# The Effectiveness of Unimodal and Multimodal Warnings on Drivers' Response Time: A Meta-Analysis
## 1 Introduction
### Central Research Questions  
As the above, the type of sensory modality and the number of applied sensory modalities in a driving warning system are two major factors influencing drivers' response to a warning signal. In addition, whether applying more modalities in a driving warning system actually brings more benefits to drivers remains uncertain. The current study aimed to explore and compare the effectiveness of driving warning systems in a holistic perspective using meta-analysis. To fill these knowledge gaps, the current study asked three layers of research questions to reveal the impact of unimodal and multimodal warnings on driver's response time. Question 1: Does each of the three unimodal warning systems (i.e., visual, auditory, and tactile warning systems) facilitates drivers' response time in a significant level compared to no warning condition? Although most studies proved that equipping a warning system would always bring benefits in improving driving safety, some studies indicated that drivers' response time under an unimodal warning condition was even longer than the no warning condition [7; 39]. It is therefore crucial for us to get a better understanding of the basics before proceeding to further analyses. Question 2: Which unimodal warning improves drivers' response time to the best degree? With the pros and cons of unimodal visual, auditory, and tactile warning systems as mentioned above, it would be valuable to better conclude the modality that can best facilitate drivers' response time. According to [PERSON] [40], the information processing of visual, auditory, and tactile signals occurs in different brain regions. Moreover, [PERSON], [PERSON], and [PERSON] (2007) believed that nonvisual sensory modalities are better for information processing due to the possible occurrences of poor visibility of the visual signal [36]; also, the processing of tactile information has fewer chances of getting interference as compared to auditory and visual information. Hence, it was hypothesized that the tactile might be the best sensory modality for delivering warning signals, where tactile resources are least consumed in a driving scenario and are less likely to be interfered by other factors. Question 3: How robust in the multisensory integration effect (MIE), and what is the relationship between MIE and the number of applied modalities in a driving warning system? As mentioned above, although MIE has been commonly acknowledged when comparing multimodal warnings with unimodal warnings [23; 24; 25; 26], several research findings still suggested contradicting evidences that showed a longer response time in the application of multimodal warnings in comparison to unimodal warnings [28; 41]. Furthermore, with both longer and shorter response time being observed in the trimodal warning condition relative to bimodal condition, it is unclear whether the effectiveness of trimodal warningsexceeds bimodal warnings significantly [8; 29]. Therefore, the relationship between MIE and the number of applied modalities could potentially show a trend as suggested by either the multisensory RSE, where drivers' response time is negatively correlated with the number of modalities applied in a driving warning system; or as conveyed by YDL, where a medium-level (i.e., bimodal) warning would best facilitate drivers' performance (i.e., response time). With the fact that multisensory information can facilitate information processing in human brains, the current study hypothesized that multimodal driving warning system can better facilities drivers' response time in comparison to unimodal warnings, and it is more possible for MIE to show a trend as described by multisensory RSE (i.e., response time decreases as the number of applied modalities increased) (see Figure 2).