What Is the Hip Flask Defence?
The hip flask defence is a statutory defence under section 15(3) of the Road Traffic Offenders Act 1988. It applies when a person consumed alcohol after they had stopped driving but before the police administered a breath, blood or urine test. The name derives from the scenario of a driver taking a drink from a hip flask after an accident but before police arrive.
The legal principle is straightforward. The prosecution must prove that the defendant was over the prescribed limit at the time of driving, not at the time of testing. If the defendant can show that they consumed alcohol after driving ceased and that this post-driving consumption was the reason their test result exceeded the limit, they have a complete defence to the charge.
Section 15(3) places the evidential burden on the defendant. The defendant must prove on the balance of probabilities that they consumed alcohol after driving, that but for this post-driving consumption they would not have been over the limit, and that they did not consume alcohol before driving in sufficient quantity to put them over the limit.
How the Hip Flask Defence Works
The hip flask defence requires the defendant to establish three things. First, that they consumed alcohol after the cessation of driving and before providing the specimen. Second, that the alcohol consumed after driving caused the specimen to exceed the prescribed limit. Third, that had they not consumed the post-driving alcohol, they would have been below the limit at the time of driving.
A forensic toxicologist is typically instructed to carry out a back calculation. The expert will consider the defendant's account of what they drank before and after driving, the time of each drink, the time driving ceased, the time the specimen was taken, and the defendant's body weight and sex. The expert then calculates whether the post-driving alcohol accounts for the reading being above the limit.
The defence fails if the back calculation shows that the defendant would have been over the limit at the time of driving even without the post-driving alcohol. It also fails if the court does not accept the defendant's account of what was consumed after driving as credible or truthful.
Evidence Required
Witness Testimony
Witness evidence is critical to establishing when the post-driving alcohol was consumed and in what quantity. Witnesses who were present when the defendant drank after driving can confirm the type and volume of alcohol consumed. A partner, friend or family member who saw the defendant drinking at home after returning from driving can provide this evidence.
Independent witnesses carry greater weight than those with a personal connection to the defendant. A neighbour, a publican or a colleague who witnessed the post-driving consumption will be more persuasive to the court. The witness must be able to give a specific and detailed account of what was consumed and when.
CCTV Evidence
CCTV footage can provide compelling evidence of post-driving alcohol consumption. If the drinking took place at a pub, restaurant or in a location covered by domestic security cameras, the footage may show exactly when and what the defendant drank after driving. This is difficult for the prosecution to dispute.
CCTV evidence should be secured as soon as possible after the incident. Most systems overwrite footage after a set period, typically 7 to 30 days. A solicitor can write to the premises or individual requesting that the footage be preserved before it is lost.
Back Calculation Evidence
Expert back calculation evidence from a forensic toxicologist is the cornerstone of the hip flask defence. The expert will calculate the defendant's likely blood alcohol level at the time of driving based on what was consumed before driving, the rate of alcohol absorption and elimination, and the effect of the post-driving alcohol on the test result.
The expert's report must demonstrate, on the balance of probabilities, that the defendant would have been below the prescribed limit at the time of driving. The calculation will typically present a range of possible values to account for individual variation in alcohol metabolism, which averages approximately 15 to 18 milligrams per 100 millilitres of blood per hour.
When Can the Hip Flask Defence Succeed?
The hip flask defence is most likely to succeed where there is strong, independent evidence of the post-driving consumption and credible expert back calculation evidence. Typical scenarios include a driver who returns home after drinking moderately, consumes further alcohol at home, and is then visited by police following a reported incident or an accident earlier in the journey.
The defence is weakened where the defendant's account of post-driving consumption is vague, unsupported or implausible. If the defendant claims to have drunk a large quantity of alcohol in a short period after driving, the court may not accept this as credible. Similarly, if there are no witnesses and no corroborating evidence, the defence faces an uphill battle.
Cases where the test is conducted many hours after driving can also benefit from this defence. The longer the gap between driving and testing, the greater the scope for post-driving consumption to have affected the result. However, the prosecution may also use back calculation to argue the defendant was still over the limit at the time of driving.
Using the Hip Flask Defence in Court
The hip flask defence is raised at trial before the magistrates' court. The defendant must give evidence and will be cross-examined by the prosecution on their account of pre-driving and post-driving consumption. The forensic toxicologist will also give evidence and may be challenged on their methodology and conclusions.
Preparation is everything. A specialist solicitor will take a detailed proof of evidence from the defendant, gather all witness statements, secure any CCTV or documentary evidence, and instruct the forensic expert well in advance of the trial date. The case must be presented clearly and consistently, as any discrepancy between the defendant's account and the expert evidence will be exploited by the prosecution.
If the magistrates accept the defence on the balance of probabilities, the defendant is acquitted. There is no conviction, no disqualification and no criminal record. Given the stakes involved, proper legal representation is essential to maximise the prospects of success.