THC likely reduces speed and reaction time in young drivers, but high-quality evidence is limited | Research

The takeaway

A systematic review found moderate confidence that THC reduces mean speed, headway distance, and reaction time while increasing lane and steering variability in young drivers aged 15-24.

Traffic safety researchers, driving educators, policymakers setting impaired driving standards, and parents of young drivers.

Moderate confidence that THC impairs multiple driving performance measures in youth

What the researchers found

Class II evidence suggests THC is likely to reduce mean speed, headway distance, and reaction time, and increase lane and steering wheel position variability in young drivers. The overall level of confidence was moderate (Level B). The review found a lack of Class I studies and insufficient attention to sex/gender differences and representative samples.

Why it matters

Young drivers already face the highest crash risk. Understanding the specific ways cannabis impairs their driving can inform targeted education and prevention rather than relying on generalizations from adult studies.

The numbers in context

Systematic review of studies on drivers ages 15-24. Class II evidence (Level B, moderate confidence). THC effects: reduced mean speed, reduced headway distance, reduced reaction time, increased lane variability, increased steering variability.

How the study worked

Systematic review registered in PROSPERO, searching 7 databases. Quality of evidence assessed using an established classification system (Class I-IV for study quality, Level A-U for confidence). Focused specifically on drivers ages 15-24.

What this study cannot tell us

Limited number of high-quality studies focused specifically on youth. Most driving performance studies use simulator environments that may not reflect real-world conditions. Sex and gender effects are understudied. Dose-response relationships unclear.

How to read the evidence

Systematic review with structured evidence grading, but limited by the small number of high-quality youth-specific studies available.

When this study was published

2021 systematic review. Identifies significant evidence gaps in understanding cannabis effects on young driver performance.

The bigger picture

As cannabis legalization expands, young drivers may face increased exposure. The evidence gaps identified here, particularly regarding sex differences and realistic dosing, need to be addressed to develop effective impairment standards.

Questions still open

How do cannabis effects on young driver performance compare to alcohol effects? Does the duration of impairment differ in younger versus older drivers? Would roadside testing protocols need to be adjusted for the specific impairment patterns seen in youth?

Common questions

How does cannabis affect young drivers specifically?

THC likely reduces mean speed, headway distance, and reaction time while increasing lane and steering variability. However, the confidence level is moderate because few high-quality studies focus specifically on drivers ages 15-24.

Is driving on cannabis as dangerous as driving drunk?

This review did not directly compare cannabis to alcohol impairment. It focused on characterizing the specific ways THC affects young driver performance, which involves different impairment patterns than alcohol.

Read the original research

Young and under the influence: A systematic literature review of the impact of cannabis on the driving performance of youth.

Accident; analysis and prevention, 151, 105961

View on PubMed ↗Publisher’s paper ↗

Citation

Alvarez, Liliana; Colonna, Robert; Kim, Sean; Chen, Caron; Chippure, Katherine; Grewal, Jasleen; Kimm, Chris; Randell, Travis; Leung, Victoria. (2021). Young and under the influence: A systematic literature review of the impact of cannabis on the driving performance of youth.. Accident; analysis and prevention, 151, 105961. https://doi.org/10.1016/j.aap.2020.105961