## 1 Introduction
### Driving Behavior and Major Design Factor of Ramp Terminals in UUR  
The ramp terminal is a road geometry environment in which driving decisions are made regarding a series of processes including traffic situation recognition such as gap acceptance, speed control, and lane changes in a relatively short distance [19]. In this road environment, drivers are expected to behave differently depending on whether they exit or enter the main line. A vehicle driving on the main line changes to the deceleration lane in diverging sections of off-ramp terminals and consistently decelerates to the ramp's speed limit level. As opposed to this, the vehicle driving on the ramp generally recognizes the vehicle driving on the main line in merging sections of on-ramp terminals, accelerates as close as possible to the main line's speed in the acceleration lane, and changes lanes to enter the main line at the same time. The speed difference between vehicles exiting and entering the main line and those on the main line not only increases the risk of accidents but is also difficult to handle once accidents have occurred [20]. Thus, drivers may be required to be more careful while driving on a ramp terminal than they would be on a main line with normal alignment.  
A growing amount of attention has also been paid to ensuring traffic safety in diverging and merging sections of UURs [21]. The ramp terminals of UUR have a similar environment as those of ground roads in terms of road alignments; however, tunnel walls may make it more difficult for drivers to recognize traffic conditions or change lanes. Unlike ground roads, in merging sections of on-ramp terminals within UURs, drivers on the ramp and those on the main line cannot be identified with each other due to the tunnel wall. In other words, it is a road environment in which a driver's visibility is restricted. As soon as the driver enters the large-section tunnel, mutual recognition is possible between the main line and ramp drivers. In such an environment, if a driver trying to merge on-ramp fails to recognize and react at the right time to vehicles driving on the main line, rapid deceleration or acceleration may occur in the acceleration lane, increasing the risk of an accident.  
Meanwhile, UUR junctions, including ramp terminals, where driver safety must be ensured, as mentioned above, are significant traffic facilities for which appropriate geometric forms and design standards must be applied during the planning and design process. There are, however, very few UUR design guidelines that describe these matters in detail worldwide. According to related research, [22] suggested that the shape of the underground road junction should allow left-side entry or exit as well as right-side in some situations in which the size of the underground space must be minimized, and since drivers cannot guess what form the ramps forming the junction are connected to, acceleration lanes should be designed so that vehicles driving on the ramps can sufficiently identify those driving on the main line. Also, an acceleration lane should be designed at an appropriate length to enable the driver to reach a sufficient speed. In an analysis of traffic behavior, it was revealed that merging speed depends on both ramp geometry and speed-change lane geometry. Lower merging speeds were associated with higher collision rates in acceleration lanes [23]. Additionally, the direction of connection at the merging sections is related to the driver's behavior when changing lanes in the acceleration lane. It is found that for the ground road case where either the leading or the following vehicle is present in the main line during merging movements, a merging starting position of 50% to 60% of the merging section on a right-side ramp corresponds to a safe situation; in contrast, a left-side ramp with a merging starting position of 30% to 40% is an unsafe situation [24]. This means that even in merging sections of on-ramp terminals at UUR junctions, driving behavior may vary depending on the direction of the ramp.