The Driver's Response to an Automated Highway System With Reduced Capability

Bloomfield, John R.; Carroll, S. A.; Papelis, Y. E.; Bartelme, M. J. · 1996 · ROSA P / United States. Federal Highway Administration. Office of Safety and Traffic Operations

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Summary

This study investigates driver performance and safety implications when an Automated Highway System (AHS) experiences reduced capability, specifically focusing on the transition of control from automation to the human driver. The research addresses a critical human factors issue in Intelligent Transportation Systems: how effectively drivers can assume steering and/or speed control when the AHS fails or is impaired, particularly in a generic three-lane configuration where automated vehicles travel in the left lane without dedicated transition lanes or barriers. The motivation stems from concerns that drivers may struggle to maintain lane position during such transitions, potentially leading to collisions or encroachment into adjacent lanes. The experiment was conducted using the Iowa Driving Simulator with 60 participants, evenly split by gender and age groups (25–34 years and 65+ years). The experimental design simulated a scenario where the driver’s vehicle, initially second in a string of automated vehicles, approached a 735-meter radius left-curving segment where AHS functionality was impaired. Drivers were required to take over steering alone, speed control alone, or both, depending on the specific failure mode. The study varied designated AHS velocities (80 mph and 95 mph) and intra-string gaps. Performance metrics included lateral steering drift, steering instability, steering oscillations, and time delays in assuming control. The results demonstrated significant degradation in lane-keeping performance when drivers assumed control. When the AHS controlled steering, the vehicle drifted only 0.16 meters to the left around the curve. In contrast, when drivers controlled steering alone, the vehicle drifted 0.66 meters to the right; when controlling both steering and speed, drift increased to 0.77 meters. This lateral drift was four times greater than under automated control and moved in the opposite direction, causing the vehicle to overshoot the lane center by up to 0.86 meters. Additionally, steering instability and oscillations increased substantially when drivers controlled the vehicle, particularly at higher speeds (95 mph). Older drivers and those controlling both functions exhibited slower vehicle speeds and greater time delays in assuming control compared to younger drivers or those controlling single functions. The findings indicate that while drivers can technically take over control with adequate warning, their performance poses significant safety risks, particularly regarding lane encroachment. The study concludes that if reduced capability scenarios are permitted in operational AHS, drivers must be encouraged to reduce speed and warned of potential overshoot. Crucially, the authors recommend against reducing lane widths below the standard 3.66 meters (12 feet), as narrower lanes proposed for AHS efficiency would likely result in drivers failing to stay within their lanes during manual takeover. These results highlight the need for robust human-machine interface designs and conservative infrastructure standards to accommodate human limitations during automation failures.

Key finding

Drivers drifted laterally 0.66 meters to the right when controlling steering alone and 0.77 meters when controlling both steering and speed, compared to 0.16 meters left drift under automated control.

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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• manual
• automation
• steering pattern
• lane positioning
• automation surprise
• takeover transitions

Information type

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• Empirical Findings: behavioral performance data
• Methodological Resource: tool software
• Theoretical Contribution: computational model