The growing trend of combining cannabis edibles with alcohol creates a public safety blind spot that controlled research has barely illuminated. This behavioral pattern affects millions of adults, yet evidence-based guidance for impairment thresholds and risk assessment has remained largely absent from both clinical practice and policy discussions. A rigorous crossover trial involving 25 healthy adults examined how THC edibles at 10mg and 25mg doses interact with alcohol at legal driving limits when consumed together. Participants completed comprehensive driving simulations, standardized field sobriety tests, and cognitive assessments across seven controlled sessions separated by washout periods. The study design eliminated confounding variables that plague observational research on substance co-use. Results demonstrated that combining even modest THC doses with blood alcohol concentrations of 0.05% and 0.08% produced measurably worse driving performance than either substance alone. The global drive score, which aggregates multiple driving metrics, showed significant deterioration with co-use compared to individual substance effects. Lane positioning variability increased substantially, indicating reduced vehicle control precision. Field sobriety test failures accumulated more rapidly with combination use, suggesting current roadside testing may underestimate impairment levels when both substances are present. This represents the first controlled evidence quantifying how cannabis edibles compound alcohol's driving impairment effects rather than simply adding to them. The findings challenge assumptions about predictable dose-response relationships and highlight gaps in current impairment detection methods. For the millions of adults who consume both substances, this research provides critical safety data that could inform more nuanced harm reduction strategies.