Human Factors in the Automated Highway System: Transferring Control to the Driver

NHTSA · 1995 · ROSA P / United States. Joint Program Office for Intelligent Transportation Systems

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Summary

This 1995 study investigates human factors related to the transfer of control from an Automated Highway System (AHS) to the driver, a critical issue for the successful implementation of "hands-off, feet-off" highway configurations. The research was motivated by the need to ensure AHS designs are usable and acceptable to the entire driving population, particularly given the increasing number of older drivers. The primary objective was to assess driver performance characteristics during the exit maneuver, where drivers must transition from automated travel to manual control in unautomated lanes. The experiments were conducted using the Iowa Driving Simulator, which provided realistic visual, auditory, and motion cues. Sixty licensed drivers participated, comprising two age groups: 24 drivers aged 65 or older and 36 drivers aged 25–34. The simulated AHS featured a three-lane highway with one automated lane and two unautomated lanes. Drivers traveled in a string of automated vehicles and were instructed to exit the automated lane after receiving a 60-second advisory. The study tested three automated design velocities (65, 80, and 95 mi/h), three inter-vehicle gaps (0.0625, 0.25, and 1.0 seconds), and two levels of traffic density in the unautomated lanes. Results indicated that higher AHS design velocities significantly increased the time required to complete the exit maneuver. Younger drivers took approximately twice as long to exit at 95 mi/h compared to 65 mi/h, whereas older drivers exhibited more stable exit times across velocities, potentially due to a desire to complete the maneuver quickly. Crucially, drivers exiting at higher speeds maintained elevated velocities upon entering the unautomated lanes; at the 95 mi/h condition, average exit speeds were approximately 70 mi/h, exceeding the manual lane speed limit by 15 mi/h. This speed differential contributed to a high rate of unsafe performance, with incursions and collisions ranging from 1.4% to 5.6%, a stark contrast to zero errors in practice trials. Despite these safety concerns, participants reported positive perceptions of the AHS, preferring larger inter-vehicle gaps and expressing willingness to use the system. The findings highlight significant safety risks associated with transferring control from high-speed automated systems to manual driving. The "carry-over effect" of velocity, where drivers fail to decelerate sufficiently upon exiting, leads to excessive speeds in manual lanes and increased collision rates. The study concludes that without a dedicated transition lane to allow for deceleration, speed differentials greater than 10 mi/h can reduce AHS throughput and compromise safety. These results underscore the necessity of incorporating human performance data into AHS design guidelines to mitigate risks associated with control transfer.

Key finding

Drivers exiting the simulated automated highway experienced collision and incursion rates ranging from 1.4 percent (younger-driver collisions) to 5.6 percent (older-driver incursions), despite zero such events in identical practice trials.

Methodology

simulator

Sample size: 60

Provenance

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Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.

Topics

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• automation
• automation surprise
• takeover transitions
• situational awareness
• manual
• rail grade crossings

Information type

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• Empirical Findings: behavioral performance data
• Theoretical Contribution: computational model, conceptual framework