# An Analysis of Visibility Requirements and Reaction Times of Near-Field Projections
## 1 Introduction  
Traffic safety in urban environments requires a successful interaction among all present road users (RUs). Approximately 69.3% of all fatalities happen in metropolitan areas in Germany. Moreover, the share of vulnerable RUs (25%), which includes pedestrians and cyclists, emphasizes the need for reducing fatalities in urban areas [1]. With the introduction of automated vehicles (AVs) in the current traffic, new challenges will arise and extend the complexity of existing conflict situations. Various studies have shown possible solutions for how AVs can be integrated into the current traffic and what implementations are necessary to improve the interaction of AVs and other RUs [2, 3, 4, 5, 6, 7, 8, 9]. Although displays mounted on AVs are a possible solution for communication and prevention of accidents, ground projections have advantages to preventing accidents or conflicts with vulnerable RUs by illustrating a projection directly in the addressed RUs field of view [4, 6, 7, 10, 11, 12, 13]. To guarantee successful communication of a projection-based human-machine interface, several studies are investigating aspects such as visibility thresholds, necessary resolution, comprehension of the projected content, and whether the projection should be in the near- or far-field of the vehicle [4, 5, 7, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]. The consideration of comprehensive near-field projections, whether symbol- or text-based, plays a significant role for the recipient of the projection. Additionally, the legibility of the content, which can be measured in reaction times, can influence the visibility and clarity of the projected message [14, 20, 21, 22, 23]. Under these circumstances, it is essential to note that those visual requirements, like visual acuity, are influenced by the complexityof the projected message, such as an increasing visual acuity of Chinese characters with increasing stimulus complexity [24; 25]. However, current studies focus on simple pattern-based projection [7; 10; 11; 12; 13; 16; 18], except for [PERSON] and [PERSON]'s [19] study which considers complex patterns such as text-based projections. The testing focuses on the resolution requirements under several ambient luminance levels in the mesopic vision with a constant contrast ratio to not influence the visual acuity perception [19]. Namely, [PERSON]'s [13] study investigated the influence of a vehicle's projected turn indicator signal in the near-field of a car in hazardous situations. Exemplarily, [PERSON] [12] defines the near-field area as a projection distance of 5 m and below around the vehicle. Several other studies by [PERSON] [11] or [PERSON] [12] also investigated this use case, as the scenario of the urban-turning accident resulted in the highest number of accidents between vehicles and vulnerable RUs in 2020, with 24.8% [1]. Therefore, the ambient luminance in the study varied to focus on the lighting conditions corresponding to dusk/dawn and nighttime, where [PERSON] examines two different projected symbol-based patterns (the rectangle and herringbone patterns) [13]. Moreover, [PERSON] et al. [16] focused their study on the cyclist's perspective and perceived safety in a low-light environment, usually during dusk/dawn or night-time. Both studies used questionnaires to evaluate the subjectively perceived brightness of the displayed ground projection [13; 16]. [PERSON] [13] concluded that low ambient lighting levels require higher contrast to detect an object. Thus, the projected symbol-based pattern type did not affect the subjects' perception of brightness. After all, the Weber contrast, which is represented by the following equation, is considered:  
\[C_{\text{W}}=\frac{L_{\text{Object}}-L_{\text{Background}}}{L_{\text{Background}}} \tag{1}\]  
where \(L_{\text{Object}}\) indicates the luminance of the projection and \(L_{\text{Background}}\) is the background luminance of the street's surface [26].