The Conspicuity Benefits of Dynamic and Static Bicycle Taillights at Night
Authors: Edewaard Darlene E. , Fekety Drea K. , Szubski Ellen C. , Tyrrell Richard A. , Rosopa Patrick J. 
Published: September 2017

A total of 729 bicyclist fatalities and approximately 50,000 bicyclist injuries were reported in the United States in 2014. Of the bicyclist fatalities, roughly 50% involved crashes that occurred at civil twilight or after dark (National Center for Statistics and Analysis, 2016). Also regarding bicyclist fatalities, Hutchinson and Lindsay (2009) found that the majority of bicyclists’ deaths resulted from collisions where the cyclists were hit from behind, especially at night, after investigating Australian bicycle/motor vehicle crashes. In addition, Bil, Bílováa, & Müller, (2010) found that the highest percentages of cyclists’ fatalities occurred on straight road segments (23%) and curved road segments (16%) after analyzing patterns found in 5428 cyclists/motor vehicle collisions in the Czech Republic.

Bicycle taillights have been found to enhance nighttime conspicuity (for a review, see Kwan & Mapstone, 2004). However, there are several gaps in the relevant literature, such as whether there is an optimal taillight mode, such as flashing or steady, and an optimal place to mount a taillight for enhancing a cyclist’s nighttime conspicuity. Although Wood et al. (2012) found no significant difference between the recognition distances of flashing and steady headlight, this comparison has never been studied empirically using bicycle taillights (which are red and less intense than headlights). With regard to taillight placement, studies have consistently found that placing lights or retroreflective material on major joints (e.g., knees, and ankles) can enhance bicyclist conspicuity (e.g., Blomberg, Hale, & Preusser, 1986; Koo & Dunne, 2012; Koo & Huang 2015; Stapleton, Stapleton, Koo, & Koo, 2017; Tyrrell, Fekety, & Edewaard, 2016; Wood et al., 2012). Indeed, there is a growing body of literature that supports the concept of biological motion (“biomotion”; Johansson, 1973); humans’ perceptual sensitivity to discriminating human joint movement patterns (for a review, see Tyrrell, Wood, Owens, Whetsel Borzendowski, and Stafford Sewall, 2016). What remains unclear is whether the bicyclist needs to be physically pedaling in order for biomotion to be perceived. Balk, Tyrrell, Brooks, and Carpenter (2008) found that pedestrians who walked in place elicited longer participant response distances than pedestrians who stood still. While this provided empirical support that biomotion configurations offer more benefits for pedestrians when their extremities are in motion, the parallel comparison for bicyclists (i.e., non-pedaling cyclist vs. pedaling cyclist) has never been made. The present study examined the conspicuity values of four taillight configurations that were systematically varied in terms of taillight mode (flashing or steady) and placement (seat post or heel) on two distinct roadway geometries.

Methods. Visually healthy participants (N = 219) were driven along a designated test route and were asked to press a button on a numeric keypad each time they became confident that a bicyclist was present. Participants encountered Cyclist 1 (who was positioned adjacent to a straight road) approximately five minutes into the drive, and approximately two minutes later, participants were driven past Cyclist 2 (who was positioned after a 90 degree curve). Upon each press of the response button, a timer on the computer was activated. The researcher stopped the timer upon passing each relevant cyclist. The time between the participants recognizing a cyclist and the vehicle passing the cyclist was used to calculate each participant’s response distance (Distance = Speed * Time). This technique has been used in numerous on-road pedestrian studies, and its accuracy has been verified (e.g., Fekety, Edewaard, Stafford-Sewall, & Tyrrell, 2016; Whetsel- Borzendowski, Stafford-Sewall, Rosopa, & Tyrrell, 2015). After passing both test cyclists, the participants were informed that the experimental session was finished, and they were then debriefed while they were driven back to campus where they were released. Each experimental session lasted approximately 30 minutes. Response distances resulting from trials where glare from oncoming vehicles interfered with the participants’ view of the bicyclists were excluded from the analyses. A response distance of 0 m was recorded whenever a participant failed to respond or responded after passing a cyclist.

Results. Two separate between-subjects analysis of variance (ANOVA) tests were performed to examine each of the four taillight configurations’ distributions for Cyclist 1 and Cyclist 2.

For Cyclist 1, the main effect of Taillight Configuration was statistically significant, F(3,168) = 19.21, η2 = .255, p < .001. Bonferroni post-hoc pairwise comparisons revealed that, when steady lights were mounted to the cyclist’s pedaling heels, participants responded from a mean distance that was 1.7 times greater than when a flashing light was mounted to the seat post and 5.5 times than when a steady light was mounted to the seat post or when lights were mounted to the cyclist’s non-pedaling heels. In addition, the mean response distance for the flashing seat post configuration was 3.1 times greater than those for the steady seat post configuration and the non-pedaling heel-mounted lights.

For Cyclist 2, the main effect of Taillight Configuration was also statistically significant, F(3,162) = 9.82, η2 = .154, p < .001. Bonferroni post-hoc pairwise comparisons indicated that when either a flashing or steady light was mounted to the seat post or the lights mounted to the cyclist’s pedaling heels the mean response distances were 1.6x greater than the mean response distance for the non-pedaling heel-mounted lights configuration.

Discussion. This study investigates the use of bicycle taillights as a way for bicyclists to help approaching drivers to more quickly recognize their presence on roadways of differing geometries at night. The results of this study suggest that, in order to maximize conspicuity while riding at night, it seems best to use two heel-mounted taillights. This configuration capitalizes on drivers’ perceptual sensitivity to the biological motion of other people. However, cyclists with only one taillight should use it on a flashing setting while mounted to their bicycle’s seat post when riding at night in order to enhance their conspicuity to approaching drivers. Bicyclists must also be informed that conspicuity aids are not effective 100% of the time, and therefore, they should always ride defensively. Still, this study empirically demonstrated that highlighting a bicyclist’s movement with lights is effective for maximizing nighttime conspicuity. The results of this study can be useful to designers of bicycle taillights, since these data offer valuable insights into how taillights can be used to maximize bicyclist conspicuity at night.