Driver Perception and Reaction Times
I usually talk about driver perception and reaction times in relation to using a signal light but it applies equally well to many other areas of driving such as following distances or why the speed limit might seem low on what appears to be a straight road. The question is "How long do I need to do something such as signalling before I change lanes?"
The Rule of Thumb
As a collision analyst I used three quarters of a second for perception and the same length of time for reaction if the true time was not known. What this meant is that a driver who was paying attention could reasonably be expected to see something, process the situation in their brain, and make a decision on what to do in that perception time period. Once decided upon, it took the reaction time period to carry out that action.
In total there was supposed to be a second and a half between seeing something and beginning to carry out the necessary action in response to it.
What Happens in the Real World
It is possible that someone could be faster, but in the real world it is far more likely that the combination of these time periods could be three or four seconds or even more if the driver were distracted by any of the many things we see or choose to do while driving.
So much for the two second rule if you are a cautious driver!
How it Applies to Signalling
What does all of this really mean? Let's go back to the example of signalling a lane change.
If you want to be sure other drivers see your signal, decide what it is that you mean to do and then act by not getting in the way, you probably need to signal for at least four seconds. Four seconds before you begin to turn your steering wheel.
Less might mean that the other driver is still discovering or contemplating your signal and too much more may mean that they have gone back to trying to decide what exactly it is you mean to do.
We're All Different
Everyone's perception and reaction times are different to some degree when we compare each other, and we vary individually according to mood, fatigue, impairment or distraction to name some familiar reasons.
How it Applies in General
Keep in mind that it is risky to do something too quickly when there is other traffic near your vehicle or sight distances are short. Never expect that everyone, including yourself, is always paying attention in the right place at the right time.