the admissibility of the evidence of DREs about drug impairment.
The applicable principles are those governing the admissibility of
expert opinion evidence. This familiar analysis is divided into two
( i) the threshold requirement; and
( ii) the cost benefit analysis.
See White Burgess Langille Inman v. Abbott and Haliburton Co.,
 2 S.C.R. 182,  2 S.C.R. 182, 2015 SCC 23.
 At the first step, the evidence must satisfy the four
requirements of R. v. Mohan,  2 S.C.R. 9,  S.C.J. No.
36, among them that the witness have expertise in the subject
matter of his or her evidence that extends beyond the experience
and knowledge of the trier of fact about that subject: R. v. Bingley,
 1 S.C.R. 170,  S.C.J. No. 12, 2017 SCC 12, at paras.
14, 15 and 19; Mohan, at p. 25 S.C.R.
 Parliament has established that a DRE has special expertise outside the experience and knowledge of the trier of fact. Said
in another way, the DRE is an expert for the purpose of applying
the 12-step evaluation and determining whether that evaluation
indicates drug impairment for the purposes of s. 254(3.1). Parliament has conclusively and irrebuttably established the DRE’s
expertise: Bingley, at para. 27.
 The statutory framework Parliament has enacted does not
undermine the trial judge’s gatekeeper function which enables
the trial judge to safeguard the trial process and to ensure that it
is not disturbed by improper expert opinion evidence. Focusing
the analysis on the DRE’s administration of the evaluation may
persuade the trial judge that the prejudicial effect of the evidence
outweighs its probative value and warrants its exclusion: Bingley,
at para. 30.
 Parliament has established the reliability of the 12-step
drug evaluation test. But it has not determined or said that the
conclusion drawn by the DRE is dispositive of guilt. The DRE
determines whether the evaluation indicates drug impairment.
This opinion is neither more nor less than a piece of evidence for
the trier of fact to consider in deciding whether the Crown has
proven that the ability of the person charged to operate a conveyance was impaired by a drug. No presumption of guilt follows
from the conclusion expressed by the DRE: Bingley, at para. 31.
 The statutory recognition of the reliability of the 12-step
drug evaluation erects no barrier to the authority of the trier of
fact to critically assess a DRE’s conclusion about impairment, or
to the right of an accused to challenge that evidence. Cross-examination may undermine the DRE’s conclusion. Unearth a bias.