The governor of Indiana signed a bill into law that amends the state’s zero tolerance per se approach to drivers found with even trace amounts of THC in their system while operating a motor vehicle.

Under Indiana’s zero tolerance per se DUI laws, it’s a misdemeanor to drive with any detectable amount of THC or its metabolites in the driver’s blood or urine. Doing so can result in stiff penalties, even if there’s no indication the driver was intoxicated at the time the sample was taken.

The bill signed into law by Gov. Eric Holcomb – SB 201 – is sponsored by Rep. Jim Lucas, Rep. Cindy Ledbetter and Rep. Health Van Natter. It ensures drivers found with THC in their blood or urine can provide an affirmative defense to Indiana’s per se laws on two bases: that the driver was not responsible for a traffic accident and that they showed no signs of being intoxicated.

Following Gov. Eric Holcomb’s approval of the measure, the law change will take effect on July 1, 2021.

Determining impairment due to cannabis is notoriously difficult compared with alcohol, as THC and its metabolites linger in a person’s system long after it’s been consumed. NORML, a marijuana reform advocacy organization, opposes THC per se limits on the grounds that “such thresholds are not evidence-based and that they may lead to the criminal prosecution of people who consumed cannabis several days previously but are no longer under its influence.”

The difficulty in determining a baseline standard of THC intoxication is why states have taken a piecemeal approach toward tackling cannabis DUIs, unlike the blood limit set federally for alcohol. Fifteen states in addition to Indiana have per se laws in place for marijuana, of which ten are zero tolerance with the other six states using various per se THC limits greater than zero. The few remaining states require prosecutors to establish the driver was actually impaired at the time of the encounter with law enforcement.

A 2019 congressional research report noted that studies to determine driving impairment owing to cannabis consumption have so far proved inconclusive. It said questions remains open as to how exactly THC affects driving skills and what level of THC could be considered safe for operating a motor vehicle.

The National Highway Traffic Safety Administration voiced similar concerns in a 2017 report to Congress about the use of per se laws to determine THC impairment. It noted that the ““poor correlation of THC level in the blood or oral fluid with impairment precludes using THC blood or oral fluid levels as an indicator of driver impairment.”

While SB 201 enjoyed success in passing through Indiana’s legislature, various other marijuana-related measures failed to make progress this session. This includes a bill to decriminalize possession of up to two ounces of cannabis, one that would establish a five year medical marijuana pilot program and another that would legalize cannabis for serious medical conditions upon a physician’s recommendation.

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