PI Brief Update

Honesty And Integrity In Drink-Drive Calculations - Professor Ronald Denney, independent forensic scientist

05/01/12. The author’s experience over many years in carrying out calculations on alcohol absorption and elimination in drink-drive cases has shown that some solicitors are ignoring basic guidelines in seeking the assistance of forensic scientists.  This paper emphasises the criteria that need to be applied by all parties in maintaining respect for these calculations.

Introduction.

For many years now under the Road Traffic Acts  (see Road Traffic Offenders Act 1988 Section 15.3) it has been accepted that ‘Back Calculations’ can be carried out in connection with post-driving drinks defences and laced drinks special reason pleas.  The purpose of such calculations is to show that the extra amount of alcohol was responsible for putting the driver over the legal limit and that the driver was below the legal limit at the relevant time.  The significance of such calculations arises not only in attempts to save driving licences but can also be an important issue in post-accident insurance claims. 

Forward calculations are also accepted in connection with offences of ‘in charge of a motor vehicle’.  The plea entered, in the hope of saving the driving licence, is that the individual concerned would not have driven until he or she was below the legal limit.

For the forensic scientist responsible for carrying out the calculations there is often the problem of how far it is possible to assist the instructing solicitor and hence the defendant without effectively enabling the defendant to construct a defence artificially by later establishing ‘witnesses’ to back up the story.

It is not uncommon for solicitors actually to ask an expert to tell them how much alcohol the defendant needed to drink after driving in order to account for the recorded breath or blood alcohol value.  Similarly, quite frequently a solicitor dealing with an ‘in charge’ will ask the expert to tell then how long it would have been before the defendant was below the legal limit.

There is no doubt that both of these requests are irregular, to say the least, and the expert complying with the request would, with the solicitor’s support, be creating a position which would enable the defendant to modify or structure the evidence to his or her benefit and eventually create a fabricated defence.

There are clear criteria which need to be applied in order that the solicitor and the expert are not put in the position of being accused of constructing a defence or a plea.

Post-driving alcohol and laced drinks.

In the case of the post-driving and post-accident drinks defences it is clear that the defendant must provide all the details of the types and volumes of drinks and the times between which they were drunk before any calculations are carried out.  The same applies to laced drinks special reason pleas.  It is essential that the full details of the lacing are established as unambiguously as possible before keys are pressed on the calculators.

When the calculations have been carried out the figures need to satisfy the following criteria:

The total claimed alcohol consumption should correspond to the recorded breath, blood or urine value, after allowing for alcohol eliminated from the body.

If the contribution due to the post-driving consumption or lacing is subtracted from the recorded value the result, the remainder which is supposedly due to pre-driving consumption, should be below the legal limit.

A back calculation, based upon the result from (2) should also still be below the legal limit.      

Due to variations in the calculation of personal body factors (the modern approach to Widmark Factors) and in alcohol elimination rates no expert is going to produce a statement which provides specific numerical values for breath, blood or urine values at any particular time.  Instead a potential range of values is likely, but when correctly applied these almost always expose the fabricated defence and plea from the genuine one.  But this requires the solicitor rigidly to seek the correct data for the expert to work on without expecting the expert to provide ‘the correct answer’ from vague data that can be later manipulated. 

The ‘in charge’ plea.

The Road Traffic Act 1988, Section 5 (3) establishes this defence if it is possible to show that that there was no likelihood of the defendant driving whilst exceeding the prescribed limit. 

Calculations with respect to these pleas depend upon the assumption that there is a linear decrease in the body alcohol level at least from the time the individual is first observed by the police and subsequently tested at the police station.  In these circumstances the forensic scientist has to be satisfied that all alcohol previously drunk had been absorbed into the body before the arrest.  Any calculations are clearly invalid if the driver is still drinking from an opened bottle of can when the police arrive at the car as alcohol absorption will continue after arrest.

It is not possible for the expert to respond to the request for the time when the driver would have been below the legal limit.  If such calculations show that the legal level would be reached at, say, 07.30 hours then the driver is almost certainly going to say I did not intend to drive until 08.00 hours at the earliest.

Before any calculations are carried out the defendant should provide a specific statement of the time he or she next intended or expected to drive.  Only then should the reduction of the alcohol level be determined over the specified period of time, allowing for the normal range of elimination rates. It is inevitable that with these criteria there will be some instances in which the intended time for driving again is earlier than the time for attaining the legal limit.  

Of course, it is the responsibility of the solicitor to act in the best interests of his client and for the final decision on the validity of the calculations to be made by the magistrates.  However, the solicitor should also ensure that the forensic scientist is not asked to participate in an action that can lead to a distortion or manipulation of evidence.  The forensic scientist can only produce a statement based upon the data provided and his or her knowledge and experience in order to assist the court not to get the motorist convicted or exonerated.

The calculations are only as reliable as the data provided for the expert to work on.   

Professor Ronald C. Denney BSc, Hon DSc, PhD, CSci, CChem. FRSC, FFSSoc.

Professor Ronald Denney has worked in the field of drinking and driving for many years and frequently appears as an expert witness in this and other forensic science areas.  He is the author of ‘None For The Road’ and many other technical books.  He is an independent forensic scientist and when not attending court can sometimes be found lecturing at Kingston University.

www.forensics-denney.co.uk

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