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How fast can you react behind the wheel?

7 min read
6 Apr 2016

An important part of road safety is being able to react quickly in an emergency. We’ll be looking at eight factors that affect driver reaction times, and what you can do to reduce the risk.

We’ll be looking at a report published by Monash University in the 1980s to investigate the various reasons why reactions might fall short of what is necessary. It’s an older report, but (i) it looks at human physiology in response to stimuli (which hasn’t varied much over the decades) and (ii) it makes for a nice jumping off point for our research.


The data in the Monash study showed that faster drivers generally took less time to react under otherwise similar conditions. However, before we conclude that faster drivers are safer, the study also states that slower drivers were likely to be in a position where they could choose to brake more slowly, or simply ease up on the accelerator, in response to a potential hazard ahead. Their reaction times were slower, but as quick as they needed to be given that they had more time to react than the faster drivers.

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A New South Wales government fact sheet states that a slower driver gives themselves more time to avoid hazards and collisions and reduces their braking distance. They report that a 30% increase in speed (e.g. from 80km/h to 105km/h) more than doubles the possibility of a crash resulting in injuries and their severity. They also point out that ‘low level’ speeding in fact causes more problems than occasional high level speeding. In urban areas, exceeding the speed limit by 5km/h doubles the likelihood of a causality crash.

Unfamiliar environments

The Monash report mentions previous studies that found “the experience and background of the human plays a part, and this is particularly important when they change their environment.” It seems reasonable to assume your reaction time will be slower when you are distracted by dealing with unfamiliar roads and junctions, different laws, new driving conditions and working out unfamiliar road signage. This is true in Australia and overseas, where you may also have to contend with a foreign language, speeds in miles per hour or driving on the right hand side of the road.

Long periods of continuous driving

The Monash report refers to a previous experiment which asked drivers to press a switch on which their foot was resting in response to a sound signal. They found “The mean reaction time to the auditory signal was just over 450 milliseconds early in the driving session but increased to about 500 ms after 3 hours of continuous driving.”

The Transport Accident Commission (TAC) states that driver fatigue is suspected to be a primary cause in more than 20% of road fatalities and also notes that reduced reaction time is one of the key factors. They recommend that all drivers avoid traveling for more than eight to ten hours a day, take regular breaks – at least every two hours – and share the driving wherever possible.

Inexperienced drivers and fatigue

Driver fatigue seems to be a particular issue for inexperienced drivers. The report refers to an earlier study that found “very inexperienced drivers showed greater reaction time increases with prolonged driving than those who had greater experience.” This slowing of the reaction time was shown to be related to the task of driving over long periods itself rather than just being in the car for an extended time (e.g. as a passenger).

As well as the general advice on driver fatigue above, the TAC also reports that “While 18 to 25 year olds represent around 14% of all licensed drivers, they accounted for more than a quarter (28%) of all fatalities on Victoria’s roads.” They also suggest that parents have a role to play by giving children an example of good driving behaviours from an early age, stating that “Positive role modelling by parents of 5-12 year olds has the potential to have a huge influence on their child’s future driving behaviour.”

Listening to the radio

We might assume that music would be a distraction when driving but in fact it may be beneficial in some situations. The Monash study noted a greater increase in reaction times among inexperienced drivers during prolonged driving also stated “Listening to a car-radio was found to reduce this increase in reaction time resulting from prolonged driving.” Another study suggested “This may occur because the radio assists the driver in broadening his attention.”

A separate Monash study looked at the use of car radio to maintain alertness, particularly to counteract the boredom of long drives, and reported there was some evidence that listening to music or current affairs shows might help maintain reaction times in those circumstances. However it is unclear whether this is equally true of driving, for instance, in more urban areas or on winding, unfamiliar roads where boredom may not be a problem. It seems reasonable, at least, to take a careful approach to changing station or volume whilst driving, especially where it involves taking your eyes off the road or removing your hand from the wheel.

Audible or visual signals

The Monash report found that “Audible signals may lead to quicker reaction times than visual ones […] They tended to be shorter for audible signals than for leading vehicle stop lights and much longer when the driver had to respond on the basis of judged headway.”

Audio warning signals have been increasingly included in car design over the last few years, perhaps for this reason. The the engineering team at Ford are dedicated to developing and testing sound warnings for a range of situations, and have done significant research in this field. According to them, One sound might alert the driver that their seat belt is undone, while another warns of an upcoming collision, so the qualities of these sounds needs to reflect the urgency of the situation. They have found that in a potential accident situation “drivers responded faster to audible alerts that sound more authoritative and are emitted in a rapid, staccato rhythm.” To help with judged headway “Using long-range radar hidden in the front of the vehicle, collision warning detects moving vehicles ahead and alerts the driver of a collision risk with an alarm and warning light.”

Positioning of pedals

We might assume a greater distance between accelerator and brake would naturally make your reaction time slower, but the Monash report points out that there needs to be a reasonable distance between the pedals in order to avoid catching the foot accidently. They also note that previous studies have suggested that, within a reasonable range, the distance between the pedals doesn’t have a significant effect on physical movement time, and accidents can in fact be caused by misplacement of the foot on the wrong pedal if they are too close.

Driver Knowledge Tests have a guide to setting up your seat and driving position to achieve better control, amongst other benefits. In terms of the pedals, you should be able to push the accelerator and clutch (if you have one) all the way down while remaining seated. When you go for the brake. your knee should not hit the steering wheel.

Eye height and night driving

The Monash report mentions an important factor for reacting effectively is the driver’s eye height. A lower eye height, such as in a sports car, means “the time available for braking or otherwise responding is reduced in the limited sight distance case.” Additionally, for night driving on rural roads, the eye height and sight distance is effectively reduced for all drivers because “headlights are mounted lower than the driver’s eye height, and an object on the road can only be seen for a vehicle approaching a crest when in the line of sight of the headlamp rather than that of the drivers eye height.”

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The New South Wales Government website gives some tips for night driving, and mentions a German study which suggested our ‘night vision’ might fool us into driving faster than appropriate in dark conditions. “The eye has night vision cells, which take over from the ordinary colour-sensitive receptors that operate in the daylight … the study showed that objects detected by the night vision cells appeared to move in slow motion. This means that if we regulate our speed based on what our eyes are telling us – rather than what the speedo is showing – then we could get into trouble.”

Clearly there are lots of factors that can affect reaction times and it might be useful to be aware of these, but we can help keep ourselves safe on the road by driving sensibly; keeping within the speed limit, staying alert, avoiding fatigue, and by taking extra care at night and in unfamiliar environments.

Test your reaction times!

How quick are your reactions? You can test yourself with this emergency stop game.

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