Ten states including Illinois have laws imposing zero tolerance per se thresholds for those that operate a motor vehicle with detectable levels of cannabinoids in the blood. Any driver with detectable amounts of cannabinoids in their system can be charged with DWI/DUI.
Illinois lawmakers are taking another look at a bill that will not only decriminalize the possession of small amounts of marijuana, but will set a standard for what is considered too high to drive. If passed, the bill will eliminate possible criminal penalties for the possession of marijuana (up to ten grams) and will also set a limit at which someone can be prosecuted for drugged driving to 5 nanograms of THC per milliliter of blood.
Source: The Chicago Tribune, “Illinois Lawmakers Give Pot Decriminalization Another Try”, March 26, 2016.