Lets break it down into the various stages.
First is the "thinking" time. You are looking ahead and see trouble, but it takes time for your brain to process this and decide what to do about this. An experienced driver, that's paying attention, will process the situation faster than someone that's tired, day dreaming, talking on their phone, drunk or stoned. Now if you are expecting to brake, like a race car driver always is, then your thinking time is less, but it's still some period of time.
So you decide, better hit the brakes. This has taken maybe a second? Hopefully slightly less, but sometime it's more.
Then you have "Reaction Time" This is how long it takes to physically move your foot and get it onto the brake pedal. Maybe 1/2 a second more.
So now the average driver has taken 1.5 seconds just to START braking.
This is where the guideline that you should be 2 seconds behind the car in front of you, no matter what the speed. If you are only one second behind, then you "thinking" and "reaction" time mean that if they suddenly brake hard, the average driver WILL hit them, before they even get their own foot onto the brake.
Once you have your foot on the brakes, then it's physics that determines the remaining stopping distance.
How good the brakes are?
How "grippy" the tires are?
What the surface of the road is like? Wet / Gravel / snow or just a poor surface has less grip
Driver skill, or the quality of the ABS system. If you lock up the wheels and skid it takes longer to stop than it should.
Thinking time is affected by distractions and driver alertness. The more alert you are. the quicker you react.
Braking distance is affected by the vehicle's speed (it takes longer to stop at higher speeds), tyres (bad tread and you may loose grip on the road), load (heavier cars take longer to stop) and braking capabilities (how good are your brakes? do you have ABS?) in addition to road conditions (i.e. is the road is slick with rain/ice?).
A thought shared.
weather conditions, how much traction the tyres have, tiredness, whether or not the driver has been drinking etc