Virtually every dangerous driving behaviour has a single common denominator. This common denominator is present in the majority of serious accidents involving speeding, reckless cornering, driving under the influence, inattention, fatigue or even a pattern of hard braking. And this factor is driver reaction time, whether it’s reductions in time available for drivers to respond appropriately, or increases in how long it takes a driver to respond appropriately to a road hazard.
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What is reaction time and why is it important?
Smart driving involves giving yourself the time, and distance, required to respond appropriately to a road hazard without being involved in, or causing, an accident. It also involves ensuring that your ability to respond as quickly as possible to a road hazard is not compromised in any way.
When drivers encounter a road hazard, three factors control how long it takes them to bring their car to a stop. These are:
- the time required to realize that a response to a hazard is required
- the time it takes a driver to take appropriate action
- the time the car’s mechanical systems require to bring it to a halt.
Any driving behaviour that increases the amount of time taken to react, or reduces the amount of time available in which to react, places you at higher risk of a serious car accident.
The average driver takes approximate ¾ of a second to recognize a threat and ¾ of a second to apply the brakes. Once they have responded appropriately their car will travel an additional distance, which is proportional to the speed at which the car was travelling when the brakes were applied.
For a car travelling at 60 km/h the typically overall stopping distance on a dry road is around 45 meters. Cars travelling at 100km/h will usually take almost 100m to come to a stop.
This is a best case scenario, and explains why anything that reduces available distance, increases driver reaction time or affects a car’s ability to brake effectively rapidly increases the risk of an accident. For example a driver who is travelling at 60km/h who takes two seconds longer to respond to a hazard will take an additional 33 meters to come to a stop – almost doubling stopping distance.
The critical role that reaction time plays in car accidents is well known to organizations tasked with researching and understanding the factors that lead to accidents. Here are four well known risky driving behaviours and how they affect driver reaction times.
Among the many dangers posed by drunk driving is its impact on driver reaction time. Even small increases in blood alcohol level can immediately affect your ability to recognize and respond to road hazards effectively. The higher your blood alcohol level goes, the more this ability is compromised until you reach the point where you are functionally unable to operate a vehicle safely.
A study by the University of Texas at San Antonio found that driving under the influence increases driver reaction times by 15%-25%. Another study found that each 10% increase in breath alcohol concentration (BrAC) among young drivers increased their reaction times by 2%.
Meanwhile the US Department of National Highway Traffic Safety Administration reported that reaction time was impaired consistently at a Blood Alcohol Concentration (BAC) of 0.06 and up. Some subjects experienced impairment at a BAC of just 0.02. The more complex the road hazard, the more impairment the driver experienced.
It is important to note that impairment in reaction time typically combines with other effects of alcohol intoxication to make drunk driving particularly dangerous. Drinking and driving also results in decreased vigilance and perception, psychomotor skill impairment and increased risk taking. Therefore reduced reaction time plays out in the context of a greater overall risk of an accident.
Mobile phone distraction
Mobile phone distraction typically reduces the window of time drivers have to respond to a road hazard while also slowing down their reaction times.
The impact of mobile phone distraction on hazard perception time is the most obvious hazard of this dangerous driving behaviour. Every tenth of a second spent starting at a phone screen in the presence of a road hazard represents an increase in the amount of time a driver will take to recognize any hazard and initiate a response to it, reducing the window in which they can react.
Hazard perception time remains affected even after a driver has disengaged from their phones. Researchers at the University of Utah found that even voice interactions could distract drivers for up to 27 seconds after a phone interaction ended.
What is less obvious is the massive impact that mobile phone distraction has on driver reaction times, specifically involving hazards arising in a driver’s periphery. A study by the Queensland University of Technology found that mobile phone usage while driving can increase reaction time to objects that emerge from the driver’s periphery by a whopping 50%
Based on this evidence mobile phone distraction is even more dangerous that driving under the influence in terms of its impact on both hazard perception and driver reaction times.
The problem posed by tailgating differs from those posed by drunken driving and inattention. Tailgating effectively reduces both the time and distance that a driver has to respond to a collision risk that arises in front of their car. This means that irrespective of how quickly a driver is able to perceive and react to a hazard, tailgating will drastically increase the risk of a collision.
The risks associated with tailgating are best illustrated with an example.
A driver is travelling behind another vehicle at 60 km/h, observing an inadequate following distance of one second. At 60 km/h this translates into a following distance of around 17 meters. The following will occur if the leading vehicle emergency brakes on a dry road:
- the leading vehicle will come to a complete stop in 20 meters
- the tailing driver will travel 25 meters in the period that it takes them to recognize that they need to brake and to then apply the brakes
- the tailing driver will then travel an additional 20 meters before their car comes to a complete stop
- the total distance travelled by the tailing driver will therefore be 45 meters, whereas the maximum braking distance between their car and the leading vehicle, including the one second of following distance, will be 37 meters.
A collision is therefore inevitable unless the tailing driver changes lanes promptly or departs the road, both of which poses additional accident hazards.
Increasing following distances rapidly reduces the risk of collision in the event of the leading car braking. For example if the driver in the previous scenario had observed a two second following distance they would have 51 meters within which to bring their vehicle to a stop, and would require only 45 meters to do so.
In practice it is advisable to allow for even more following distance, with many car safety expertsrecommending a minimum three second following distance. This not only increases available braking distance in the event of unexpected braking of the lead vehicle, but also means that the tailing driver will not be required to emergency brake – which reduces the risks associated with this action.
Note that faster speeds require greater following distances. Two cars travelling at 110 km/h with a two second following distance will collide if the leading vehicle emergency brakes. The same applies to wet roads, as soon as the road surface is wet car stopping distances will increase, requiring additional space between two vehicles.
Driver fatigue is a silent killer on Australian roads and one of the ‘fatal five’ contributors to the Australian road toll. The state of Victoria has reported driver fatigue as the main factor in 20% of fatal accidents while statistics released by the state of New South Wales showed that fatigue was responsible for 18% of all fatal accidents between 2012 and 2016..
In some cases driver fatigue can reduce the amount of time a driver has to react to road hazards to zero, as fatigued drivers easily fall asleep at the wheel for several seconds at a time. In these cases they can travel large distances in very short periods of time without being responsive to road hazards. For example at 100 km/h a driver who nods off for five seconds will travel 138 meters.
However, fatigued driving is hazardous even when drivers are able to remain awake, as it impacts reaction time. A study by the Centre for Sleep Research in South Australia found that driving after 17 hours without sleep placed drivers at the same risk of an accident as a blood alcohol level (BAC) of 0.05, meanwhile driving after 24 hours without sleep was equivalent to driving with a BAC of 0.1.
These findings are in line with a number of studies involving athletes, which have shown a clear correlation between increases in fatigue and reaction times.
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