 tunnels are quite rare \[[@pone.0201732.ref010]--[@pone.0201732.ref012], [@pone.0201732.ref023], [@pone.0201732.ref031]\]. Previous studies focus mostly on: drivers making the decision to starting evacuating \[[@pone.0201732.ref011]\], walking speed in smoke \[[@pone.0201732.ref004], [@pone.0201732.ref012], [@pone.0201732.ref031]\], marking of evacuation exits \[[@pone.0201732.ref011], [@pone.0201732.ref012]\] and single person moving behavior in dense smoke \[[@pone.0201732.ref010], [@pone.0201732.ref012], [@pone.0201732.ref023]\]. In this research we analyze tunnel evacuation in the context of group behaviors and interactions between evacuees. According to our knowledge, this is the first tunnel evacuation experiment reported in a journal, that considers evacuation from a bus including the impact of smoke and grouping behaviors.

We observed the decision making of pedestrians in different densities of artificial smoke. During experiments: 0 (no smoke), 1 (light smoke *Cs* = 0.1-0.2 *m*^−1^) and 2 (moderate smoke *Cs* = 0.4---0.5 *m*^−1^), when visibility was good we observed that only the first group of passengers leaving the bus in the tunnel made a decision about escape direction. While during experiments 3 and 4 with heavy smokiness *Cs* = 0.8---0.9 *m*^−1^ and *Cs* = 1.0---1.1 *m*^−1^, respectively, each small group has to make a decision independently. Formation of such small groups is visible at check-points ([Fig 5a and 5d](#pone.0201732.g005){ref-type="fig"}), additionally participants reported in questionnaires and during informal surveys the feeling of uncertainty or fear, when exposed to increasingly dense smoke.

The grouping behavior of pedestrians is an important issue in our experiments. In heavy smoke, grouping behavior depends on evacuees' density. For low crowd density, participants tend to keep in small groups ([Fig 5b and 5d](#pone.0201732.g005){ref-type="fig"}), while in higher crowd densities (experiments 3 and 4) *river like, double-line patterns*, patterns characteristic for bi-