But a major victory for proponents of the legalisation has been accompanied by a new provision affecting drivers who use marijuana, including people who have long had legal access to the drug for medicinal purposes. In the wake of the drug's legalisation, Colorado has, for the first time, set a limit on the amount of THC – the active ingredient in marijuana – acceptable in a driver's blood.

Until now, Colorado drivers could only be convicted of driving under the influence of marijuana if prosecutors could prove that the THC in their system was recently taken and was impairing their faculties. The new law quantifies that: anyone with more than 5 nanograms of THC per millilitre of blood is deemed under the influence.