Driver Situation Awareness and Cognitive Workload Effects of Novel Interchange Configurations and Associated Signage
Authors: Liu Yunmei , Pyo Kihyun , Cunningham Christopher , Chase Thomas , Kaber David 
Published: June 2022

In recent years, there has been a push towards use of grade-separated interchange (GSI) design to increase the overall capacity of intersections. The primary recommendation has been to resolve physical intersection constraints, including signalized left turns (in the U.S.). However, few, if any, investigations have made comparisons of driver situation awareness (SA) and cognitive workload in navigating novel grade-separated configurations and how to effectively implement associated signage to promote driver and traffic safety at different types of interchanges. To address this research gap, this study designed and conducted a driving simulation experiment to compare driver SA and cognitive workload in negotiating standard GSIs vs. novel GSI conditions, including contra-flow and quadrant configurations. All GSIs accommodated cross-traffic flows (north, south, east, and west) with four-lane roadways running in each direction through urban environments. The experiment also manipulated driver exposure to lane assignment (LA) signs (present and absent) and decision point (DP) signs with either overhead or right-side mount configurations. Forty-eight (48) licensed drivers participated in the experiment with each driver experiencing each GSI configuration in two trials for a total of six experiment trials for each participant (total of 288 trials). Participants in the experiment were divided into two groups according to age, including young (18-24 yrs.) and middle-aged (25-64 yrs.). The participants were also assigned to unique combinations of LA and DP signs (LA present + DP overhead; LA present + DP right-side mounted; LA absent + DP overhead; LA absent + DP right-side mounted), which remained consistent across GSI configurations for each driver. A high-fidelity and full-motion driving simulator was used in this study. During each trial, a driver was required to maintain posted speed limits and to achieve a pre-identified destination (“Garden St. North), as posted on the LA and DP signs. At specific stopping points in each test trial, driver SA was assessed using the Situation Awareness Global Assessment Technique (SAGAT). The simulation scenario was frozen and drivers were posed with multiple queries addressing perception, comprehension, and projection of roadway conditions, vehicle and traffic states, and routes. Qualtrics survey software was used to present questions in an electronic format (using driver mobile devices) with all being randomly selected from a large pool of questions on the driving environment. Driver responses to queries were graded based on recordings of ground-truth simulator settings. That SAGAT output as a percentage of correct responses to all queries delivered at a simulation freeze with range [0,1]. Driver cognitive workload was assessed using the NASA Task Load index. The purpose of using this index was to determine the cognitive load imposed on drivers by the signage conditions in negotiating the various types of GSIs. At the beginning of the experiment, participants ranked the importance of six workload demand components, including mental, physical, temporal, performance, effort, and frustration for the driving task. At the end of each test trial, participants rated their perceived mental workload, according to the various demand components on a 100-point scale. The NASA TLX was calculated as the rank-weighted sum of the demand ratings scaled from 0 to 100 points. The results revealed driver SA and workload to significantly differ among GSIs. The standard and contra-flow GSIs were not different in driver SA but both were superior to the quadrant configuration. There were no significant differences in SA detected for the use (LA) and placement (DP) of signs. Regarding cognitive workload, results corresponded with SA findings, indicating the standard and contra-flow GSIs produced lower cognitive demands for drivers than the quadrant configuration. However, there were no significant differences in cognitive workload detected between the use and placement of signs. No interactions were detected among the GSI configurations and use and placement of signs for both SA and cognitive workload. In addition, correlation analyses were also applied to the SA and workload responses. Results indicated that SA and workload were complimentary in the context of the present experiment and they represent unique methods for assessing human behavior/performance in driving research.On the basis of these results, it was concluded that novel GSI designs influence driver SA and workload responses compared with standard interchanges; however, the presence of LA signs and positioning of DP signs does not appear to positively influence these responses. There is a need for additional empirical driving research to determine what aspects of GSI geometry and other traffic control devices may serve to promote comparable levels of driver SA and workload for new designs as compared to standard interchanges.