Drug driving enforcement has expanded rapidly across the UK, with roadside testing becoming a routine part of policing. While many assume a positive roadside test automatically proves guilt, the legal reality is far more complex.
Following recent public debate, and our own viral video on roadside drug testing and false positives - one issue has come up repeatedly:
"Can innocent drivers be pulled into drug-driving investigations due to flawed screening results or procedural errors?"
The short answer is: yes, they can, and understanding how the process works is crucial.
How Drug Driving Testing Works in Practice
In UK motoring law, police cannot randomly drug test drivers without reason. Officers must have reasonable grounds to suspect either:
- drug use in the body, or
- impairment while driving
Once suspicion exists, testing generally follows two distinct stages.
What Roadside Drug Tests Actually Look For
Despite what many people believe, roadside drug screening in the UK is limited in scope. In most cases, police saliva screening devices are designed to test primarily for:
- Cannabis (THC)
- Cocaine
These are the two most commonly detected substances at the roadside because they are prevalent and can be identified quickly using portable equipment. If a driver tests positive for either substance, this will usually lead to arrest and a further evidential blood test.
However (and this is an important point) a negative roadside drug test does not necessarily mean the driver is free to go. Where officers still suspect impairment, they can carry out a Field Impairment Test (FIT). This involves a series of physical and cognitive exercises designed to assess whether a driver’s ability is affected by drugs or alcohol, such as:
- balance and coordination checks
- eye movement assessments
- divided-attention tasks
If a driver fails the FIT test, police may still arrest them on suspicion of driving while unfit through drugs, even if the saliva test for cannabis and cocaine is negative.
In short:
- Roadside drug tests screen for specific substances
- FIT tests assess impairment more generally
- Both can lead to arrest and further investigation
This is another reason why roadside testing is not as straightforward as many people assume, and why strict legal procedure matters at every stage.
CBD, Prescription Medication & “But I Haven’t Taken Drugs”
We are seeing increasing questions about CBD use.
Hemp-derived CBD products may legally contain up to 0.2–0.3% THC depending on jurisdiction. Repeated use can potentially lead to detectable THC levels in the bloodstream.
There are also situations where:
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Medicinal cannabis patients exceed legal limits
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Drivers take prescribed benzodiazepines or stimulants
-
Individuals unknowingly ingest trace substances
The law distinguishes between:
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Specified limit offences (section 5A) – strict concentration limits
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Impairment offences (section 4) – driving while unfit
In section 5A cases, impairment does not need to be proven; only that the concentration exceeds the statutory limit.
That’s why understanding the science matters.
The Evidential Stage: Blood Testing
Once a driver is taken into custody, police will usually require an evidential blood sample to be taken by a qualified healthcare professional. This is the critical stage of a drug-driving investigation.
The sample is divided into two sealed containers:
• One sample is sent to an accredited forensic laboratory for prosecution testing
• The second is securely stored and made available for independent defence analysis if required
This safeguard exists to ensure transparency and allow results to be verified or challenged where appropriate. Laboratory blood analysis uses highly sophisticated techniques capable of measuring drug concentrations down to very small margins. It is far more precise than roadside screening devices and is regarded by the courts as the evidential standard.
However, scientific accuracy alone is not enough. For the result to be admissible and reliable in court, strict legal procedures must be followed at every stage, including:
- correct collection by trained personnel
- proper sealing and labelling of samples
- documented chain of custody
- compliant storage and transportation
- testing within statutory timeframes
Any failure in this process can compromise the integrity of the evidence and, in some cases, render it unreliable or inadmissible.
This is why drug-driving cases are not decided purely on numbers on a laboratory report, but on whether the prosecution can prove that the entire testing process was carried out lawfully and accurately from start to finish.
Where False Positives Actually Fit In
Much of the public confusion comes from mixing up roadside screening with evidential testing.
False positives are primarily a roadside screening issue - not a blood analysis issue.
Saliva screening devices, like all immunoassay-style tests, can be affected by:
- sensitivity thresholds
- cross-reactivity with substances or medication
- contamination
- handling issues
- calibration problems
Their purpose is to flag possible drug presence, not to prove an offence.
Why Procedure Still Matters - Even With Blood Tests
While laboratory blood testing is highly accurate, prosecutions can still fail where legal safeguards are not followed properly.
Common procedural issues include:
- breaks in chain of custody
- contamination risks
- incorrect storage or transport
- time limit breaches
- disclosure failures
The science may be strong; but the legal process must be flawless. This is why motoring defence focuses so heavily on procedure as well as results.
Why Public Concern Is Growing
It’s important to be clear: Police usually wait for blood results before formally charging a driver. However, once a roadside test or FIT leads to arrest, drivers can still experience:
- seizure of their vehicle
- release under investigation or bail
- months of uncertainty
- stress over potential bans and criminal records
Even before a final evidential result is fully examined.
The concern is not that blood testing is unreliable. The concern is that expanding roadside testing increases the number of people drawn into a legal process where any procedural failure can have life-changing consequences.
Case Study Example:
A medicinal cannabis patient tests positive at the roadside because cannabis can remain detectable days after use and is arrested and taken through the full custody process. Their blood result later comes back below the legal limit, meaning the “over the limit” drug driving offence cannot proceed. However, police still charge the driver with driving whilst unfit based purely on officer observations, with the IDPC often containing little more than those observations alongside the roadside screening result, which is not evidential. Without legal advice, the driver assumes the positive roadside test means automatic guilt and pleads guilty, even though impairment evidence could have been challenged and the case may have been highly defendable. As a result, a driving ban is imposed; not because laboratory evidence proved drug driving, but because screening triggered the prosecution and the legal process was never properly tested.
MYTH:
“Only guilty drivers get investigated.”
REALITY:
Wider roadside testing inevitably brings more people into the legal system, where mistakes can and do occur.
The Bottom Line
False positives matter, not because they replace scientific evidence, but because they start a legal process that must be followed perfectly. When it isn’t, cases that should never reach court can progress far too far.
Drug driving laws exist to protect public safety, and rightly so. But fairness in enforcement depends on:
✔ accurate science
✔ lawful procedure
✔ proper evidence handling
At MAJ Law, we regularly challenge cases not on technicalities, but on whether the prosecution has actually met the strict legal standards required for conviction.
Because in motoring law, process is just as important as results.
