Urgency-Based color coding to support visual search in displays for supervisory control of multiple unmanned aircraft systems

This study derives and validates a color-coding concept for system states in icon-based displays for supervisory control of multiple unmanned aircraft systems. In aviation, safe indications are typically coded with green, caution indications with amber and warnings with red. This study proposes to adjust the salience of indications according to their intended urgency. For this purpose, a numerical measure for target salience was developed based on findings from the visual search literature. The measure uses differences in color space between target, distractor and background colors to estimate target salience. The measure is computed by dividing the color difference between target and distractor by the color difference between distractor and background and multiplying the result by the division of the target-distractor and distractor-background color differences. The resulting target salience value increases when decreasing the difference between distractor and background color as well as increasing differences between target and distractor and target and background colors. The results from applying the target salience measure suggest to code safe indications with grey instead of green, caution indications with yellow and warning and advisory indications with red and cyan, respectively. Two visual search experiments were conducted to validate the target salience measure and the proposed color-coding concept. Experiment one (n = 32) was a within-subject design comparing reaction time differences between yellow and amber targets among green and grey distractors for four set size conditions. Experiment two (n = 62) was a mixed-subject design comparing reaction time differences between red, yellow and cyan targets among grey and green distractors for four set size and 18 eccentricity conditions. The results show that target saliences diminish with increasing distractor salience, which could particularly be observed for yellow targets among green distractors. Red and yellow targets among less salient, grey distractors allow for a fast, parallel search. Further, the results show that cyan targets among grey distractors facilitate parallel search, although the reaction times are longer than for red and yellow targets. The findings support the proposed numerical measure for target salience and favor the color-coding concept thus suggesting to use grey for safe, and yellow and red for safety critical indications. Cyan is suited for coding advisory indications that are less critical. The results are transferable to the design of icon-based displays in similar domains, such as highly automated ground and maritime transport systems.