Human Factors Aspects of the Transfer of Control from the Driver to the Automated Highway System

Abstract

DTFH-61-92-C-00100The third in a series of experiments exploring human factors issues related to the Automated Highway System (AHS) investigated the transfer of control from the driver of a vehicle entering an automated lane to the AHS. Twenty-four drivers aged between 25 and 34 years drove in the Iowa Driving Simulator--a moving base hexapod platform containing a mid-sized sedan with a 3.35-rad (180 deg) projection screen to the front and a 1.13-rad (60 deg) screen to the rear. The experiment focused on a generic AHS configuration in which the left lane was reserved for automated vehicles, the center and right lanes were reserved for unautomated vehicles, and in which there was no transition lane and no barrier. The driver took the simulator vehicle onto a freeway, moved to the center lane, and then, after receiving an "Enter" command, drove into an automated lane and transferred control to the AHS. Then, the AHS moved the vehicle into the lead position of the string of vehicles approaching it from behind. RESULTS: The entering response time, lane-change time, entering exposure time, and string-joining time data were used to determine the minimum inter-string gap required to enable the driver's vehicle to enter the automated lane without causing a delay to the string it joins. The required minimum inter-string gap varied with the design velocity and the method of transferring control. With the partially automated transfer method, the required minimum inter-string gap time increased from 1.14 s for the 104.7-km/h (65-mi/h) design velocity, through 3.38 s for the 128.8-km/h (80-mi/h) design velocity, to 7.33 s for the 153.0-km/h (95-mi/h) design velocity. The hourly capacity when the design velocity is 104.7 km/h (65 mi/h) is likely to be four times greater than when the design velocity is 153.0 km/h (95 mi/h) (the hourly capacity for the latter would be only slightly more than the traffic flow that could be achieved without an AHS). It is not the design velocity of 104.7 km/h (65 mi/h) per se that produces the higher capacity--it is the relatively low velocity differential between the design velocity and the speed limit in the unautomated lanes. If the transfer of control from the driver to the AHS were to occur before the driver moved into the automated lane, the required minimum inter-string gap times should be reduced--a possibility that is being investigated in the next in the experimental series. No collisions occurred, suggesting that the drivers were able to join the automated lane safely--a suggestion reinforced by the responses to a questionnaire indicating that the drivers felt safe and believed they controlled the vehicle well during the entry maneuver