One other note on drugged driving.
We have Certified DREs (Drug Recognition Experts/Examiners). They can look at eye pupil size, pulse rate, blood pressure, tongue and other spook stuff and tell if a person is under the influence of:
a) Narcotics (Heroin, Oxy, Codeine, etc)
b) Central nervous system depressants (Downers)
c) Stimulants (Meth, speed, cocaine)
d) Cannabis (Marijuana, hash)
Their observations provide the requisite Probable Cause to arrest for Drugged Driving, although a blood/urine test is required for confirmation and conviction.