Users of Level 4-5 connected autonomous vehicles (CAVs) should not need to intervene with the dynamic driving task or monitor the driving environment, as the system will handle all driving functions. CAV humanmachine interface (HMI) dashboards for such CAVs should therefore offer features to support user situation awareness (SA) and provide additional functionality that would not be practical within non-autonomous vehicles. Though, the exact features and functions, as well as their usability, might differ depending on factors such as user needs and context of use. The current paper presents findings from a simulator trial conducted to test the usability of a prototype CAV HMI designed for older adults and/or individuals with sensory and/or physical impairments: populations that will benefit enormously from the mobility afforded by CAVs. The HMI was developed to suit needs and requirements of this demographic based upon an extensive review of HMI and HCI principles focused on accessibility, usability and functionality [1,2], as well as studies with target users. Thirty-one 50-88 year-olds (M 67.52, three 50-59) participated in the study. They experienced four seven-minute simulated journeys, involving inner and outer urban settings with mixed speed-limits and were encouraged to explore the HMI during journeys and interact with features, including a real-time map display, vehicle status, emergency stop, and arrival time. Measures were taken pre-, during-and post-journeys. Key was the System Usability Scale [3] and measures of SA, task load, and trust in computers and automation. As predicted, SA decreased with journey experience and although cognitive load did not, there were consistent negative correlations. System usability was also related to trust in technology but not trust in automation or attitudes towards computers. Overall, the findings are important for those designing, developing and testing CAV HMIs for older adults and individuals with sensory and/or physical impairments.
IntroductionWe are moving at a rapid pace towards fully autonomous vehicles that handle all driving related functions without the need for user intervention. Level 5 fully connected autonomous vehicles (CAVs) are defined by SAE International [4] as systems that can handle 'the full-time performance by an automated driving system of all aspects of the dynamic driving task under all roadway and environmental conditions that can be managed by a human driver', whereas Level 4 highly automated vehicles are similar but only have such system capabilities for some driving modes and may require human intervention for some non-driving related actions. Both levels assume that the user is not an active driver and can therefore have eyes and mind off the road. This means they will not require driving input devices such as a steering wheel and pedals. Instead, user interaction with Level 4 and 5 CAVs will be through human-machine interface (HMI) dashboards providing vehicle-related (e.g., speed, time/distance to destination, local area information)...