Had a quick look into this, and found this on a website of a (self-proclaimed) "national firm of Expert Motoring Lawyers".
Anyway, according to this, when the government was considering the cocaine limit for driving "it enlisted a panel of advisers ranging from specialists in pharmacokinetics (the science of what the body does to a drug), pharmacology and psychopharmacology, forensic toxicology, misuse of drugs, clinical practice, mental health, addiction science and transport safety."
Ashworth Motoring Law wrote:
After considering all of the options, and the evidence relating to the point at which a user would become at risk of a road traffic accident or impaired driving, the recommended cocaine limit for driving that they came to was 80 micrograms of cocaine per litre of blood and 500 micrograms of benzoylecgonine per litre of blood; much higher than the limits that are in place today. The actual limits are set at only 10mg for cocaine and 50mg for the metabolite.
With particular reference to the fact that the panel of experts recommended a limit of 500mg for benzoylecgonine, the metabolite so as to exclude prosecution for cocaine consumption that occurred several days ago, it will be left in the mind of the reader to determine whether the drug driving limits are unfair. It is also with noting that this trend continues throughout all of the illicit substances listed under the new offence.
So basically the experts recommended a limit of 500 micrograms of benzoylecgonine, which would mean that someone who took cocaine a couple of days previously would be under the limit.
So our man's reading was 520 micrograms, therefore marginally over the recommended limit. But because the threshold was actually set very low (effectively to catch out those who've used cocaine in the last few days, but who the expert group obviously didn't think should be penalised) the headline figure is 'ten times over the limit'.
Which is kind of what I was getting at - it makes it sound like he was off his face while sitting in the street waiting for unsuspecting passengers, or was actually snorting a line while sitting there.
Suspect in fact he hadn't taken cocaine for a couple of days.
And I suspect it wouldn't be that uncommon for members of the public to be driving with those kinds of levels in their body, but the likelihood of them showing any obvious signs of impairment would be remote, and thus very unlikely to be clinically tested for cocaine, even in the unlikely event that the had to take (and failed) a roadside impairment test.
However, in the particular circumstances here (caught in possession, plying for hire, former cab driver) he was blood tested and was obviously over the legal limit.