ISO 26022: Road Vehicle HMI — Occlusion Testing and Distraction Assessment

ISO 26022 specifies test methods and performance requirements for human-machine interface (HMI) systems in road vehicles, focusing on the evaluation of visual behaviour and task demands while driving. Published in 2010, this standard provides a standardized methodology for assessing the visual and cognitive workload imposed by in-vehicle information and communication systems. This article examines the test protocols, performance criteria, and engineering design implications of ISO 26022.

The Occlusion Testing Methodology

The central methodology defined in ISO 26022 is the occlusion testing technique, which simulates the intermittent visual sampling behaviour of drivers. In this method, test participants interact with the HMI system while wearing occlusion goggles that alternate between transparent and opaque states. The standard specifies a 1.5-second open interval followed by a 1.5-second closed interval, modelling the typical pattern of a driver glancing at an in-vehicle display and then back at the road. The total task time under occlusion conditions is compared to the task time under full vision conditions to calculate the occlusion ratio.

The occlusion ratio, defined as the time taken to complete a task under occluded conditions divided by the time taken under full vision conditions, serves as the primary performance metric. ISO 26022 specifies that the occlusion ratio should not exceed 1.5 for tasks acceptable for performance while driving. This means a task that takes 10 seconds with full vision should be completable within 15 seconds under the intermittent viewing conditions created by the occlusion goggles. Tasks exceeding this threshold are considered to impose excessive visual demand for safe concurrent driving.

Static and Dynamic Test Conditions

ISO 26022 defines two categories of test conditions for HMI evaluation. Static testing is conducted in a laboratory or stationary vehicle, using the occlusion methodology described above. This approach provides a controlled, repeatable environment for comparing different HMI designs and configurations. Static testing is typically used for early-stage design validation and comparative benchmarking of interface alternatives.

Dynamic testing involves real or simulated driving conditions, incorporating a secondary driving task that the participant performs concurrently with the HMI interaction. The standard recommends using a lane-change test (LCT) or a car-following task as the primary driving task. Performance degradation on the driving task while interacting with the HMI provides a direct measure of the distraction potential. The dynamic testing protocol captures both visual distraction (eyes-off-road time) and cognitive distraction (mental workload that degrades driving performance even when eyes are on the road).

Engineering Insights for Distraction-Minimized HMI Design

ISO 26022 provides quantitative targets that directly inform HMI design decisions. The 1.5 occlusion ratio limit translates to concrete design guidelines: each HMI interaction should be completable within a maximum of five occlusion cycles (15 seconds total), and individual glance durations should not exceed 2 seconds. This drives design toward simple, hierarchical menu structures with no more than three levels of depth, large touch targets (minimum 20 mm), and high contrast ratios (at least 7:1 for critical information).

Voice interfaces present unique challenges under ISO 26022 evaluation. While voice interaction avoids visual demand, the cognitive load imposed by speech recognition errors and dialogue management can significantly degrade driving performance. Engineers should implement voice interfaces with short, unambiguous command structures and provide clear visual feedback of voice recognition results. Haptic feedback on touchscreens has been shown to reduce occlusion ratios by 12-18% compared to non-haptic screens by reducing the need for visual confirmation of successful inputs.