Ancillary Powers Doctrine
The common law ancillary powers doctrine permits a police officer to interfere with a person's liberty or privacy during the lawful execution of their duty as long at the actions satisfy the following (The Waterfield test):
- the police be acting in the execution of their duties under common law or statute; and
- conduct constitutes a justifiable interference with individual liberty or privacy.
Under the first stage, " police powers are recognized as deriving from the nature and scope of police duties", including, “the preservation of the peace, the prevention of crime, and the protection of life and property”. 
The second stage balances "the competing interests of the police duty and of the liberty interests at stake". This aspect includes:
- "whether an invasion of individual rights is necessary in order for the peace officers to perform their duty", and
- "whether such invasion is reasonable in light of the public purposes served by effective control of criminal acts on the one hand and on the other respect for the liberty and fundamental dignity of individuals."
- the importance of the performance of the duty to the public good
- the necessity of the interference with individual liberty for the performance of the duty; and
- the extent of the interference with individual liberty
If these "factors, when weighed together, lead to the conclusion that the police action was reasonably necessary, then the action in question will not constitute" an "unjustifiable use of... police powers".
Interpretation of Ancillary Powers
This common law test is to be interpreted with s. 31 of the Interpretation Act in mind.
(2) Where power is given to a person, officer or functionary to do or enforce the doing of any act or thing, all such powers as are necessary to enable the person, officer or functionary to do or enforce the doing of the act or thing are deemed to be also given.
Powers to be exercised as required
(3) Where a power is conferred or a duty imposed, the power may be exercised and the duty shall be performed from time to time as occasion requires.
R.S., 1985, c. I-21, s. 31; R.S., 1985, c. 27 (1st Supp.), s. 203.
There is always a balance between police powers and individual liberties. There are no bright line rules and each each will turn on the facts.
R v Waterfield,  3 All ER 659
R v Stenning, 1970 CanLII 12 (SCC),  SCR 631, pp. 636-637 - first application of waterfield in Canada
Brown v Regional Municipality of Durham Police Service Board, 1998 CanLII 7198 (ON CA)
Dedman v The Queen, 1985 CanLII 41 (SCC),  2 SCR 2
R v Mann,  3 SCR 59, 2004 SCC 52 (CanLII), at para 26
Dedman, supra at p. 32
Mann, supra at para 26
R v MacDonald, 2014 SCC 3 (CanLII) at para 36
Mann, supra at para 39
Clayton, supra at paras 21 and 29
MacDonald, supra at para 37
- Mann, supra at para 39
- Dedman, at p. 35; Clayton, at paras. 21, 26 and 31
- Dedman, at p. 35
MacDonald, supra at para 37
- Brown v Regional Municipality of Durham Police Service Board at para 62
Police have a common law duty to preserve peace, prevent crime, and protect life and property.
R v Mann, 2004 SCC 52 (CanLII),  3 SCR 59 at para 26
The justification of police conduct depends on factors such as:
- the duty being performed;
- the extent to which interference of liberty is necessary to perform duty;
- importance of the duty to the public good;
- the liberty interfered with; and
- nature and extent of the interference.
These considerations must be balanced in the context of all available information, "the existence of any less intrusive alternative, and the strength of the police belief relating to the exigency or danger said to justify an extraordinary intrusion and a necessitous departure from conventional investigative measures".
A police officer is expected to act reasonably in the circumstances. The officer must evaluate the "totality of circumstances" when deciding to act. This includes changes in their circumstances which must be re-evaluated over time. New information cannot be ignored. The officer can only rely on objective and articulable circumstances, and not on "profile characteristics" that undermine the assessment of the circumstances.
Police are permitted to:
Police should be given "latitude" when exercising discretion and judgement in difficult or fluid circumstances.
Hill v Hamilton-Wentworth Regional Police, 2007 SCC 41 (CanLII) at para 58
R v Wilhelm, 2014 ONSC 1637 (CanLII) at para 113
Wilhelm at para 113
R v Chehil, 2013 SCC 49 (CanLII) at para 40
Wilhelm at para 114
Cornell at para 35
MacKenzie at para 15, 16, 62 to 64
Cornell at para 24
Jones at para 42
R v Kelsy, 2011 ONCA 605 (CanLII) at para 56, 57
Kephart at para 10
Cornell at para 23
R v Burke, 2013 ONCA 424 (CanLII) at para 44, 45
Wilhelm at para 115
Examples of Established Intrusions
There are several established situations that have warranted intrusions of police:
- Investigative detention
- Search incident to investigative detention
- Search incident to arrest;
- random vehicle stops;
- emergency roadblocks;
- search incident to emergency roadblocks;
- roadside sobriety tests;
- sniffer-dog searches;
- residential search incident to 911 phone calls
An officer may seize a cell phone incident to detention the purpose of officer safety or the potential loss of evidence.
Regulatory and provincial laws can diminish or eliminate any reasonable expectation of privacy.
Provincial regulatory Acts that authorize police to inspect vehicles will reduce the expectation of privacy.
The police have a common law power to impound vehicles when enforcing the Ontario highway traffic act.