This post is not conclusive but meant to provide some guidance to officers conducting vehicle stops.
A motorist in Canada is legally obligated to stop their vehicle when directed to do so by a police officer: see, for example, Highway Traffic Act s. 253(2) PE; Motor Vehicle Act s. 83(1) NS; Motor Vehicle Act s. 105 NB, Highway Traffic Act s. 201.1(1) NL; Highway Traffic Act, R.S.O. 1990, c. H.8, ss. 216(1)-(2), etc.
A driver has no choice but to comply with such a direction. When a motorist does so, they are “detained” from a Charter standpoint: see R. v. Grant, 2009 SCC 32,  2 S.C.R. 353, at para. 30; R. v. Orbanski; R. v. Elias, 2005 SCC 37,  2 S.C.R. 3, at para. 31; R. v. Hufsky,  1 S.C.R. 621, at pp. 631-632; R. v. Therens,  1 S.C.R. 613, at pp. 641-644, but that detention is justifiable under s. 1 of the Charter provided that the stopping of the motorist is for reasons related to traffic safety, this includes things, “such as checking the driver’s licence and insurance, the sobriety of the driver and the mechanical fitness of the vehicle”: R. v. Ladouceur,  1 S.C.R. 1257, at p. 1287. See also, Hufsky. It also includes the authority to detain a motorist whom a police officer observes or reasonably suspects of committing an offence: R. v. Wilson,  1 S.C.R. 1291, at p. 1297 (a.k.a. articulable cause).
The Supreme Court of Canada has held that a police officer who lawfully detains a motorist for traffic safety reasons may also harbour ulterior criminal investigative purposes for the detention. Provided that traffic safety remains a motivation for the detention, the fact that the officer is also interested in discovering evidence of another offence does not in itself invalidate the detention. That said, in such cases, a police officer must be careful not to exceed the limits of his or her traffic-safety powers. If they do so, they violate the Charter: see R. v. Nolet, 2010 SCC 24,  1 S.C.R. 851, at paras. 4, 23, 32-41.
A police officer’s authority, “must be determined having regard to the police power actually exercised and not by reference to some other police power which may have been, but was not, exercised.” R. v. Stevenson, 2014 ONCA 842, at para. 56, leave to appeal refused  S.C.C.A. No. 37.
A police officer is empowered to briefly detain a person if the officer has reasonable grounds to suspect that the individual is connected to a recently committed or still-unfolding criminal offence and the detention is reasonably necessary in all of the circumstances: R. v. Mann, 2004 SCC 52,  3 S.C.R. 59, at paras. 34, 45. The power to detain is not confined to crimes known to the police but includes crimes that are reasonably suspected: R. v. Chehil, 2013 SCC 49,  3 S.C.R. 220, at para. 35; R. v. Nesbeth, 2008 ONCA 579, at para. 18, leave to appeal refused  S.C.C.A. No. 10.
Section 10(a) of the Charter guarantees everyone the right “on arrest or detention” to be “informed promptly of the reasons therefor”. This constitutional right imposes an informational duty on police that they can discharge with relative ease. It merely requires a police officer on detaining a person to tell them in “clear and simple language” the reason(s) why: Mann, at para. 21; R. v. Evans,  1 S.C.R. 869, at p. 888.
This obligation applies whether a police officer is detaining a pedestrian or a motorist: Orbanski & Elias, at para. 31. Compliance with s. 10(a) assumes added significance when police detain a motorist because the right to counsel does not apply during motor vehicle stops motivated by traffic safety concerns: Orbanski & Elias, at para. 60. As a result, a detained motorist is often, “wholly reliant on the police to provide him with the information he requires to be able to make informed choices.”: R. v. Mueller, 2018 ONSC 2734, at para. 29. Given this, the obligation on police to inform a motorist of the reason for their detention is especially important during traffic safety stops.
In terms of when the person detained must be told of the reason(s) for their detention, the text of s. 10(a) instructs that this take place “promptly.” The case law makes clear that this means immediately: R. v. Nguyen, 2008 ONCA 49, at paras. 16-22. The only justification for delay is where the police must first gain control over a detainee whose actions are creating a dangerous situation: R. v. Boliver, 2014 NSCA 99, at paras. 15-20.
Driving is a licensed activity that is subject to regulation and control in the interests of public safety. The Supreme Court of Canada has upheld a variety of police powers meant to combat the threat posed by impaired, unlicensed and uninsured drivers, as reasonable limits on the constitutional rights of motorists under s. 1 of the Charter: Orbanski & Elias, at paras. 54-60; Hufsky, at pp. 636-637; Ladouceur, at pp. 1279-1288; R. v. Thomsen,  1 S.C.R. 640, at pp. 653-656.
Nevertheless, a person who is lawfully entitled to operate a motor vehicle has a reasonable expectation of privacy in their vehicle, albeit of a diminished nature as compared to a dwelling or a private office. Given this, under s. 8 of the Charter, they enjoy the right to be secure against unreasonable search or seizure in their vehicle: see R. v. Belnavis,  3 S.C.R. 341, at para. 19; R. v. Grant,  3 S.C.R. 223, at p. 242; R. v. Mooiman and Zahar, 2016 SKCA 43, at para. 39. In assessing what is constitutionality permissible during the detention of a motorist, much depends on a police officer’s purpose and actions.
If traffic safety is amongst a police officer’s purposes for detaining a motorist, the officer can take a variety of steps without engaging the driver’s s. 8 Charter right. For example, a police officer may visually inspect the passenger compartment of the vehicle, including with the aid of a flashlight, require the driver to produce their license, car registration and proof of insurance, and also inspect the vehicle to assess its mechanical fitness. Such measures do not encroach upon a motorist’s reasonable expectation of privacy during a lawful traffic stop: Hufsky, at p. 638; Belnavis, at para. 28; R. v. Mellenthin,  3 S.C.R. 615 at pp. 623-625. These steps do not engage s. 8 even when a police officer also harbours an ulterior criminal investigative purpose for the detention, provided the officer’s traffic safety motivation for the detention persists, and they do not act in excess of the limits on their associated powers: Nolet, at paras. 32-41.
If during the lawful detention of a motorist for traffic safety purposes a police officer happens to observe an item that is immediately recognizable as evidence of a crime or illicit contraband, the plain view doctrine provides the officer with authority to seize the item: Criminal Code, R.S.C., 1985, c. C-46, s. 489(1); Controlled Drugs and Substances Act, S.C. 1996, c. 19, s. 11(8); R. v. Buhay, 2003 SCC 30,  1 S.C.R. 631, at para. 37; R. v. Law, 2002 SCC 10,  1 S.C.R. 227, at para. 27. Alternatively, if while operating within the parameters of their traffic safety powers, a police officer acquires the grounds necessary to undertake more invasive investigative steps, then they are entitled to act on such grounds: Nolet, at para. 28.
Once a police officer’s interest in traffic safety ends, however, any intrusion on a motorist’s reasonable expectations of privacy must comply with s. 8 of the Charter. Ordinarily, this means that the police officer will require reasonable grounds to search the vehicle for evidence of a crime: Mellenthin, at pp. 624-625; Nolet, at paras. 28, 39, 43. Alternatively, if the motorist is lawfully arrested, the vehicle may be searched for evidence or weapons incidental to that arrest, provided the vehicle is connected to the reason for arrest and there is a reasonable prospect that evidence will be located in the vehicle: see, generally, R. v. Caslake,  1 S.C.R. 51. Short of this, a protective weapons search could potentially be justified, but only if the police officer believes on reasonable grounds that his or her safety is at stake and that such a search is necessary: R. v. MacDonald, 2014 SCC 3,  1 S.C.R. 37, at para. 41. Depending on the circumstances, this may justify searching a motorist’s vehicle for weapons: see, e.g., R. v. Plummer, 2011 ONCA 350, at para. 65; R. v. Lee 2017 ONCA 654 at para. 43.
Entirely different considerations apply if traffic safety does not truly factor into the officer’s decision to detain and is merely offered as a pretext (a false justification) to detain a motorist and look for evidence of a crime. Without any traffic safety justification, not only will the pretextual detention result in a violation of the motorist’s s. 9 Charter right from the outset, any resulting intrusion on the motorist’s reasonable privacy expectations will also be unlawful and violate s. 8 of the Charter: see, e.g., R. v. Ladouceur, 2002 SKCA 73, described as “fatally flawed from the outset” in Nolet, at para. 25. Where traffic safety is not the reason to conduct the stop, the officer must have at least articulable cause to stop the vehicle, meaning… “A constellation of objectively discernible facts which give the detaining officer reasonable cause to suspect that the detainee is criminally implicated in the activity under investigation”: Simpson. The threshold is lower than the threshold for an arrest, which is reasonable grounds, but it is something more than an officer’s hunch based on intuition gained by experience.