Which is why it's the job of the police to collect as much evidence as reasonably possible, and more importantly, it's the prosecutor's job to explain the the jury why the evidence is sufficient and what expectations are reasonable.
In one case I had, a 24 year old man was stopped for reckless driving. He acted strangely and smelled of alcohol. He was hyperactive with a flushed face, pupils about 12 mm, and pulse of 120. (Cocaine or Meth).
The County Lab expert testified that the man had a gazillion nanograms of Cocaine in his blood, and a .07
Blood Alcohol content.
The Defendant TESTIFIED-under oath- that he snorted Cocaine three times that day. He had about half a thimble full just before he started driving, and he had put that between his lower lip and his gums.
Two old bluehaired ladies hung the jury. They insisted that the prosecution had not shown that the Defendant "was under the influece of alcohol and/or drugs while driving"