Cannabis Lane-Weaving Measurements and Crash Risk: What Alcohol Calibration Can Establish
Lane weaving is a sensitive experimental measure of THC-related driving effects, but it is not itself a count of crashes. A recent driving review explains that the standard deviation of lateral position, or SDLP, has often been interpreted through alcohol calibration rather than direct validation against crash risk.
What SDLP actually records
SDLP describes variability in a vehicle's lateral position during a driving task. Metrik and colleagues reviewed controlled human studies in which THC impaired this aspect of lane control. That is functional evidence under the tested conditions. It differs from merely detecting THC in blood, and it also differs from measuring whether a real-world collision occurred.
Alcohol calibration provides an indirect comparison
The review describes experiments that relate changes in SDLP to changes observed after alcohol. Because alcohol exposure has a separate evidence base connecting it with crash risk, that calibration can help interpret the magnitude of a performance decrement. The review explicitly notes that SDLP has not been directly validated against crash risk. The calibration therefore contains an inferential step that should remain visible.
Comparable lane weaving does not make every effect equivalent
Two exposures producing similar SDLP changes need not have identical effects on every cognitive task, driving maneuver or crash circumstance. Nor does a study-level calibration generate a universal blood-THC equivalent to a particular blood-alcohol concentration. The review discusses variation in route, dose, timing and user history. Those conditions matter when applying a laboratory comparison outside the experiment.
Interpreting the evidence
When interpreting an SDLP finding, describe the measured lane-control effect, the comparator and the calibration method. That supports a more precise opinion than equating centimeters of lane weaving with a directly measured crash probability or an individual legal impairment threshold.
Related reading: validation claims in cannabis driving research.
Research reference
Jane Metrik; Nicholas Bush; Rachel L. Gunn; Denis M. McCarthy (2026). Recent Advances in the Science of Cannabis-Impaired Driving. Current Addiction Reports. doi:10.1007/s40429-025-00712-0. Evidence basis: review/commentary abstract and selected explanatory passages.
