What Is the Legal Threshold for Impairment in Canada?

How Does Canadian Criminal Law Define Impaired Driving?

A person may be charged based on alleged impairment, prohibited blood alcohol concentration, prohibited drug concentration, or failure to provide a required sample. Each type of allegation has its own legal issues.

Criminal Code Concepts That May Apply

In impaired driving cases, the Criminal Code may involve concepts such as:

• Impaired operation by alcohol
• Impaired operation by drugs
• Impaired operation by alcohol and drugs together
• Blood alcohol concentration at or over the prohibited limit
• Drug concentration limits
• Refusal or failure to comply with a lawful demand
• Reasonable suspicion
• Reasonable grounds
• Approved screening device demands
• Breath or blood sample procedures

A DUI lawyer may look at whether police had the legal authority to demand a sample, whether the accused person understood the demand, and whether testing was done properly.

Impairment vs. Over 80

Impairment and “over 80” are related but different legal issues.

Issue	Impairment	Over 80 / Blood Alcohol Concentration
Main focus	Whether ability to operate was affected	Whether alcohol concentration met or exceeded the prohibited level
Common evidence	Driving, behaviour, speech, balance, officer observations	Breath, blood, or approved testing results
Defence focus	Whether impairment was actually proven	Whether the demand, testing process, and results are legally reliable

Can Someone Be Charged Without Bad Driving?

Yes. Bad driving may support an impaired driving investigation, but it is not always required for a charge.

Police may rely on other evidence, such as:

• Odour of alcohol
• Red or glassy eyes
• Slow responses
• Difficulty following instructions
• Balance or coordination concerns
• Admissions about drinking or drug use
• Roadside screening results
• Breath testing results
• Drug-related observations

A DUI lawyer in Calgary can review whether the evidence truly supports the allegation or whether there are weaknesses in the investigation.

What Evidence May Police Look For?

Police may investigate alcohol impairment, drug impairment, or impairment caused by both alcohol and drugs. The type of evidence can differ depending on the allegation.

The investigation may begin with a traffic stop, collision, checkstop, citizen report, or police observation. From there, officers may look for signs that support further testing or arrest.

Alcohol Impairment Evidence

In alcohol-related cases, police may look for physical signs, driving behaviour, and testing results.

Common evidence may include:

• Weaving or lane drifting
• Delayed response to police lights
• Odour of alcohol
• Slurred or unusual speech
• Poor balance
• Difficulty producing documents
• Confusion or delayed answers
• Approved screening device results
• Breath sample results

These signs are not always conclusive. For example, a person may have a strong odour of alcohol but not be impaired. Another person may appear nervous or tired during a police interaction.

The defence may review whether the officer’s observations were detailed, consistent, and supported by the rest of the evidence.

Drug-Impaired Driving Evidence

Drug-impaired driving cases can involve different issues than alcohol cases. Drugs may affect people in different ways, and the evidence may be more complex.

Police may rely on:

• Driving behaviour
• Physical appearance
• Eye observations
• Coordination concerns
• Statements by the accused person
• Standardized field sobriety testing
• Drug recognition evidence
• Blood or oral fluid evidence, where applicable

Drug influence can occur when someone is under the influence of marijuana, illegal drugs, prescription drugs, or a mixture of any drugs. One may even get into legal troubles for using a drug that has been legally prescribed to him/her because he/she cannot drive properly.

Combination Alcohol and Drug Impairment

Some impaired driving allegations involve both alcohol and drugs. Police may allege that the combined effect affected the person’s ability to operate a vehicle.

This can create additional legal and evidentiary issues. The defence may need to review:

• What substances were allegedly involved
• When they were consumed
• Whether testing was done properly
• Whether the evidence connects the substances to impairment
• Whether police followed the required legal process
• Whether the timeline supports the allegation

Combination impairment cases should be handled carefully because the Crown may rely on the overall pattern of evidence rather than one single test result.

Evidence Problems a DUI Lawyer May Review

A DUI lawyer may examine the investigation for legal, procedural, and evidentiary problems.

Key questions may include:

1. Was the traffic stop lawful?
2. Did police have reasonable suspicion or reasonable grounds?
3. Was the demand made correctly?
4. Did the accused understand the demand?
5. Were breath or drug tests properly administered?
6. Were there delays in testing?
7. Do police notes match video or audio evidence?
8. Were Charter rights respected?
9. Does the evidence prove impairment at the time of driving?

FAQs  

Can I be charged if my blood alcohol level is below 80?

Yes, a person may still face an impaired driving allegation if police claim alcohol or drugs affected their ability to operate a vehicle.  

Do I need a criminal lawyer for an impaired driving charge?

A criminal lawyer can be important because impaired driving cases often involve technical evidence, Charter issues, licence consequences, and criminal penalties.