Many drivers report that driving at an unusually slow speed tends to make them bored or sleepy. Equally most people would agree that driving very fast is exciting or scary. Somewhere between these extremes is a 'normal' speed.  How do these observations fit into road safety? How are they likely to be related to driver performance?

It turns out that hundreds of psychology papers explore the relationships between "work rate", attention, concentration, arousal, boredom and task performance.

But despite the obvious connections between chosen speed and driver work rate no science explores the proposed relationship between speed and driver performance.

Ask yourself:

• Why is it frustrating to be stuck behind a slow vehicle?
• Why is it boring to be stuck in a traffic jam?
• Why is it scary or exciting to go very fast?

This inverted 'U' relationship (first postulated by Yerkes-Dodson in 1908) is highly prominent in the psychological literature. Other relationships are postulated but for reasons that we'll explore don't apply to the driving case.

Although the inverted U is challenged in quite a bit in modern psychological literature, the simplest of basic logical analysis confirms that it must apply to the proposed speed / driver performance relationship. Taking extreme cases to establish the end points on the chart we see:

• A driver at 20 mph on an empty motorway in otherwise good conditions will not concentrate well on the task. At its simplest he'll be too bored to concentrate. We should conclude that his performance at the task is likely to be poor. This establishes a point at the low end of the curve.
• A driver at 250 mph on an empty motorway would be under a great deal of stress. He might perform well by his own personal standards, but the high speed also demands much more from him. Few people would doubt that his risk of crashing is high, but it isn't obvious or certain whether the crash risk comes from the speed or the stress. Nevertheless we have established a point at the high end of the curve.
• A driver at a typical comfortable speed on an empty motorway has a very low crash risk. He'll be interested in what he's doing and have no difficulty concentrating. He can vary his speed to establish his own comfort. Low crash risks prove that this process works - at least on some level. We've established a middle and reasonably optimal point on the curve.

Since the introduction of HGV speed limiters over the period 1994 to 1996 there has been an increase in fatal crashes affecting HGV drivers. We believe that this is likely to be due to lower attention and concentration levels at the new lower speeds. We would expect that many more HGV crashes involve falling asleep in the speed limiter era. The fatality figures tend to indicate that this effect is stronger than any benefit that might accrue from the reduced speeds.

information rate

How fast the brain receives information for processing. Higher speeds mean that information arrives more rapidly.

work rate

How hard and fast you have to work. Higher speeds mean that you have to work harder to keep up with events. Too fast may mean that you simply can't keep up with the work.

arousal

Where you are on the scale of 'asleep' to 'full panic'. Normal arousal provides optimum performance for long periods. In some cases 'panic' may enable short periods of very high performance, but these are not sustainable.

stimulation / interest

These are the main input factors to arousal - if you are interested you don't fall asleep or lose concentration so easily. Stimulation may come from information rate or work rate. Being 'over stimulated' makes you jittery, worried or stressed and less able to perform well.

task performance

How well you do at a task - in this case driving. Good performance at driving means fewer mistakes and a reduced risk of crashing. Clearly we're looking for road safety policies that optimize drivers' (and riders') performance at the task of driving.

adrenaline

The body's arousal regulator hormone. When you're under pressure or at risk the body quickly releases adrenaline increasing arousal.

stress

Feeling under pressure. It isn't usual to use the term stress until arousal is above typical levels. Stress causes and is closely related to high levels of arousal.

Inattention

Many crashes are caused by inattention. Some inattention crashes are cause by distraction and many are caused by its overlapping close cousin: poor concentration. A typical motorway inattention crash may be not noticing that traffic ahead has slowed and 'shunting' the vehicle ahead. Another common error resulting from poor concentration is lane changing without checking the rear three quarters blind spot. These two errors together probably account for half of all motorway crashes. Clearly we already don't have enough driver attention to avoid these crashes that are taking place and any change in driver attention levels will be directly reflected as a change in the number of crashes. In other words we're already working on a slope - if driver attention levels go up then crashes will go down. If driver attention levels go down then crashes will go up.

Falling asleep at the wheel

On some motorway stretches at least 30% of crashes are caused by drivers falling asleep. This is going to work very similarly to inattention. We're already on a slope where driver sleepiness causes dangerous crashes. Anything that tends to increase driver sleepiness will cause extra crashes.

Any policy that tends to reduce vehicle speeds is very likely to tend to provoke crashes that happen through inattention of driver sleepiness. The relationships described on this page suggest a strong link.

The Bike article was published without our approval or editorial input and fails to make some of the important points clearly. It was based entirely on Safe Speed work and soon after publication Safe Speed and Bike magazine issued a joint press release. (click here)

Much of the science considers 'stress' as the input factor. Since there is no stress as such at normal levels of arousal these studies are only working on the upper half of the curve. We shouldn't be surprised that they find task performance reducing as stress increases.

Most studies show that task performance reduces as stress increases, but some show stellar levels of performance at high stress levels. The reason for the difference can usually be traced to the the duration of the task. We're capable of remarkable feats in an emergency, but these high levels of performance can only be sustained for a few tens of seconds at a time. With driving we're looking at long term performance - often over hours - and temporary feats are reserved for occasional emergencies. We're interested in long term performance levels - how effective are we at not getting into trouble when driving?

Safe driving depends on sustaining high (or at least adequate) levels of vigilance for extended periods of time. Clearly any interruption to vigilance is dangerous. The literature about driving frequently refers to concentration and observation as highly important behaviours - these are clearly the same ideas expressed from the perspectives of drivers and psychologists.

Driving is an unusual task because few things that we do require sustained and uninterrupted vigilance, and fewer still become extremely dangerous if vigilance fails. Even a pilot can take breaks from the task but a driver who loses concentration for even 10 seconds is highly likely to crash.

Drivers naturally regulate their work rates and information rates by varying their speed to establish a comfort zone. There are not many tasks in modern life where you have a control that influences your work rate! We use the accelerator to find a comfort zone - neither too slow for comfort, nor too fast. 

The main reason for publishing this page is the fear that speed enforcement policy may move some drivers out of their zone of optimum performance and make the roads more dangerous as a result. Take the case of motorways. We apparently have 57% of car drivers exceeding 70 mph and 20% exceeding 80 mph. (DfT figures from VSGB 2003) We should assume that many of those 80mph+ drivers are somewhere near their zone of optimum performance and forcing them to slow to 70 mph might well make them more likely to make mistakes and crash.

Until this subject is fully researched it would be extremely unwise to try and influence vehicle speeds on motorways. Our motorways are the safest roads in the world despite huge proportions of traffic speeding. One would need very good evidence to risk changing anything, and the camera operators certainly don't have the evidence.

There are proposals in the UK and Europe to fit motor vehicles with satellite controlled speed limiters. The (rather absurd) theory is that if drivers are prevented from exceeding the speed limit by technology then the road will become safer. But of course the proposers have not taken account of the relationship between speed and driver performance proposed on this page.

We're certain that ISA is an extremely dangerous and flawed idea that would reduce driver's crash avoiding performance by lowering levels of work to well outside of the optimal zone resulting in worse concentration and more mistakes.

The relationship discussed here isn't the beginning and the end of drivers' speed choice. It's one important underlying relationship in a range of relationships. It's even inter linked with others - for example, the safe speed rule. Driving at a speed greater than the safe speed is frightening for any experienced driver.

The will be unusual individuals at both ends of the scale who tend to select inappropriate speeds also at both ends of the scale. A very nervous driver will choose low speeds to keep away from the extra stress at higher speeds. An overconfident driver might choose very high speeds because the sensation of risk at normal speeds isn't enough to stimulate.

Most of us lie between these extremes of course.

We asked web visitors how a low speed would affect their concentration and the risk of falling asleep. (click here)

Here are the responses (at the time of writing):

• "If I were forced to drive at 55 mph on a quiet motorway in good daylight conditions my concentration would be likely to be affected as follows..."

Only 4 out of 57 respondents (7%) felt able to state 'there would be no effect on my concentration'.

36 out of 57 respondents (63%) felt that their risk of falling asleep would be increased, with 17 (30%) stating that their risk of falling asleep would be 'much increased'.

Clearly our web respondents well recognise the effects we're talking about making it all the more surprising that they have never been examined by science.

Our first publication of these ideas was on October 23rd 2004 in the Safe Speed forums (click here).

An excellent report by NASA provides much of the background linking into the psychological literature. (click here).

We are quite certain that there is a real and important relationship between speed and driver performance that bears directly on 'how road safety works'. Policy must not intervene and make road safety worse by neglecting to account for real world practical and important effects on driver performance levels.

The poor results in the recent crash history of HGVs with speed limiters should be setting alarm bells ringing all around the world.

Nothing in road safety is as simple as it first appears and vital psychological factors are frequently and dangerously overlooked.

Clearly drivers recognise these effects - yet science has never explored them. This is evidence that research objectives are not being set properly. It is vital that skilled and experienced drivers are involved when research objectives are being set.

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