An empirical investigation of a dynamic brake light concept for reduction of rear-end collisions through manipulation of optical looming

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Abstract

The concept of dynamically manipulating the optical looming cues of a lead vehicle's brake lights is investigated as a means of potentially reducing the frequency of rear-end collisions in automobile driving. In a low-fidelity driving simulator, 40 participants were instructed to follow a leading vehicle (LV) and appropriately respond to braking of the LV, under three visibility conditions: day, night-time with following vehicle (FV) headlights, and night-time without FV headlights. During some LV braking events, separation and size of the brake lights of the LV were expanded or contracted, by a nominally imperceptible amount, to simulate an effective virtual time shift in the headway of the LV. Results show that this manipulation was most effective for very poor visibility conditions: at night with no headlights, for which LV brake lights were most salient. When confronting a LV with expanding or contracting brake lights, subjects generally braked sooner or later respectively, in comparison with the no manipulation case. The concept shows some promise for causing drivers to brake sooner in emergencies.

Introduction

This paper introduces a proof-of-concept study that aims to reduce the high rate of rear-end collisions, which contribute to an increasingly large number of crashes in automobile driving. The concept is a dynamic brake light system, which would serve to trigger faster braking reactions from following drivers (FDs) responding to hard braking leading vehicles (LVs), by means of expanding the size and separation of the brake lights of the LV during braking. The study was designed with the intention that, if this concept were found effective at positively affecting subjects’ braking behaviour in response to emergency braking LVs on a simulated roadway, the results of the study could provide the basis for seriously considering such brake lights in future rear signalling applications.

According to US National Highway Traffic Safety Association (NHTSA, 2005), in 2003 alone rear-end collisions accounted for 29.6% of all crashes (1.9 million) and caused 5.4% of total fatal crashes (2076), 29.6% of all injury crashes (0.57 million), and 29.8% of all property-damage-only crashes (1.3 million). The major causal factor of rear-end collisions occurs when the FD does not react correctly to the behaviour of the LV, due to either inadequate or late detection of LV deceleration. This has been attributed variously to factors such as inattention, inadequate perceptual discrimination, incorrect interpretation about traffic movements, or inadequate headway to allow the FD to react appropriately to emergency braking by the LV (Rumar, 1990; Knipling et al., 1993; Kostyniuk and Eby, 1998).

Although current ‘binary’ brake light systems indicate whenever the driver of a LV has a foot on the brake—thereby immeasurably assisting in detection of LV deceleration—this information is frequently insufficient for helping the FD to properly judge the rate of deceleration. For such judgements as when to brake, how hard to brake, or whether it is necessary to brake at all, drivers have to rely on direct visual information about how rapidly the following vehicle (FV) is closing in on the LV (Liebermann et al., 1995). Whereas under most circumstances such directly perceived visual information may be adequate, in extreme situations such as emergency LV braking it is clearly not adequate, because there is always a danger that the FD might be dangerously too close to the LV before he is able to pick up adequate closing information.

The braking response—moving the foot to the brake pedal—is only the first stage; the braking adjustment stage is just as critical. Normally a FD does not initiate full-power braking as soon as he/she ascertains that a LV is braking since, among other things, this would risk a skid and/or being run into from behind. Rather, drivers normally adjust their braking on the basis of the perceived urgency of a situation. In an emergency, for example, it is important, and often critical, to adjust braking to an appropriate level early enough to avoid rear-end collisions, because the sooner deceleration is initiated, the more effective it is. Therefore, whenever the LV brakes very hard, and/or when lead distances are very short, any supplemental information that will allow the FD to control his/her braking profile appropriately should be of obvious benefit.

Much consideration has been given to whether, and how, the intensity of a LV braking manoeuvre can be indicated to the traffic behind, including a number of proposals for informing the FD about braking emergencies through LV rear signal enhancement. One example is the advanced braking warning system (Shinar, 2000), which activates LV brake lights in an anticipatory fashion whenever the lead driver (LD) suddenly removes his foot from the accelerator pedal while going at high speeds, prior to actually applying any pressure on the brake pedal. Another example is the addition of an imperative brake light signal on the rear of the vehicle (Tang, 1989), which would light up only when the emergency brake is activated. Another suggestion is flashing lights, which either would communicate the deceleration rate of the LV to the FD—i.e. gentle stops would produce slow flashes and rapid stops rapid flashes (Voevodsky, 1974)—or would indicate sharp decelerations of the LV to the FD by switching on additional flashing lamps—such as centre high mounted brake lights (Browne and Chin, 1991; Horowitz, 1994), rear fog lamps, stop lamps, and hazard warning signals (Alferdinck, 2004). Gail et al. (2001) suggested a two-stage brake force display, for which the area and luminance of LV brake lights would change from a normally small area and low luminance to a larger area, higher luminance state for any LV deceleration rate greater than 7 m/s2.

These examples share the common concept of communicating an emergent situation explicitly to the FD so that he can immediately be aware that the brakes of the LV have been applied in an (apparent) emergency. One potential disadvantage of such systems, however, is that detection and comprehension of a warning in these settings could be distracting or even time-consuming, in the sense that FD would have even more information to process in times of emergency. Moreover, it would not be unexpected for a significant number of FDs to adjust their subjective acceptable risk thresholds (Wilde, 1982) to accommodate to what they perceive to be the operational properties of such emergency braking systems, after having encountered them in action a number of times during driving. With these considerations in mind, we propose that an ideal system would be one for which the FD would be unaware that he is being warned to brake more rapidly, but would nevertheless be compelled do so—involuntarily so to speak.

TTC is the time remaining to a collision when approaching a stationary or moving object, assuming that the vehicle's direction and closing velocity will remain constant. (We can therefore define TTC=Xrel/Vrel, where Xrel is relative separation between vehicles and Vrel is the relative velocity.) Based on Lee's (1976) seminal work, optical looming, which is defined as the symmetrical expansion of the retinal image size of an approaching object on a collision course, can specify TTC through an optic variable Tau, defined as the inverse of the relative expansion rate of the retinal image, assuming a constant closing velocity. Lee argued that TTC information, directly specified through Tau, could in principle be used to judge when to start braking and how to control braking. In other words, a driver could initiate braking at a certain Tau margin and control braking simply on the basis of the rate of change of Tau. Lee's hypothesis about the use of Tau through optical looming during automobile driving has been supported by experimental evidence (van der Horst, 1990, van der Horst, 1991; van Winsum and Heino, 1996).

In our own simulated car driving experiments, we have also found a direct relationship between optical looming cues and control of braking (Li and Milgram, 2004a, Li and Milgram, 2005). In those experiments FDs were found to advance or delay their braking behaviour in a predictable fashion while following a size-changing LV whose optical image was experimentally manipulated. The hypothesis supported in those studies was that, when the FD brakes sooner in response to an expanding LV (or later to a contracting LV), this behaviour is a result of manipulation of the optical looming cue. Similar patterns of behaviour have been observed also for other psychomotor tasks, for example in animal studies (Sun et al., 1992), for ball catching involving dynamic size changing (Savelsbergh et al., 1991, Savelsbergh et al., 1993; Wann and Rushton, 1995a; van der Kamp, 1999), and for braking while approaching a size-changing barrier in a bicycle simulator (Sun and Frost, 1998).

In actual driving situations there is no obvious way to manipulate the size of a real LV, as can be done with a virtual vehicle in a driving simulator. Research has shown, however, that the brake lights of a LV are a very important source of information for FDs to regulate speed and inter-vehicle distance, on the basis of changes in the perceived distance between LV brake lights. This is especially true during reduced visibility situations, such as night-time, fog and rainy weather (Janssen et al., 1976; Liebermann et al., 1995). Theoretically then, it should be possible to manipulate the optical looming of the image formed by the LV brake lights, in order to shorten the FD's perception of TTC with the LV and cause him/her to brake sooner.

With this in mind, a dynamic automobile brake light system has been proposed, as a means of potentially reducing the rate of occurrence of rear-end collisions (Li and Milgram, 2004b). According to that proposal, whenever the LV brakes rapidly (assuming that sensors within the LV can detect and estimate FV distance), both the separation and size of the brake lights of the LV will automatically expand, continuously and gradually, to amplify the natural optical looming which occurs to the FD during braking. The proposed concept is based on the hypothesis that exaggerating the apparent rate of optical looming whenever a LV brakes very hard, and/or when lead distances become very short, will provide the FD with the intuitive illusion of a (artificially) more rapidly approaching LV, which may then subconsciously prompt the FD to decelerate more quickly. Clearly, a decrease of even a few tens of milliseconds in braking time could have a significant impact on overall traffic safety, when aggregated over the extremely large number of FV braking incidents.

Other successful applications of turning human visual illusions or distortions to advantage in order to improve transportation safety do exist. One of these is the redesign of a dangerous traffic circle in Scotland, where drivers had tended to overspeed, with a high accident rate as a consequence (Denton, 1980). Lines of diminishing separation were drawn across the roadway to “trick” the driver's perceptual system; whenever approaching the circle at an excessive speed, drivers would experience the “flow” of transverse line texture passing the vehicle as signalling an amplification in speed. Because of the essentially automatic nature of such percepts, it was predicted that drivers would instinctively brake in response to the augmented speed percept, bringing it to within a safer range. This is exactly the effect that was observed after the marked pavement was introduced, resulting in a substantial reduction in fatal accidents at that traffic circle, a result that has been sustained for several years (Godley et al., 1997).

The objective of the experiment reported here was to empirically investigate whether and how the dynamic brake light concept described above might influence drivers’ braking behaviour, in a manner similar to the marked pavement described above. Specifically, using a low-fidelity diving simulator, we have investigated whether manipulating the separation and size of the brake lights of the LV while it is decelerating can make FDs modify their braking behaviour accordingly. In addition, to ascertain whether such a phenomenon, if observed, might be a consequence simply of the presence of our manipulation, rather than of its prescribed pattern, our experiment comprises manipulations in both directions, that is, both expansion and contraction (even though any practical operational system would logically comprise only expansion), and at two levels for each. Our hypotheses are thus: (i) that significant effects on FD braking behaviour can be expected through manipulation of only brake lights; (ii) that LV brake light expansion will cause advanced braking, while contraction will retard braking; and (iii) that modifying the magnitude of the manipulation will cause changes in braking behaviour which correspond to the size of the manipulation. A fourth aspect of our investigation involved a comparison of daytime vs. night-time driving, under different illumination conditions, with the hypothesis being (iv) that manipulation of the brake light optical looming properties will have its greatest impact for lowest visibility conditions, for which the brake light cue is most relevant for assisting in FD braking.

Section snippets

Participants

Forty male paid volunteers participated in the experiment. They were 18–36 years old (mean=24; SD=5.1) with normal or corrected-to-normal vision and naïve to the purpose of the experiment. All had full driving licenses, with 3–15 years of driving experience (mean=6; SD=3.3). Our reason for limiting the subject population to young male drivers was to give more power to our experiment, by allowing us to focus on the optical cue manipulation factor. In doing so, any existing significant

Time of taking foot off gas pedal and time of first pressing brake pedal

Referring to the top of Fig. 5, for time of first taking foot off gas pedal, a three-way within-subjects ANOVA (viewing condition×manipulation×deceleration rate) indicated only a significant viewing condition main effect (F(2,118)=30.9, p<0.001), without any two-way or three-way interactions among viewing condition, deceleration rate and manipulation. Generally, it seems that subjects released the gas pedal later in the night-time viewing condition with FV headlights. The absence of any other

Discussion

The results reported here generally support our hypothesis: subjects braked sooner, to an extent corresponding to the magnitude of manipulation while viewing a LV whose brake lights are artificially expanding, at a rate compatible with its being closer to the LV in terms of effective virtual time shift (ΔTTCvirtual). To complement this finding, subjects braked later while viewing a LV with brake lights artificially contracting, to an extent compatible with the magnitude of the contraction and

Acknowledgements

The authors gratefully acknowledge the support of the Institute of Robotics and Intelligent Systems (IRIS), Precarn, and the National Science and Engineering Research Council (NSERC).

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