The statue at issues is Wis. Stat. s. 346.63(1)(am). The substance most often detected is Delta 9 THC, the active component of THC. Other states, including Minnesota, exclude THC from the list of restricted substances.
According to the Court of Appeals, the statute does not violate due process even though it does not require the driver to (1) actually be impaired; or (2) knowingly or intentionally ingest THC/restricted substances. Statutes that require "knowledge" or "intent" are said to have a "scienter" element. Those that do not, like section 346.63(1)(am) are called "strict liability" offenses. The Court of Appeals decision can be summarized as follows, "[B]ecause no reliable measure of impairment exists for many illicit drugs, the legislature reasonably could conclude that the more prudent course was to ban any amount in the driver’s system."
In a previous blog, I gave my reasons why section 346.63(1)(am) leads to police abuse. It is my sincere hope that the Supreme Court will see things differently than the Court of Appeals. However, I am not holding my breath.