Driving Distracted after Cannabis Use

Timothy Brown

January 11, 2025

Driving Safety Research Institute

Acknowledgements

Collaborators

• University of Colorado Anschutz Medical Campus
• Ashely Brooks-Russell, Sam Wang, Michael Kosnett
• Emory University
• Julia Wrobel
• University of Iowa
• Gary Gaffney, Gary Milavetz

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Introduction

• Taking eyes and attention off the road increases risk of crashes
• Cannabis impairs driving performance
• Observed impacts vary between individuals
• Cannabis users report different levels of confidence in driving after use based on usage patterns

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• What happens when mixing cannabis use with distracting tasks while driving?

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High Speed Freeway Driving

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Intake

Baseline Simulator Drive

Cannabis Dosing

Post-Dose Simulator Drive 1

Post-Dose Simulator Drive 2

Conditions

• Use Condition
• Non-users (NU)
• Daily Concentrates (DC)
• Occasion Flower (OF)
• Daily Flower (DF)
• Time Points
• Baseline, prior to use
• Post-dose 1 (near peak) – ~60 minutes after dosing
• Post-dose 2 (late) – ~120 minutes after dosing

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• Tasks
• HVAC
• Music Selection
• Text Reading
• Difficulty
• Easy
• Hard

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Prior to Use

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Are there interaction differences prior to use?

• Engagement durations with the tasks differed based on difficulty
• NU, OF & DC - Hard tasks ~1 second longer
• DF – Hard task 2.8 seconds longer
• DF has shortest duration for easy task and longest for hard task.
• Speed reduction also varied based on difficulty

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Inconsistent differences in speed reduction

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Are there visual attention differences prior to use?

• Complex interactive effect for use condition with task type and task difficulty for
• Average percent road center
• Maximum percent road center
• Interactive Effects between use condition and task difficulty for
• Variability in horizontal gaze
• Occasion flower users did not show increased variability in horizontal scanning for harder tasks

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Average Percent Road Center

• Overall Non-users spent less time on roadway as task difficulty increased
• For HVAC task
• Less time on road for cannabis users for more difficult task
• Other tasks have a more muddled interaction

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Are there eye behavior differences prior to use?

• Interactive Effects between use condition and task type for
• Average eye opening
• Occasional users had smaller eye opening across tasks
• Interactive Effects between use condition and task difficulty for
• Blink frequency
• Non-users blinked more
• Daily users showed increased blink frequency with more difficult tasks
• Average eye opening
• Daily users didn’t differ from non-users but occasional flower users had smaller
• Variability in pupil diameter
• Daily users didn’t differ from non-users, but occasion flower users had more variability for the hard tasks only.

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After Use

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Are there interaction differences right after use?

• No differences in time engaged with tasks
• Differences in start of task engagement
• For Baseline and near peak, cannabis users started task sooner than non-users
• For later timepoint, non-users started sooner
• Differences in speed reduction independent of task
• No differences in lane control

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Larger and Longer Speed Reduction by OF

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Continued Larger Speed Reduction by OF

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Are there visual attention differences after use?

• Some inherent effects associated with users of cannabis, but don’t change after use
• Minimum Percent Road Center (17 seconds)
• Daily flower users generally have a greater minimum PRC then non-users and daily concentrate users.
• Variability in horizontal gaze
• Occasion flower users generally have less variability than non-users and daily concentrate users

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Are there eye behavior differences right after use?

• Blink Frequency - Complex interaction but generally more blinks by non-users
• Average Eye Opening – Decreases after use for cannabis users and rebounds for daily users
• Variability in Eye Opening – Generally less for cannabis users. Increasing variability over time for non-users, but no similar pattern for cannabis users.
• Pupil Diameter – Non-users decrease over the visit, but for cannabis users, it increases after use.

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Discussion

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Differences in Approach

• There were some systematic differences between non-users and cannabis users that were evident even absent cannabis use
• How much they slowed down when engaging in distracting tasks
• Daily flower users had shortest engagement durations for easy tasks and longest durations for hard tasks
• Time spent focused on forward roadway decreased for non-users but patterns of cannabis users did not track non-users
• Some eye measures showed difference from non-users even without acute cannabis use (e.g., fewer blinks)

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Effects of Acute Cannabis use

• Total task engagement time didn’t vary, but time to start the task did.
• Task accuracy wasn’t captured, so it is possible tasks were abandoned
• Persistent greater speed reduction during task after cannabis use
• Occasional cannabis users seemed to scan less back and forth between the road after use

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More Work Needed

• Explore abandoned or inaccurate tasks
• Deeper look at eye scanning
• Number of glances to the secondary in-vehicle display
• Total eyes-off-road time
• Additional studies should explore longer tasks
• Phone conversations
• Speech to text messaging

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Driving Safety Research Institute

Driving Safety Research Institute

2401 Oakdale Blvd.

Iowa City, IA 52242

319-335-4673

dsri.uiowa.edu

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