DENVER — With the recreational use of marijuana now legal in Colorado, officers who patrol the state’s roads face a new set of challenges. Though smoking or possessing small amounts of cannabis is no longer breaking the law, anyone who drives while impaired is still subject to arrest.

Which raises a knotty question: How many tokes can a driver take before the ability to control a vehicle is compromised to the point of being a danger on the road?

Unlike alcohol, which has an undisputed — and usually quite apparent — influence on driving, there is no clear-cut consensus on the amount of marijuana that must be consumed to impair a driver’s ability.

This year, as Colorado lawmakers worked out regulatory matters, including taxes on sales, they also passed legislation that set legal limits on marijuana levels in the bloodstream. Under the new law, which took effect on May 28, a driver is assumed to be impaired if a blood test shows a level of tetrahydrocannabinol, or THC, that is five or more nanograms per milliliter. A nanogram is a billionth of a gram.