Being in charge of a vehicle whilst unfit by reason of excess alcohol is defined as driving or attempting to drive a motor vehicle on the public highway or a public place whilst under the influence of alcohol exceeding the prescribed limit. Even if you are not driving the vehicle, but are in the vehicle on the public highway/public place, you can be deemed to be "in charge" of the vehicle.
The current limit is 35 micrograms of alcohol per litre of breath or 80 millilitres of alcohol per 100 millilitres of blood. However, if the lowest reading is 39 micrograms or below, you should be released with a warning.
Between 40 and 50 micrograms you must then be given the option of blood/urine tests. It is for the police to decide which of these options to use. A urine sample is on the basis of 2 samples within 1 hour. A blood sample must be taken by a police surgeon. You can demand 2 blood samples are taken which can be useful in defending the charge. If you are not given this option you may escape further prosecution.
A roadside breath test can then be administered and if the test is positive you will be arrested and taken to the police station where you will be asked to provide a further 2 specimens of breath for analysis using approved equipment. The lower reading is the one that will be used.
If you are over the limit during the police station breath test you will then be charged, cautioned and bailed to attend Court.
The term "road" is defined at section 142 of the Road Traffic Regulation Act 1984 as any length of highway or other road to which the public has access and includes bridges over which a road passes. The Concise Oxford Dictionary defines "road" as a line of communication for use of foot passengers and vehicles; while in Oxford -v- Austin  RTR 416 it was said to be a definable right of way between two points.
The expression 'on a road or other public place' is employed frequently in Road Traffic legislation, for example, in the drafting of moving traffic offences at sections 1-6 RTA. A public place is a place to which the public, or part thereof, have access.
The onus is on the prosecution to establish that a particular location was a `road' or `other public place'.
If you claim that the proportion of alcohol in a breath or laboratory specimen provided by you is above the legal limit because you had consumed alcohol after you had ceased to drive, you will need to rebut the presumption contained in section 15(2) Road Traffic Offenders Act 1988 that the proportion of alcohol in your breath, blood or urine at the time of the alleged offence was not less than in the specimen. The presumption will be rebutted if you prove the matters set out at section 15(3) on a balance of probabilities. In order to do so you will usually rely upon expert scientific evidence to establish that his alleged post driving consumption of alcohol accounts for the excess found in your sample, which took you over the legal limit.
However, in (Dawson v Lunn  RTR 234) it was held that the decision in (Pugsley v Hunter  RTR 284), the leading authority on 'laced drinks', was equally applicable to this defence. Hence, the defence must call medical or scientific evidence on the point unless it is obvious to a layperson that the post offence consumption of alcohol explained the excess. Experience has shown that this is frequently argued in cases where the defendant claims to have consumed no alcohol prior to driving. Even here it will not be 'obvious' where the quantity of alcohol subsequently consumed is not consistent with the measured breath, blood or urine sample.
Conversely, if you provide a specimen some hours after the time of the alleged offence which is below the legal limit the Forensic Science Service (FSS), or a private laboratory, may advise that by means of back calculations based upon rates at which the human body eliminates alcohol it is possible to establish that you were in excess of the legal limit when the offence occurred (see Gumbley v Cunningham (1989) 1 All ER 5.
The following information is relevant, where available:
When your reading is between 40 and 50 a driver may choose to replace a breath specimen by supplying a blood or urine sample (section 8(2) of the Road Traffic Act 1988). The police:
If the officer has failed to inform the driver of his option to have a blood or urine test you will be acquitted of the relevant allegation.
The first generation of Evidential Breath Testing Instruments (EBTIs)were replaced in 1999. All forces are now equipped with the Intoximeter EC/IR, the Camic Datamaster or the Lion Intoxylizer 6000UK. They detect and record a wider range of information when analysing breath samples.
These three makes of instrument are type approved by the Secretary of State for the purposes of the Road Traffic Act. Any challenge of that type approval must be made by way of an application for Judicial Review, not in the course of a summary trial relating to the performance of a particular instrument: (see DPP -v- Brown and DPP -v- Teixeira  EWHC Admin 932, 166 JP 1)
In the case of a breath specimen there is a statutory assumption at section 15 RTA that the instrument concerned performed reliably. However, that assumption may be challenged by evidence relevant to the circumstances of that particular case. In order to convict in the face of such evidence the Court must remain satisfied that the instrument provided a reading upon which they can rely. See Cracknell -v- Willis (1998) 1AC 450 at 467, and DPP -v- Brown; DPP -v- Teixeira.