Analysis of drivers’ mental workload in V-shaped undersea tunnels based on heart rate variability

•The impact of uphill and downhill slope on the driver’s mental workload in undersea tunnels was studied.•The effects of different illumination and slopes on drivers’ mental workload in undersea tunnels were analyzed.•A model was constructed to quantify the relationship of the SDNN values of drivers with illumination and slope.•Proposed value intervals are given for the combined design of illumination and slope in undersea tunnels. To explore the effects of illumination and tunnel longitudinal slope on driving safety in V-shaped undersea tunnels, we conducted a study to analyze changes in drivers’ mental workload caused by Illumination and longitudinal slope variations within the tunnel through field data. The typical V-shaped undersea tunnel crossing Jiaozhou Bay in Qingdao, China, was selected. Thirty drivers were recruited to participate in the driving tests during off-peak hours and 120 sets of electrocardiographic (ECG) signals were collected from drivers under natural driving conditions. The standard deviation of the Normal-to-Normal intervals (SDNN) was used and an outlier test was performed using the box plot. The drivers’ SDNN was used to index drivers’ mental workload under different Illumination and longitudinal slope conditions in the V-shaped undersea tunnel. A model was established to quantify the relationships of illumination and longitudinal slope with the SDNN. The results showed a significant negative correlation between the illumination change rate and the SDNN, i.e., the greater the change rate of illumination was, the smaller the SDNN was (and the greater the mental workload was). While the mental workload of drivers varied on tunnel longitudinal slope compared with illumination. With increased steepness of longitudinal slopes, the driver’s SDNN value decreased, but the SDNN of drivers in the downhill section were smaller than SDNN in the uphill section, reflecting a greater mental workload. Finally, a proposed value interval for the combined design of illumination and slope in undersea tunnels is given in the context of drivers’ mental workload variations.