Effects of taillight shape on conspicuity of vehicles at night

Highlights

• A taillight shape of a longer length can most effectively enhance vehicle conspicuity than that of a shorter length.

• The long line-shape taillight (striplight) is recommended as the best ergonomic solution to facilitate vehicle conspicuity.

• An enclosed contour-shaped taillight can more effectively enhance vehicle conspicuity than an open contour-shaped taillight.

• The vehicle–observer distance, driving experience and gender of a driver must be comprehensively considered.

Abstract

Taillight shape in a vehicle provides an essential lighting signal that enables the vehicle to be seen from the rear at night, thereby preventing rear-end crashes. This study aims to investigate the effects of taillight shape on vehicle conspicuity, and proposes ergonomic taillight shape solutions to vehicle designers and manufacturers. Two complementary experiments were conducted to examine three types of taillight shapes at three design levels. The first experiment was designed to investigate the detection speed of a driver and the fixation duration and fixation counts on leading vehicles with different taillight shapes, based on an eye-tracking methodology. The second experiment was designed to investigate the dynamic visual searching performance of a trailing driver for leading vehicles with different taillight shapes, based on a visual search task. The experimental results indicated that a long line-shaped taillight (striplight) was the optimal ergonomic solution for enhancing vehicle conspicuity. Vehicles with an enclosed contour-shaped taillight were more salient than those with an open contour-shaped taillight. Moreover, the experience and gender of the driver and the vehicle–observer distance were found to be closely related to vehicle conspicuity, and therefore, must be considered by vehicle designers when applying a specific taillight shape design. This study provides insights into the taillight shape design that not only aid vehicle designers or manufacturers in enhancing vehicle safety but also enable potential vehicle buyers to choose a safe lighting system.

Introduction

In numerous countries, rear-end crashes are the most frequently occurring type of collision (Lee et al., 2007; Wang et al., 2016; Watanabe and Ito, 2007). Rear-end crashes occurring at night account for a large proportion of the total crashes worldwide (Qi et al., 2013; Yuan et al., 2017). During nighttime driving, vehicle appearances, such as size, contour, and color, are typically diminished because of the decrease in the ambient illuminance, and the taillights on the rear of the vehicles are the primary signals that influence the driving behaviors of the trailing vehicles (Boonsim and Prakoonwit, 2014). Low visibility of leading taillights is one of the main reasons for rear-end crashes as the trailing driver does not notice the preceding vehicle or miscalculates its position (Buchner et al., 2006; Tobitani et al., 2016).

To enhance taillight detection by drivers, several human factor studies have been conducted on taillight configurations. Muttart et al. (2017) and Dinakar et al. (2018) investigated the impact of taillight width (spacing) and brightness on the recognition of the closing or separating speed and distance from the leading vehicle, and demonstrated that a wider or brighter taillight configuration was perceived as the vehicle being closer. Cavallo et al. (2001) and Buchner et al. (2006) studied the relationship between taillight width (spacing) and height and the driver perception of vehicle distance in a nighttime fog scenario. They revealed that the perception of vehicle distance could be significantly improved using wider taillights (two fog lights with maximal spacing), and the distance estimates were consistently larger with a higher vertical taillight position than with a lower vertical taillight position. Theeuwes and Alferdinck (1995) investigated the effects of various layout factors (geometric arrangements and distances between lights) of taillights and center high-mounted stop lamps (CHMSLs) on drivers’ brake light detection performance. They proposed that a higher CHMSL with taillights on the horizontal plane resulted in better light detection performance than a CHMSL located adjacent to the horizontal plane. McIntyre et al. (2012) and McIntyre and Gugerty (2014) proposed that a taillight of another color (amber) instead of the currently mandated red would enhance red brake light detection. Previous studies have mainly focused on the physical features of taillights, such as width (spacing), height, brightness, layout, and color. However, there has been little research on taillight shapes. Taillight shape has a significant effect on visual perception (Taddei-Ferretti et al., 2008). With the increased use of the light-emitting diode (LED) technology in vehicle lighting systems and diverse vehicle design trends, taillight shape designs have become increasingly flexible and varied. A prominent taillight shape design enhances vehicle conspicuity, which enables it to be more easily detected by the trailing driver and decreases the risk of rear-end collision.

Conspicuity can be defined as the property of an object to attract attention or make it readily located by search (Cole and Hughes, 1984), and it differs from the concept of visibility. The visibility of an object refers to its properties, such as size, color, type, and location, whereas conspicuity refers to the extent of an object embedded in its environment (Wertheim et al., 2011). Two aspects of conspicuity are commonly distinguished (Hancock et al., 1990): sensory conspicuity (or visual conspicuity (Engel, 1976)) and cognitive conspicuity. Sensory conspicuity is a bottom-up process in human perception, and can be defined as the ability of an object to attract attention through its physical characteristics (Pinto et al., 2014) and the extent to which it is embedded in its environment (Wertheim, 2010; Wertheim et al., 2011). Previous studies on sensory conspicuity during driving have primarily investigated motorcycle front lights (Cavallo et al., 2015; Rößger et al., 2012), bicycles (Costa et al., 2017; Edewaard et al., 2020), and traffic signs (Costa et al., 2018; Porathe and Strand, 2011). Although Tobitani et al. (2016) proposed that a taillight shape that evokes angry emotions can offer better visibility, their study was mainly focused on visibility instead of conspicuity. The distance between a leading vehicle and the trailing observer was noted to be a factor contributing to vehicle sensory conspicuity in several studies (Gershon and Shinar, 2012; Hole et al., 1996; Wertheim, 2010). Therefore, the impact of different taillight shapes on vehicle conspicuity can be significantly affected by the vehicle–observer distance.

Cognitive conspicuity (Rößger et al., 2012) is a top-down process in human perception. It refers to the distinction of an object under the strong influence of the experiences, interests, and expectations of an observer (Wulf et al., 1989). Hughes and Cole (1984) further categorized cognitive conspicuity into two types: attention conspicuity, which refers to the ability of the target object to attract an observer's attention when it is not cued to search for a specific target, and search conspicuity, which refers to the ability of the target object to be detected when the observer is cued to actively search for it. Gershon et al. (2012) and Gershon and Shinar (2012) investigated the attention and search conspicuity of powered-two-wheels (PTWs) and proposed several potent conspicuity treatments, such as incorporating a helmet-mounted alternating-blinking light system in the rider's outfit, or using an appropriate rider's outfit that distinguishes him/her from the background scenery.

The driving experience of an observer is a factor that is related to cognitive conspicuity. Many studies have investigated the effect of driving experience on driving performance. For example, novice drivers require longer fixation durations than experienced drivers under dangerous driving circumstances (Chapman and Underwood, 1998); Driving instructors have a shorter processing time, increased sampling rate, and broader scanning of the road in comparison with learner drivers (Konstantopoulos et al., 2010); Secondary tasks are better dealt with among experienced drivers than novice drivers (Klauer et al., 2014), and it takes longer for novice drivers to fixate hazards on the road than for experienced drivers (Crundall et al., 2012a). Gender has also consistently been reported to be related to driving performance. Accidents have typically been found to be a function of gender (Whissell and Bigelow, 2003). Male drivers are more likely to speed than female drivers in fatal crashes (NHTSA, 2007). Substantial differences in driving behavior are related to gender, and male drivers are more likely to violate traffic regulations (González-Iglesias et al., 2012). However, in some studies, no differences have been found in driving behavior based on gender (Deffenbacher et al., 2003; Vignali et al., 2019a). The driving experience and gender of an observer may have several impacts on the driver's perception of the leading vehicle.

The visual attention of a human is often investigated by analyzing the characteristics of the visual search task (Mizuhara et al., 2000). Such tasks are extensively conducted in research concerning object conspicuity in a driving scenario (Gershon et al., 2012; Tobitani et al., 2016). Eye tracking has been extensively implemented in the visual search process under a driving scenario, and has been demonstrated as a means of better understanding how people approach and engage in a variety of tasks and situations (Van Gompel et al., 2007). Measurement of the eye gaze behavior is an effective method of evaluating the conspicuity of objects objectively (Kapitaniak et al., 2015; Vignali et al., 2019b). In general, conspicuous objects attract early fixations (Underwood et al., 2008). Underwood et al. (2011) investigated the effects of saliency on fixation prior to the inspection of critical vehicles and demonstrated that high saliency can attract early fixation. Rößger et al. (2012) revealed that shorter times were needed to first fixate a motorcycle with a T-light configuration, indicating a better attention-attracting ability in comparison with motorcycles with solo headlight configurations. Wood et al. (2017) proposed that a biomotion configuration is significantly faster to attract drivers’ attention than a vest, based on the metric of time-to-first fixation (1.1 vs. 3.5 s). Interpreting longer fixations requires careful consideration of the experimental circumstances. Dukic et al. (2013) found that electronic billboards attracted more visual attention based on longer dwell time, longer maximum fixation duration, and a greater number of fixations when driving past an electronic billboard in comparison with other signs on the same road stretches. Crundall et al. (2012b) identified longer fixation durations on motorcycles by dual drivers in comparison with car drivers while inspecting the road scenery at a t-junction, whereas dual drivers showed shorter fixation durations than car drivers when the subjects had to detect motorcycles in the rearview mirror or motorcycles approaching in front of the subject. Longer fixation durations may indicate different underlying circumstances (Just and Carpenter, 1976); for instance, they indicate that the extraction of visual information is more difficult for the subject, and they also indicate that the object is more engaging for the subject (Rößger et al., 2014). As mentioned above, although several studies have been conducted on visual attention based on eye-tracking methods in the driving scenario, the taillight shape effects on vehicle conspicuity have not been determined thus far.

This study focuses on determining the effect of the design pattern of the taillight shape on vehicle conspicuity; it does not evaluate specific taillight shape designs. Three design features of taillight shapes are empirically selected from popular vehicle brands. The first feature is based on the enclosed contour design. With the application of LED technology, the enclosed contour taillight shapes have transformed from solid patterns (e.g., the Benz R107) to more artistic patterns, such as textured (e.g., BMW 5 series 2010–2017) or contoured designs (e.g., Audi Q5 2009–2015). The second feature is based on the linear design. A line-shaped taillight is commonly employed in the current vehicle market (e.g., Lincoln Navigator 2019). However, the lengths of such taillights vary. In previous studies, a vehicle with a wide spacing was found to be closer to the trailing vehicle (Buchner et al., 2006; Muttart et al., 2017), indicating that the length of the taillight has a specific effect on the visual attention of the driver. The third feature is based on the open contour design. There are numerous taillight shapes in an array style, including those combining line shapes (e.g., Lamborghini Aventador), contour shapes (e.g., Chevrolet Camaro 2019), or mixed shapes (e.g., Volkswagen Golf 2016). Table 1 lists the three design features of taillight shapes in the vehicle market.

Subsequently, according to the existing taillight shapes, three typical types of taillight shapes with three levels of shape variations were empirically defined based on expert evaluations performed by seven vehicle designers with at least three years of design experience (Table 2).

• Type A (square shape) is classified into three design levels according to the degree of filling: solid, texture, and contour square shapes.

• Type B (line shape) is classified into three design levels according to the length of the line: short, medium, and long lines.

• Type C (array shape) is classified into three design levels based on the constituent elements: line array, contour array, and mixed array shapes.

Our first hypothesis predicted that a vehicle with a taillight of an enclosed contour shape (e.g., square shape) will have better conspicuity than that with a taillight with an open contour shape (e.g., array shape). According to Mathes and Fahle (2007), closed contours are often better perceived than those not fully enclosing an area (e.g., open contours). Additionally, Donnelly et al. (1991) and Elder (1992) found that an enclosed contour shape (or closure with good continuation) could support a more efficient visual search than an open contour shape (closure but not good continuation). Additionally, we hypothesized that a vehicle with a taillight shape of a long length (e.g., long line shape) will have better conspicuity than a vehicle with a short length (e.g., medium line shape and short line shape). Rensink (2011) demonstrated that a shape with a long length is more noticeable than a shape with a short length. Moreover, a long-length taillight has a large illumination area, which enables the corresponding leading vehicle to be better detected by the trailing drivers than that with a short-length taillight. Furthermore, experienced drivers typically detect hazards earlier (Castro et al., 2019) or with less effort (Crundall, 2016) than novice drivers. Thus, it can be assumed that non-novice drivers (intermediate and experienced drivers) are better at detecting a leading vehicle than novice drivers, regardless of the taillight shape.

Two laboratory experiments were conducted by viewing nighttime photographs on a computer monitor to investigate the effect of taillight shape on vehicle conspicuity. The first experiment focused on the taillight shape effects on the attention conspicuity of the vehicle—the ability to detect leading vehicles with different taillight shapes when the driver's attention is not cued to search for a specific target. The eye-tracking method was implemented to analyze the ability of a taillight to draw the driver's attention. The second experiment focused on the taillight shape effects on the search conspicuity of the vehicle—the ability to detect leading vehicles with different taillight shapes when the driver is cued to actively search for them. This study aims to propose taillight shape ergonomic design recommendations for vehicle manufacturers and designers, which not only enhance the vehicle attention-capture feature for drivers at night but also provide an impressive visual signature for vehicle design. Moreover, taillight shape design insights can offer a driver or a potential vehicle purchaser a safer lighting proposal.