Diverging drivers’ visual attention and search behavior in closely spaced tunnel-interchange structures: a field study

OBJECTIVE: Closely spaced tunnel-interchange (CSTI) structures are prevalent in mountain freeways, resulting from constraints imposed by topography, geological conditions, and cost considerations. The study aims to explore the visual attention and search behavior of drivers executing diverging maneuvers immediately after exiting tunnels. METHOD: An on-road experiment was conducted to collect eye movement data at four CSTI structures with varying net distances and at one conventional interchange. K-means++ and DBSCAN (Density-Based Spatial Clustering of Applications with Noise) were utilized to segment drivers' areas of interest (AOIs). The proportion of fixation time allocated to each AOI was used to identify primary AOIs, while fixation duration and frequency were used to assess visual attention engagement. Saccade duration, saccade frequency, fixation frequency, one-step transition probability matrix, stationary gaze entropy, and gaze transition entropy were used to analyze visual search behavior. RESULTS: The near-forward, far-forward, and overhead roadway areas constituted the primary AOIs within the CSTI structures, upon which visual search of diverging drivers was predominantly concentrated. Compared with drivers in conventional interchanges, those in the CSTI structures exhibited faster but less efficient visual search behavior. As the net distance decreased, drivers' visual attention engagement declined, while visual search pace increased. CONCLUSIONS: This study reveals how diverging drivers navigate the challenges of CSTI structures by adjusting their visual attention engagement, visual attention allocation, and visual search strategies. The findings can offer a reference for designing and placing guide signs in CSTI structures.