Roadside Drug Tests: The Hidden Legal Flaw That Can Collapse a Drug Driving Case
By MAJ Law on 31 March 2026
Most drivers assume that a positive roadside drug test means the case is over. In reality, that’s often where the legal issues begin.
At MAJ Law, we regularly act for drivers who are convinced they have no defence because they tested positive at the roadside. But in many cases, the key issue isn’t what was found in their system, it’s how the police reached that point in the first place.
Because in drug driving cases, procedure isn’t just important. It can be everything.
What a Roadside Drug Test Actually Does (And What It Doesn’t)
Roadside DrugWipe tests are used by police as a preliminary screening tool. They are designed to detect the presence of certain substances, most commonly cannabis (THC) and cocaine. They are quick, widely used, and often treated as decisive. But legally, they are anything but....
A roadside drug test does not prove that you were over the legal limit. It does not confirm impairment. And it is not, on its own, evidence of guilt. Instead, it is simply a gateway, a way for police to justify taking the investigation further, usually by requiring a blood sample at the police station.
That distinction is critical. Because if that gateway isn’t lawfully opened, everything that follows can come into question.
Our View: Most Drug Driving Cases Turn on Procedure - Not Just Results
There’s a common assumption that drug driving cases are purely scientific. That once a substance is detected, the outcome is inevitable. In practice, that’s rarely how these cases are won or lost.
From a defence perspective, the focus is often on the process - how the test was carried out, whether proper procedures were followed, and whether the police had a lawful basis to proceed at each stage. We regularly see cases where drivers have accepted their position too quickly, assuming that a positive test result leaves no room for challenge.
But the reality is, a flawed process can undermine even the strongest-looking evidence. And in many cases, it does.
Why the Roadside Test Matters More Than You Think
The roadside test is not just an early step in the process. In many cases, it is what gives the police the legal authority to continue the investigation. Without that authority, the requirement for a blood sample may not be valid. That raises a crucial question:
Were the police lawfully entitled to require a blood specimen at all?
If the answer is no, the implications are significant. Because if the procedure leading to the blood test is flawed, the evidence obtained from it may also be challenged.
Where Things Commonly Go Wrong in Practice
Although DrugWipe tests are designed to be simple, they are highly sensitive devices that must be used correctly. In reality, that doesn’t always happen.
We frequently identify issues such as failures to observe waiting periods before testing, problems with how samples are collected, and inconsistencies in how results are interpreted. In some cases, the device itself may not have been properly checked, whether in terms of expiry dates, packaging, or control indicators.
Timing is another key issue. These tests must be read within specific timeframes, yet in practice, this isn’t always adhered to. Even small deviations can affect the reliability of the result.
There are also cases where results are misread entirely, particularly where faint or unclear indicators are involved.
Individually, these issues might seem minor. But collectively, they can raise serious questions about whether the test can be relied upon at all.
Cannabis and Cocaine Cases: Why the Law Isn’t as Straightforward as It Seems
Drug driving law in England and Wales is based on strict legal limits, not impairment.
For cannabis, the threshold is extremely low. This means drivers can feel completely normal, show no signs of impairment, and still be over the legal limit.
Cocaine cases can be even more complex. The substance often detected is benzoylecgonine (BZE), a metabolite of cocaine. It is not psychoactive and can remain in the system after the effects of the drug have worn off.
This creates situations where a driver may be over the legal limit without being under the influence at the time of driving.
👉 You can explore this in more detail in our guide to drug driving offences and legal limits
Case Study: When a Positive Test Isn’t Enough
We recently represented a client who had been stopped by police and subjected to a roadside DrugWipe test, which returned a positive result for cannabis.
Following this, they were arrested and required to provide a blood sample. When the results came back, they showed a reading above the legal limit. At that stage, the client assumed the case against them was effectively complete and that a conviction was inevitable. However, once we were instructed, our focus wasn’t just on the result, it was on how the police had reached that point.
We began by reviewing the full sequence of events leading up to the arrest. This included disclosure of the officer’s statement, custody records, and importantly, body-worn video footage from the roadside. What quickly became apparent was that the procedure surrounding the roadside test had not been properly followed.
There were inconsistencies in the officer’s account of the testing process, particularly in relation to the conditions immediately prior to the test being administered. The footage suggested that the required waiting period (during which the driver should not eat, drink, smoke or vape) had not been clearly observed or recorded. This matters because substances in the mouth can interfere with saliva-based testing devices, potentially affecting the reliability of the result.
We also identified issues with how the test result itself had been interpreted and documented. The recording of the result did not clearly align with what could be seen on the device, raising further questions about whether the test had been read correctly.
Individually, these points may not have seemed decisive. But taken together, they raised a fundamental legal issue:
Did the police have a lawful basis to require a blood sample at all?
If the roadside test could not be relied upon, then the justification for requiring the evidential sample was significantly weakened. This argument was put forward to the prosecution, supported by the inconsistencies in the evidence and the lack of clear procedural compliance.
As a result, the prosecution were unable to proceed with the case.
👉 Outcome: No conviction, no disqualification.
Why This Case Matters
What makes this case particularly important is that the client was, on the face of it, over the legal limit. But that alone was not enough.
The case ultimately turned on whether the correct legal process had been followed, and whether the evidence had been obtained lawfully.
This is something many drivers don’t realise - a positive test result does not automatically make a case watertight. If the procedure leading to that result is flawed, it can open the door to a defence that wouldn’t otherwise be considered.
▶ Watch: How Roadside Tests Can Be Challenged
Our director, Conor Johnstone explains how roadside drug tests are reviewed in practice, including the common procedural errors that can impact a case.
The Legal Test: Was the Blood Sample Lawfully Required?
Under the Road Traffic Act 1988, police cannot simply require a blood sample without justification. They must rely on one of two lawful routes.
The first is a valid preliminary test, such as a properly conducted roadside DrugWipe. The second is a medical opinion that the individual is under the influence of drugs. In practice, both routes can be problematic.
We often see cases where the roadside test is flawed, or where the medical assessment is vague, poorly recorded, or lacking sufficient detail.
If neither route is properly established, the requirement for a blood sample may be unlawful, and that can fundamentally weaken the prosecution case.
Why Early Advice Can Make the Difference
One of the biggest mistakes drivers make is assuming that a positive result means there is nothing to challenge. In reality, early legal advice can make a significant difference.
At MAJ Law, we look beyond the result itself. We examine how the test was conducted, whether proper procedures were followed, and whether the evidence has been obtained lawfully.
This can involve reviewing body-worn footage, custody records, and the full testing process in detail.
Because in many cases, the defence doesn’t lie in denying the result, it lies in questioning how it was obtained.
Final Thoughts
Drug driving cases are often presented as straightforward. But once the detail is examined, they rarely are.
A positive roadside test does not automatically mean a conviction. And where procedure has not been followed correctly, the entire foundation of the case can come into question.
Need Advice?
If you’ve been stopped, tested, or are under investigation for a drug driving offence, it’s worth understanding your position before making any decisions.
What appears clear-cut on paper is often far more complex in reality.
📞 Get in touch for free initial advice.
If the procedure is flawed, the prosecution may be too.