Driver Perception and Reaction Times

StopwatchI 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.

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How Long before signalling

If I might just add a comment that I find it extremely helpful if a driver "signals" before braking. brake lights on their own initiate several instinctive responses, and place the driver at the rear in a series of pending decision making scenarios (albeit mostly sub-conscious, but never-the-less they are tension forming as one anticipates the next action of the braking vehicle infront).
A simple 1 second indicator period, prior to braking, sets the scene, and the subsequent brake lights are to be expected, rather than placing the driver behind into a decision making situation.
This is even more confusing with North American combined brake/indicator lights - I would really like to see a complete change-over to seperate yellow indicators for all vehicles.
Thanks for the opportunity to comment


Geoff, do you mean that the driver should signal left or right before braking, even if they are not turning?

Would tapping the brake pedal to flash the brake lights if time permits not be a better choice?

As Much Notice as Possible

Good information Tim but to take it a bit further. When I learned to drive I was told that when turning I should signal in time to give the people both ahead and behind me as much notice as possible so that they could make good safe decisions. 1/3rd of a block was often used as an example. Waiting to the last second to signal a right hand turn for instance would not give a vehicle turning right into the lane I am vacating enough time to smoothly merge into the lane. Leaving it to the last moment to signal a left turn on a four lane (2 each way) where there is no left turn lane doesn't give the person behind you opportunity to safely switch lanes to go around you. Often leading to quick moves without fully checking to see if someone is in the blind spot. (this also applies to the right turn above)

Ahead and behind

I was also taught to try to drive for those ahead of me and behind me also. In other words, try to give as much time as you can so that those not only behind you can be informed of your intentions, but do those in front of you can have time to notice your I tentions also. Unfortunately, not many ppl at the front of the lines bother to check on what's happening at the back of the line anymore. And that's where our modern day traffic problems are starting now.

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