What to expect during a roadside breath test?

As described in our article on “are random drink or drug tests legal”, you can be asked to undergo a roadside breath test if either:

  • The officer reasonably suspects you’ve consumed alcohol;
  • The officer reasonably suspects you’ve committed a motoring offence;
  • You’ve been involved in an accident.

There are currently fifteen, home office approved roadside breath testing kits in use on Britain’s roads. The below are the instructions that are typical to most devices in use today.

What should happen during a roadside breathalyser test?

You should be asked when your last drink was. The police officer should wait at least 20 minutes before conducting the roadside breath test after your last drink. Officers typically wait 2-5 minutes after your last cigarette.

The officer should attach a fresh mouthpiece to the breathalyser and should explain the test to you. The officer should instruct you to fill your lungs and blow in one continuous breath through the mouthpiece.

In some cases, you’d have to blow strongly enough to bring on light A, and long enough to bring on light B. If you fail to bring on light B then you’d be deemed to have provided an unsatisfactory sample. In other cases, you’d need to cause the machine to create a continuous beep whilst blowing and would have to stop once you hear a double beep. It all depends which type of breathalyser device you blow into.

In under a minute, the officer will take the reading from the machine.

A green light, or the words “Zero” or “Pass” means you’ve hardly got any alcohol in your system and you’ve passed the test. You can go on your way.

An amber light, or the word “Warn” means you’ve got more alcohol in your system, but you’ve passed the test. You’ve been lucky and should go on your way.

A red light, or the word “Fail” is bad news. You’re over the limit and will be taken to the police station for evidential testing.

The do’s and don’ts of roadside tests that the police should adhere to:

Your knowledge of these procedures could help establish whether the police have breached procedure, and whether or not the roadside breath test result can be challenged by your legal team.


  • Ensure that it has been at least twenty minutes since your last drink before performing a roadside breath test;
  • Allow at least two minutes since your last cigarette before performing a roadside breath test;
  • Use a new mouthpiece for every test;
  • Ensure that a “ready check” is obtained before performing the breath test;


  • Allow you to hold the breathalyser device yourself;
  • Allow tobacco smoke to be blown into the mouthpiece;
  • Store the breathalyser device in an environment that is too hot or too cold;
  • Don’t subject the machine to severe mechanical shock (e.g. throwing it on the ground).

If you’ve been subjected to a breath test and you think the police have failed to follow the above guidelines, please contact a member of our legal team as soon as possible on 0330 33 22 770. There are numerous defences to drink driving, whether the police have breached procedure at the roadside, police station or hospital. It may not always be obvious that the police have made a mistake, so it’s vital to gain expert opinion on whether you have a case to fight. It could make the difference between walking away from Court with your licence intact or facing a minimum twelve-month ban from driving and a criminal record.

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Article written by Alison Ashworth, Specialist drink and drug driving solicitor and Managing Director of Ashworth Motoring Law. Alison has a first-class honours degree in law and has appeared on various TV and Radio programs to discuss drink and drug driving. Widely considered the “go-to” expert in the field of motoring law, Alison has outstanding rates of securing not guilty verdicts in cases involving drink and drug driving. Contact 0330 33 22 770 for an initial opinion on your case for free today.

Random stop for breath test or drug test

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