Association between length of upstream tunnels and visual load in connection zones of highway tunnel groups

To investigate drivers' visual load and comfort in the distance between adjacent tunnels (tunnel group connection zones), the maximum transient vibration value (MTVV) of the pupil area is used in this study as the index to analyze the visual load characteristics of the driver throughout the connection zones in highway tunnel groups. Data was collected using field driving experiments during which the pupil area change rate is measured as an additional indicator to evaluate the sufficiency of the length of the connection zones from the perspective of drivers’ visual adaptation. The findings show that the length of the upstream tunnel affects the visual strain of the drivers when they enter the connection zone. The visual load and its association with the length of the upstream tunnel appeared to be in the following descending order: short > extra-long > long > medium tunnel. The visual discomfort level in the short upstream tunnel has shown to be “uncomfortable,” while the level of comfort slightly rises to “fairly uncomfortable,” in the connection zone when the upstream tunnel is extra long and long. Departing from medium upstream tunnel resulted in the highest level of comfort “a little uncomfortable level” in the connection zone. When the upstream tunnels are short and medium in length, the required time for light adaptation is 5 s. The connection zone length threshold which is the minimum length of connection zone in order for two consecutive tunnels not to affect each other in terms of visual load of drivers is calculated to be 713.89 m. The driver's pupil area change during light adaptation when the upstream tunnel is short and medium is in the range of 30–40 %. When upstream tunnel is long and extremely long, the light adaptation time is 8 s and 9 s, respectively, and the respective thresholds for connection zone are 797.22 m and 825 m. The drivers' pupil area change in long and extremely long tunnels during light adaptation is in the range of 38–50 % and 43–50 %, respectively. Findings in this study can be used for the design of connection zones between tunnels in a highway tunnel group.