Headlight Glare (HLG)

in Night Driving

Importance of the problem

HLG is a major public concern as evidenced by the continual high level of complaints registered with the government. The National Highway Traffic Safety Administration (NHTSA) opened an internet forum in 2001 to gather comments. Within three months, this forum received more than 4,000 comments (NHTSA, 2001). This was much larger number of public comments than NHTSA has received on any other safety topic. In addition, studies have found that older drivers perceive HLG as their primary nighttime driving hazard (Mace et. al, 2001), and one of the main causes for refraining from nighttime driving (Brabyn et al., 2005). Fatalities per mile traveled are three to four times higher at nighttime than at daytime (Sullivan & Flannagan, 2002a; Varghese & Shankar, 2007). Although nighttime crashes involve other complex factors (e.g. speeding and alcohol), reduced visibility has been shown to be a major contributor to fatal nighttime crashes involving pedestrians and bicyclists (Owens & Sivak, 1996; Sullivan & Flannagan, 2002b). Car visibility and conspicuity actually increase at night due to head and tail lights, so increased car-to car accidents at night are less likely to be due to visibility or vision deficits. Oncoming headlight glare (HLG) negatively affects driving in two ways:

1. 1)Light scattering within the eye causes bright luminance veiling on the retina and directly reduces image contrast, thus reduces overall visibility (Disability glare). This visibility reduction can make it difficult to perform various necessary visual tasks related to driving safety (Pulling et al. 1980; Vos et al. 2002; Vos, 2009).
   

2. 2) The visual distraction and annoyance caused by bright light and the resulting discomfort sensation (Discomfort glare) may affect driving by causing averting eye or head movements that may affect steering (Boyce & Beckstead, 1991).
   

Cataract

Cataract is one of the most common causes of age related vision impairments. More than 26% of people over the age of 55 in the U.S. develop bilateral cataracts and this figure expands to more than 56% for people over 65 (Acosta et al., 2006). Cataracts reduce visual acuity (VA) and contrast sensitivity (CS). Patients with cataract are particularly affected by oncoming HLG because the increased scattering of light in the eye causes bright veiling glare that further degrades their poorer visual function under low luminance conditions (Elliott et al., 1996). Overall reduction of ambient luminance and reduced conspicuity of pedestrians or other on-road objects, and other driving related features (e.g. lane markings, traffic signs) are expected to be more severe in cataract patients. However, prior studies have not provided direct evidence of the impact on driving safety of HLG for patients with cataracts. Impact of cataract on safety has been demonstrated in epidemiological studies (i.e., Owsley & McGwin, 1999). Older drivers (55-85 years) with cataracts were 2.5 times more likely to have been involved in crashes in the prior 5 years than older drivers who were free of cataracts. However those studies did not differentiate night and day crashes, nor did they distinguish between car-to-car and car to- pedestrian crashes, where only the latter are presumed to be more affected by HLG.  The presence of cataracts has been shown to reduce visual performance in detecting and responding to road hazards. Wood & Carberry (2006) conducted a daytime closed-circuit driving performance test, and found that patients with bilateral cataract had significantly poorer performance (e.g. sign recognition, hazard recognition, hazard avoidance, and gap perception) than normal vision (NV) controls, but their performance was improved to the NV level after both cataracts were removed. Similar results were found in a video-based hazard perception and detection test with two different levels of simulated cataract goggles (Marrington et al., 2008). However, they did not address the more challenging (and riskier) driving condition, the nighttime driving and associated impact of headlight glare (HLG). Wood et al. (2010) demonstrated that nighttime driving performance is better predicted by a driver’s CS than by their VA in a nighttime closed-circuit driving study, which compared the driving performance of subjects with simulated cataract (SC) goggles and VA-matching optical blur (OB) goggles. The SC group hit about twice as many hazards (traffic cones) as the VA-matched OB group, who in turn hit about twice the number of hazards as the NV control group. Recently, Wood et al. (2012) found similar performance degradations with SC where detection of pedestrian presences is only about 30% for SC and about 52% for OB condition, and for those detected cases, recognition distance of SC and OB condition is 5.5 times and 3.6 times shorter than NV condition, respectively. Although these studies provided important data about the impact of cataracts on overall nighttime driving performance, they have not specifically addressed the impact of HLG. In these studies, the pedestrian always showed up near the HLG and there was no control data measuring performance without HLG. There is a need to fill this knowledge gap regarding the direct effect of HLG on driving relevant tasks by comparing performance with and without HLG, so that the analysis can reveal any direct effect of HLG on performance. Another important question is the relative impact of unilateral vs. bilateral cataract surgery. A surgical procedure such as CE-IOL implantation, even on a single eye, restores binocular VA and CS to NV levels (Desai, 1999; Busbee et al., 2002; Norregaard et al. 1998; Lum et al., 2000). However, there is concern that if cataract surgery is only done on one eye, significant visual function defects still remain. Various studies have compared visual function before and after the second eye surgery and found that patients have improved binocular VA and CS, as well as stereo acuity, after the second surgery (Talbot & Perkins, 1998; Castells et al., 2000; Laidlaw & Harrad, 1993; Bissen-Miyajima et al., 1995). A recent survey on daily life difficulties in cataract patients found that after the first cataract surgery, around 60% of participants still complained about visual impairments, but this was reduced to about 7% after the second eye cataract surgery (Avakian, 2005). Although it is clear that the first cataract surgery improves visual functions of individual patients (as a result of a better vision in the operated eye), it is still unclear whether the presence of cataract in one eye is sufficient to cause significant HLG that may impair vision, and thus affect a monocular cataract patient’s nighttime driving performance. A study by Owsley et al. (2001) analyzed the interactions between police crash reports of elderly drivers (with and without cataract) and their monocular visual function, and concluded that a CS deficit (due to cataract) in only one eye was also significantly associated with higher crash involvement, 2.7 times more than the drivers without any CS deficit. The results suggest that the second eye cataract surgery may be the important factor for driving safety, but the study did not differentiate between daytime and nighttime accidents and thus did not address the impact of cataract on nighttime driving with oncoming HLG. It also did not account for the fact that cataract patients are likely to self-regulate their nighttime driving. Thus the real-world impact of single eye cataract on nighttime driving performance may be even higher than what was found in this study. Currently, there is no driving performance data directly addressing nighttime driving risk, and its interactions with HLG caused by cataract on one eye, after a single corrective surgical intervention in bilateral cataract patients. Numerous studies have addressed the HLG impact on NV drivers. For example, HLG source intensity and exposure duration (Akashi & Rea, 2001; Chen, 2004; Van Derlofske, 2005; Flannagan, 1999; Stiles & Crawford, 1937), HLG source color and size (Flannagan, 1999; Bullough et al, 2003), and HLG source type and aiming direction, such as HID (High-intensity discharge) and low/high beams (Flannagan et al., 2000; Bullough et al., 2002). Dynamic visual acuity was found to decrease by a factor of six if the oncoming car’s headlights were on high-beam, compared to low-beam (Anderson & Holliday, 1995). However, these studies often do not capture the full nature of nighttime and HLG driving conditions, because most studies are done in highly artificial in-laboratory environments, or in very simplified outdoor setups, where the simulation of glare source is either static or shown with limited mobility.

Studies of nighttime driving

In 2002, the European Glare project was formed to study disability glare in driving and its impact on nighttime driving. Several studies (Van den Berg et al., 2009; Van Rijn et al., 2005) have demonstrated how intraocular straylight measures may be a good predictor for nighttime driving performance in presence of HLG. However, since the intraocular straylight measure just measures the physical magnitude of straylight on a subject’s retina under given lighting conditions, these studies depended on a series of assumptions, which oversimplifies the dynamic nature of the HLG encounter in nighttime driving environment to predict the impact of glare on nighttime driving performance. Furthermore, the validity of the use of the intraocular straylight measure to predict nighttime driving performance was not sufficiently supported by actual driving study data, where an individual driver’s behavioral responses to the oncoming glare source may either reduce or increase the impact of HLG.

Eye tracking data were used to detect drowsiness of driver (Hayami et al., 2002), driver’s initiation of lane change (Salvucci & Liu, 2002), scanning efficiency (Pradhan et al., 2005), and difficulty of driving road condition (Victor et al., 2005). Studies have also examined the interaction between gaze movements and steering (Doshi et al., 2009; Doshi et al., 2012). However, it is not clear how oncoming HLG affects eye movements, and how the eye movements in turn affect driving behavior.

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