Reaction time stoned: 350 ms
You probably won't notice the difference.
Warning given: 400 ms -> you're OK, stoned or not
Warning given: 275 ms -> you're screwed, stoned or not
Warning given: 320 ms -> Sober, you're OK. Stoned, you're screwed.
If all your emergency situations have been of the 400-ms variety, then you might not be aware that your safety margin had been reduced.
Most drivers overestimate their ability, and half the people out there on the road shouldn't be there, even sober.