Two Defences to Dangerous Driving: Automatism and Mechanical Defect

This article will consider two specific defences which can be pleaded in relation to dangerous driving: automatism, and mechanical defect.  Usually, when raising a defence, the defendant admits doing the acts he is accused of but relies on other circumstances to justify those acts.  One example would be “self defence”, for example where A admits punching B and breaking his nose, but argues that this is justified because B was pointing a loaded gun towards him and was about to pull the trigger.

However, automatism and mechanical defect are very different defences because they involve a (partial) denial of the elements of the offence: to be convicted of most crimes the defendant’s actions must be voluntary, and these two defences are examples of how a defendant’s actions can be considered involuntary.  (It should also be noted that these defences can apply to numerous other offences as well as dangerous driving.)



Automatism is the classic ‘involuntary act’ defence, and arises where a driver experiences a “total destruction of voluntary control”[1] at the wheel of his vehicle.  It must be noted that the loss of control must be a complete and total loss of control: a partial loss of control is insufficient.  The loss of control must also relate to the driver’s control of his own actions rather than to the driver losing control of the car due to factors such as speed or poor driving conditons, for example: losing control as a result of having an epileptic fit is clearly different to ‘losing control’ by skidding on a patch of ice.  It should, of course, be emphasised that self-induced automatism does not count: the state of automatism must not be caused by the defendant in any way.  For this reason, fatigue arising from a long journey is incapable of amounting to automatism.

The scope of this defence is broad, but Hill v Baxter[2] gave numerous examples of the type of situation where automatism can be made out.  These included (non-exhaustively):

(a)    Where the driver suffers from a stroke or epileptic fit at the wheel;

(b)   Where an ‘act of God’ intervenes;

(c)    Where the driver is struck by a stone; and

(d)   Where the driver is attacked by a swarm of bees.

In 1998, the Magistrates Court heard a case where an HGV driver had crashed into a queue of stationary traffic on the M62, causing damage to 7 other vehicles and serious injury to other drivers.[3]  The driver stated that, when he was 60 feet away from the queue, he suffered from a severe sneezing fit, consisting of 4-5 sneezes in rapid succession, which made him disorientated and unable to control his vehicle so as to avoid a collision.  In that case, the Magistrates held that, on these specific facts, the automatism defence was made out because sneezing can produce a state of automatism.  This case is very fact-specific, and so it is highly unlikely that every sneeze would be severe enough to amount to automatism in the circumstances of the individual case.

It goes without saying that being under the influence of alcohol or drugs whilst driving cannot amount to a defence of automatism.

Mechanical Defect

Broadly speaking, it is a defence to show that the dangerousness of the driving was caused by a mechanical defect which caused a “sudden total loss of control in no way due to any fault on the part of the driver”.[4]

However, the law is very clear in that the defence does not apply where the driver knew (or ought to have known) that the defect existed before the sudden loss of control occurred.  For example, in Spurge[5] it was held that the mechanical defect defence did not apply where the defendant had bought a car which he knew swerved to the right whenever he applied the brakes.  Accordingly, proving the defendant knew the vehicle was dangerous at the relevant time (or that he ought to have known it was dangerous) would be something the prosecution would likely focus on at trial if this defence was raised.

It should be noted that raising the mechanical defect defence without very strong evidence can be disastrous: if it is shown that the vehicle did in fact suffer from a defect, and that the defendant knew or ought to have known that the defect existed, then not only would the defence fail but it would provide the prosecution with another route to conviction.  Equally, if the mechanical defect defence was raised in relation to a less serious offence (such as speeding, for example) it would usually be possible for dangerous driving to be added to the charge sheet or indictment as a second offence.  This is because the definition of dangerous driving includes driving a vehicle where it is reasonably ‘obvious’ to a competent and careful driver that driving the vehicle in its current state would be dangerous.

It is clear that both of these defences are exceptions rather than the rule: they are usually only successful in the most rare and unusual cases.  However, if you are accused of a driving offence and think that either (or both) of the defences discussed in this article could apply to your case, please contact us for advice – you may be able to avoid conviction.


The contents of this article should not be relied upon in isolation.  Each case is fact specific and this article should not be treated as legal advice or as a substitute for legal advice.


[1] Coley [2013] EWCA Crim 223 at [22]

[2] [1958] 1 QB 227

[3] R v Wooley [1998] CLY 914

[4] Spurge [1961] 2 QB 205

[5] [1961] 2 QB 205

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