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Sure. The blood test looks for a blood cannibinoid level greater than 2ug per liter.
The panel of advisors that the government used to help establish their new drug driving policy recommended 5ug as the lower limit as they felt that would catch the majority of drivers who were indeed intoxicated. The government ignored them. The new roadside tests, from a medical perspective, are likely to show positive 24 hours after using cannabis for a significant proportion of people being tested.
Stonehedge
Roadside tests aren't sufficient evidence for a ban, are they?
A positive will lead to a blood test, which has far better sensitivity & specificity.
Not trying to be pedantic, but the police have powers to stop drivers for any (or no) reason in the UK.