Meady v. Greyhound Canada Transportation Corp., 2015 ONCA 6:
 In considering the admissibility of the expert evidence, the trial judge correctly identified the test in R. v. Mohan,  2 S.C.R. 9, as explained by this court in R. v. Abbey, 2009 ONCA 624, 97 O.R. (3d) 330, leave to appeal to S.C.C. refused,  S.C.C.A. No. 125. The test has four requirements: (a) the evidence must be relevant; (b) it must be necessary to assist the trier of fact; (c) it must not be subject to an exclusionary rule; and (d) the expert must be properly qualified.
 The issue on this appeal is the second requirement – the necessity of the evidence to assist the trier of fact. In Mohan, at p. 23, the Supreme Court explained that necessary evidence must be outside the knowledge and experience of the trier of fact:
This pre-condition is often expressed in terms as to whether the evidence would be helpful to the trier of fact. The word "helpful" is not quite appropriate and sets too low a standard. However, I would not judge necessity by too strict a standard. What is required is that the opinion be necessary in the sense that it provide information "which is likely to be outside the experience and knowledge of a judge or jury": as quoted by Dickson J. in R. v. Abbey, 2 S.C.R. 24]. As stated by Dickson J., the evidence must be necessary to enable the trier of fact to appreciate the matters in issue due to their technical nature. In Kelliher (Village of) v. Smith,  S.C.R. 672, at p. 684, this Court, quoting from Beven on Negligence (4th ed. 1928), at p. 141, stated that in order for expert evidence to be admissible, "[t]he subject-matter of the inquiry must be such that ordinary people are unlikely to form a correct judgment about it, if unassisted by persons with special knowledge".
 The application of the necessity criterion asks whether the trier is able to form a correct judgment about the issue without the assistance of persons with special knowledge.
 The general standard of care of a professional, such as a police officer, is a question of law, but the content of the standard of care in a particular case is a question of fact. As such, the content of the standard will generally require expert evidence: Krawchuk v. Scherbak, 2011 ONCA 352, 106 O.R. (3d) 598, at para. 125, 133, leave to appeal to S.C.C. refused,  S.C.C.A. No. 319.
 This is, however, subject to the exception for “nontechnical matters or those of which an ordinary person may be expected to have knowledge”: Krawchuk, at para. 133, referring to Zink v. Adrian (2005), 2005 BCCA 93, 37 B.C.L.R. (4th) 389, at para. 44.
 The trial judge invoked this exception in refusing to admit the expert evidence. He quoted the British Columbia Court of Appeal in Burbank, above, at para. 79:
Expert evidence would only be necessary (and therefore admissible) to establish the standard of care in a negligence case of this kind if the conduct in question gave rise to considerations beyond common understanding. Evidence that need not invariably be expert evidence might be adduced to prove the training police officers are given, or to explain police practice, or where needed to interpret and explain the application of requirements contained in legislation or policy with which the trier of fact is unlikely to be familiar. These are just examples of the kind of evidence that where needed might go toward assisting the court in establishing the standard of care. But unless the nature of the inquiry into the conduct of a police officer is actually beyond the common understanding or experience of judge or jury, evidence of the standard of care, particularly expert opinion, is not required and should not be admitted. It is not otherwise necessary to adduce evidence that a police officer failed to meet the standard of care of a competent police officer.
 There has been growing recognition of the responsibility of the trial judge to exercise a more robust gatekeeper role in the admission of expert evidence – see: Lisa Dufraimont, “New Challenges for the Gatekeeper: The Evolving Law on Expert Evidence in Criminal Cases” (2012) 58 C.L.Q. 531; and Report of the Inquiry into Pediatric Forensic Pathology in Ontario (Toronto: Queen’s Printer for Ontario, 2008), vol. 3, c. 18: “Role of the Court”. This observation holds true for both civil and criminal contexts. Although much of the discussion has focused on increasing scrutiny of threshold reliability at the gatekeeper stage, it is equally important to ensure the evidence is genuinely necessary.
 There are compelling rationales underlying this approach. Unnecessary expert evidence distracts the trier of fact from the issues at hand, complicates the proceeding, prolongs the trial and increases the cost of litigation. In Masterpiece Inc. v. Alavida Lifestyles Inc., 2011 SCC 27,  2 S.C.R. 387, Rothstein J. stated at para. 77:
If a trial judge concludes that proposed expert evidence is unnecessary or irrelevant or will distract from the issues to be decided, he or she should disallow such evidence from being introduced.
 In a similar vein, Moldaver J.A. (as he was then) stated in Johnson v. Milton (Town), 2008 ONCA 440, 91 O.R. (3d) 190, at para. 48:
Trial judges should do their best to perform the gatekeeper function they have been assigned, if for no other reason than trial economy. Permitting experts to give evidence on matters that are commonplace or for which they have no special skill, knowledge or training wastes both time and resources and adds stress to an already overburdened justice system. It is also legally incorrect.
 As Doherty J.A. observed in 2009 in Abbey, at paras. 93-95, in assessing whether the evidence is necessary to the proper adjudication of the facts, the trial judge conducts a cost-benefit analysis – whether the benefits of admission are sufficiently strong to outweigh the associated costs. This is part of the judge’s gatekeeper function and involves an exercise of discretion. It is an analysis that does not necessarily admit of a “yes” or “no” answer: Abbey, at para. 79.
 In one of his rulings, the trial judge referred to Bryant, Lederman and Fuerst, The Law of Evidence in Canada, 3rd ed. (Markham: LexisNexis, 2009) at pp. 799-800, where the authors observe that there is “no bright line” to determine whether the subject matter of expert evidence falls within the normal experience of a particular trier of fact. The authors quote the judgment of this court in R. v. D.S.F. (1999), 43 O.R. (3d) 609 (C.A.), in which O’Connor J.A. observed, at p. 625 :
There is no exact way to draw the line between what is within the normal experience of a judge or a jury and what is not. The normal experiences of different triers of fact may differ. Over time the subject matters that come within the normal experiences of judges and juries may change. The normal experiences of those in one community may differ from those in other communities. In the end, the court in each case will be required to exercise its best judgment in deciding whether a particular subject matter is or is not within the normal experience of the trier of fact.
 For these reasons, deference is owed to the exercise of the trial judge’s gatekeeper function in excluding unnecessary evidence. The trial judge is best equipped to appreciate the issues in the context of the evidence as it unfolds and to determine the extent to which, if at all, expert evidence is required to assist the trier of fact in the disposition of the issues: R. v. D.D., 2000 SCC 43,  2 S.C.R. 275, at paras. 12-13.
 The tragic events in this case unfolded in a small northern community situated on the highway between Kenora and Thunder Bay and on a bus serving that and similar communities. The key issues involved the standard of care to be observed by police officers and by a bus driver operating in those communities. In my view, the trial judge was uniquely positioned to decide whether he needed expert evidence to determine those standards of care and his rulings attract deference.