Driver perception and reaction times

Staying aware on the road

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 you need to do something such as signalling before you change lanes?

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 that meant was a driver who was paying attention could reasonably be expected to see something, process the situation in his or her brain and make a decision about what to do in that perception time period. Once decided upon, it took the reaction time period to carry out the 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.

It is possible 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 was 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.

What does all of that 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 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 the other driver is still discovering or contemplating your signal and too much more may mean her or she has gone back to trying to decide what exactly it is you mean to do.

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.

Keep in mind it is risky to do something too quickly when there is other traffic near your vehicle or sight distances are short.

Never expect everyone, including yourself, is always paying attention in the right place at the right time.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.

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About the Author

Tim Schewe is a retired constable with many years of traffic law enforcement experience. He has been writing his column for most of the 20 years of his service in the RCMP.

The column was 'The Beat Goes On' in Fort St. John, 'Traffic Tips' in the South Okanagan and now 'Behind the Wheel' on Vancouver Island and here on Castanet.net.

Schewe retired from the force in January of 2006, but the column has become a habit, and continues.

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The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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