The title implies cannabis involvement in the crashes, but this is not at all clear, from TFA:
Most cannabis tests do not distinguish between any past use and acute intoxication, and implementing standardized thresholds is challenging due to tolerance from regular use.
"“Our testing methods for cannabis remain suboptimal and individuals can test positive for cannabis weeks after they have consumed it,” says study lead author Marlene Lira, an epidemiologist at BMC."
Most cannabis tests do not distinguish between any past use and acute intoxication, and implementing standardized thresholds is challenging due to tolerance from regular use.