Even at 40 miles per hour your speed is 58 feet per second (or, at 60kmh you speed is 16 meters per second).
So it's either "the driver had sufficient warning" or "the lane change from Tesla was dangerous". As you don't expect cars that are dangerously changing to suddenly slow down.
The average driver’s reaction time is 0.2 to 0.3 seconds. This is before the driver’s foot moves.
Then there is approx 1 second before the brakes take effect (foot movement, applying the force, brakes responding)
So 1.5 second before the car starts breaking.
There is a rule of “be at least 2 seconds behind the car in front” which gives a safe distance to handle any emergency braking.
Of course everything depends on driver’s and car’s conditions.
However you count, if Tesla did a dangerous lane change and started braking, how is this the fault if the driver behind?
1. Fault in an accident can be spread across every party that was involved, and no matter how negligent one part was, that has no influence over how negligent every other party also potentially was.
2. A driver is typically responsible for being aware of all potential hazards on the road, not just the ones immediately ahead in the same lane. For instance a car that has started indicating to move into their lane from another lane (as this Tesla did ~7 seconds before the collision), and a car in their lane that is coming to a stop (this Tesla had completed the lane change pretty much 3 seconds before the collision).
Obviously the Tesla is really pushing the limits of what would be considered a safe gap, and coming to a stop in that location without a proper reason is obviously negligently dangerous. But the negligence of both parties contributed to this accident, regardless of who was most at fault.
At 60 mph it will take many meters and seconds to deaccelerate, assuming the driver can take exactly the correct action:
https://www.wolframalpha.com/input?i=time+to+brake+at+70+met...
Estimate here is about 4 seconds, under perfect conditions. 5 seconds is more than reasonable, considering you also need to realize the other driver is acting erroneously.
You seem to have your terminology confused. Reaction time doesn’t even necessarily have anything to do with driving. I suggest a quick web search next time before assigning people remedial physics.
No, reaction time means the time from noticing the need to brake to the time brake pressue is getting applied.
