Impaired Driving Investigation

A reference text for
          police officers

Henry Waldock
Last updated: 2016-03-18

Here are some simple concepts for investigators to remember when investigating impaired drivers.  This page discusses only the simplest concepts.

If you want more information, there's a book written for police officers on this topic.  Unfortunately, the original author became a judge.  Fortunately, others took over and continued updating it.  They finished the second edition at the end of 2012.  It contains lots of valuable tips and information.


Suspicion or Belief

To make a breath or blood demand you need reasonable grounds to suspect or believe:

  1. who (driver)
  2. when (within 3 hours)
  3. what (driving or care or control)
  4. how (drugs or alcohol in body or impaired by alcohol or a drug)

What you can demand depends upon how strong the evidence is:

To make a blood analysis demand, you must also believe that the person is "incapable of", or it would be "impracticable to" obtain breath.

When you stop a vehicle, you observe the first three. But at an accident scene, you must investigate. Try to get the answers to these questions from all possible sources, not just the driver:

When you have reasonable grounds to suspect that a driver's body contains alcohol or drug, you may:

Although “reasonable suspicion" that a driver contains alcohol may arise from hearsay, observation, or inference, usually it comes from smelling liquor on the drivers breath. Many judges prefer you to isolate that smell from other possible sources. Removing the driver from the car usually achieves this.

When you have reasonable grounds to believe that alcohol impairs a driver you may:

You don't need proof that the driver is drunk, but you need more than a strong odour of liquor. A smell just means the driver drank; the judge needs reason to believe the drinking impaired the driver's ability to drive.

Care or Control

... Is about risk that the suspect will pose to the public by operating the vehicle.  If you find the suspect sitting in the driver's seat, the court may presume that the driver had care or control, unless the driver says he wasn't intending to set the vehicle in motion.  But drunks sometimes change their minds.

Hurry, hurry

S. 254 requires you to make breath demands promptly after you form the necessary “suspicion" or “belief". And once you make a demand, a clock starts ticking. You must test the suspect's breath (or take his blood) without delay.

Do what must be done at roadside: public safety, police warning, Charter warning, officer safety steps. Delay until later any time-consuming steps you can: writing up the file etc.  Don't wait for tow-trucks to arrive if other officers can manage the scene.

After an analysis demand, do not take breath or blood samples from a suspect who wants legal advice until the suspect gets a “reasonable opportunity" to get legal advice in private.  In this context, the suspect may choose from among lawyers who are reasonably available to give legal advice, but may not use the right to counsel to defeat a proper investigation.  Your obligation to allow the suspect to get legal advice applies equally at the hospital as it does at the police station.

Investigate, investigate

The procedure for testing breath provides only evidence of a blood-alcohol level. Only investigation can describe how that alcohol affected the driver. Therefore:

Keep investigating while you have the driver in your presence.  Even at court when the driver appears for trial observe how does this driver walks and talks when sober (assuming he's sober at court).

I like quantification. Instead of writing “the driver stumbled", try recording “on the 3rd step toward police car, driver's right foot moved 18 inches to right". Instead of writing “unsteady on her feet", try “head rotating clockwise 5 inches diameter".  If the driver slurs words, record the slurring. (“Offisher, I washn't drunking thish evning.")

The Demand

When making a demand, never suggest that the suspect has a choice. (“Are you going to blow?") Proceed as if the suspect will blow; give up trying to get breath samples only when the suspect makes it absolutely clear that he or she won't blow.

Refusal is an offence. The suspect's words and actions are the crime. You are the witness.  Document or record as much of this as you can. I like video and audio recordings.

Silence is not a refusal.  When the suspect refuses to speak, or gives ambiguous or contradictory information about whether he or she will blow, bring the suspect to the instrument and try to test their breath.

After a screening demand, the suspect has no right to talk to a lawyer unless you must delay to find a screening device.

After a breath analysis demand, the suspect has the right to talk to a lawyer. Don't accept the refusal unless the suspect waives or consults with counsel and still refuses to blow.

Taking Blood

Before taking blood, a medical practitioner must be satisfied that taking a blood sample will not endanger the suspect's health. You must tell the suspect that no blood sample will be taken if it would endanger his or her health. You can make a demand before getting the medical opinion that it would be safe to take the blood.


A “fail" result on a screen device tells you something about the drivers blood-alcohol level. The screening device does not say the driver is drunk.

Therefore, a “fail" means you believe the driver is over .08.

Qualified toxicologists told me that a blood-alcohol level of .10 or more impaired everyone. If you know that, then a “fail" gives you grounds to arrest and detain the driver for impaired driving.

Arrest or detain?

Unless the driver injured or killed someone, the offence of impaired driving may be prosecuted summarily or by indictment. For such hybrid offences, s.495 of the Criminal Code prevents you from arresting people when all public interest concerns are met. Never arrest unless you have a reason. But in most cases, there will be a reason.


Ordinarily, when you detain someone you must tell them about their rights to counsel and hold off eliciting evidence until they exercise or waiver of that right. But the courts make an exception in the case of quick roadside questioning. You must tell the driver of the reason for the stop, but you can ask questions and demand a breath screen without going into rights to counsel.

The downside of this is that none of the answers the driver gives you before you satisfy s.10(b) are admissible to prove the driver was drunk.

For example, suppose at roadside, you ask a driver “how much have you had to drink?" And the driver responds “I'm drunk man!" This answer justifies a breath demand, but the judge will not use it to determine whether alcohol impaired the driver.

What the driver tells you after waiving or exercising the right to counsel is admissible.

Equipment & Documentation

Don't trust a screening device until you know it's calibrated. For that reason, your office procedures should document the results of screening device maintenance.

Recent decisions say defence are entitled to just about all documentation concerning breath testing equipment and the technicians who operate it. Keep clear records and make them available for disclosure.

BC Administrative Driving Prohibition

Peace officers must submit sworn documentation to the Superintendent of Motor Vehicles.  Murray v. British Columbia (Superintendent of Motor Vehicles), 2013 BCCA 363

Accident Cases

The same principles of investigating drivers you stopped apply to investigating drivers whose vehicles stopped before you got there. The differences are: you start with less evidence about who was driving and when. Injuries often complicate the crime scene and your investigation.


Identifying the driver can be done many ways, but the riskiest way to gather this evidence is by asking the driver to incriminate himself. Try to rely on other ways first:


Grounds for a demand may be collected only within the 3 hours following driving. s.254(3) (Curiously, the demand itself may be delivered after this time frame, so long as the officer who makes the demand for his or her opinion before the 3 hours expired, and makes the demand as soon as practicable.)  Deruelle.

There are many ways to determine the recency of driving. Again, asking the suspect is the riskiest because of the right to silence.  For example:

Police officers often assume that the time of dispatch determines the time of the accident. This is a mistake. Nobody called 911 until after the accident occurred.  Dispatchers then responded after the 911 calls. Unless the 911 callers testify that they called 911 immediately, and the dispatchers testify that they dispatched immediately, the court will near no evidence of the time lapse between accident and dispatch.


At the time of driving, was the driver's ability to drive impaired by drugs or alcohol?

After the crash, the suspect may appear unsteady or dim-witted. Is it shock, head injury, or booze? A combination of investigative techniques may shed light on these questions.  For example:

Keep watching the suspect. Changes in symptoms, or the absence of change of symptoms can make a big difference in the case. Saul, 2015 BCCA 149.


When deciding whether to comply with breath or blood demands, the driver should know how much harm was done. Therefore, to the extent that you know whether other people were injured or killed, you must tell the driver before he or she provides bodily samples, and before he or she gets legal advice.

And if you learn that an injured victim succumbed, you should tell the detained driver about the new development, and permit further access to counsel.

Doctors & Medical Records

Doctors owe their patients a duty of confidentiality. If you go to the hospital, don't ask or permit the doctor to tell you “informally" what blood-alcohol concentrations their lab reports showed.  Dersch, [1993] 3 SCR 768.  Get a search warrant or production order.

Scene of the crime

The crash is where the crime stopped. The route the suspect drove is the scene of the crime. When investigating the crime, photograph the scene of the crime - as much of the route the suspect drove as is relevant to the offence - not just the crash site.