# Individual Differences in Signal Perception for Takeover Request in Autonomous Driving
## 2 Related Works
### Warning Signal Design in Autonomous Vehicle  
Warning signals can be implemented in a variety of ways within the context of autonomous vehicles in order to convey vital information to the driver or user. During manual driving, warning signals typically signify problems with vehicle systems or interactions with other vehicles or the surrounding environment. A collision warning may be activated, for instance, when the vehicle approaches the vehicle in front of it, when it deviates from its lane, or when a rear-view camera is used for parking. At Level 3 and higher, when the vehicle is partially capable of performing the entire driving task, alerts can be provided continuously to keep the driver notified of the driving situation. These alerts also prompt the driver to regain control when the system loses control of the vehicle. In a scenario of fully autonomous driving, where the vehicle handles the entire driving task and the human assumes the role of a service user, notifications can be used to indicate critical situations or to provide content-related alerts during service interactions. In-vehicle alerts are typically categorized as visual alerts displayed on instrument panels or screens, auditory alerts delivered via in-vehicle speaker devices, and tactile alerts conveyed via vibration pads affixed to the steering wheel or seat. Individuals may receive these notifications via a single stimulus or via a combination of multiple modalities, known as multimodal alerts. Each modality of autonomous vehicle warning signals has its own advantages and disadvantages. Visual alerts can convey information through words or icons, displaying the alert's content continuously [2]. Contrary to auditory and tactile alerts, visual alerts can manipulate salience through the use of hue, luminance, and size [1]. Auditory signalshave the advantage of capturing immediate attention upon presentation. According to the multiple resource theory [3], auditory stimuli can be perceived with minimal interference to the manual driving task. Experimental research has shown that auditory stimuli elicit faster responses compared to visual stimuli, enabling quicker reactions to hazards [4]. Moreover, auditory alerts can provide better location information compared to tactile alerts [5; 6]. In driving situations, visual and auditory tasks are frequently secondary tasks, making tactile sensations less susceptible to distracting drivers [7]. Unlike visual stimuli, tactile alerts can be physically stimulated, allowing for quicker attention switching [8]. According to previous studies, tactile stimuli are effective for forward collision warnings (FCWs) [9; 10], and varying the intensity can convey meaningful degrees [11]. However, the dynamic characteristics of driving can influence the effectiveness of warnings. Perception can be difficult if a stimulus requires the use of the same sensory organs for distinct tasks. A visually prominent alert on the in-vehicle human-machine interface (HMI) may go unnoticed if the driver is focused on their phone or the road ahead. Similarly, it can be challenging to perceive an auditory alert while engaged in a phone conversation. When the road surface is unstable or when the vehicle's vibrations are forceful, tactile alerts can be ignored. In addition, auditory alerts are dependent on time, so if one misses them, they must be repeated. If the working memory is engaged in a secondary task when a meaningful auditory signal, such as a word or sentence, is presented, the meaning of the auditory signal may not be comprehended due to limited cognitive resources [12]. Tactile notifications can provide stimulation, but it is more difficult to convey their meaning and orientation compared to auditory alerts [6]. Numerous studies have demonstrated the effectiveness of multimodal warnings in compensating for deficiencies and providing redundant information. Several studies have demonstrated that multimodal alerts decrease false positives [13; 14].