Driving blind: Investigating crashes caused by sun glare
Dazzling sun blindness can be a contributing factor to vehicle accidents, especially in the spring and fall.
Ask drivers about sun glare, and they all agree that it is challenging to drive when dazzling sunbeams trigger momentary blindness. It makes sense then to wonder if sun blindness, and other sources of glare, are significant factors in motor vehicle accidents. Does anecdotal evidence correlate with crash statistics/?
Unfortunately, drilling down to that answer is difficult because crash statistics gathered by law enforcement agencies do not supply an adequate level of detail. It is a rare state that offers much beyond “obscured vision” as an accident factor, and that refers to lighting or weather conditions rather than sun glare. Arizona, a state with record-leading sunny days, is the only one to offer sun glare as an option to check under “Vision Obscurement” in the state’s standard motor vehicle accident report.
Sun glare in accidents
Despite the scarcity of supporting statistics, the inescapable reality is that glare shining into the gaze of anyone actively controlling a moving vehicle can cause dangerous visual deficits, albeit temporary ones. Most drivers are able to mitigate that momentary impairment with defensive driving maneuvers like slowing down and deploying windshield visors and polarized sunglasses to protect their eyesight. Clean windshields devoid of cracks and chips also help to minimize sunlight’s dazzling effects. Choosing an alternate route or changing the timing of a drive to avoid sun-glare periods are also safe-driving coping mechanisms.
The most important attribute of sun glare is that it is temporal in nature: The angle of the sun must be low enough in the sky to shine light into a driver’s eyes. That means that the most vulnerable times of the day for glare challenges coincide with the rising and setting of the sun, which, coincidentally, also fall into rush hour timeframes in urban zones.
It is that aspect that has spurred traffic researchers to see if they can find upticks in crashes during those hours. A representative study looked at accidents occurring on east/west roadways used by commuters in Tucson, Arizona, a city with more than 300 sunny days annually. Drawing from crash reports at signalized intersections over a three-year span within the peak glare time frame, statistical analysis showed upticks in collisions in the direction of travel most exposed to sun glare. Empirical evidence from this study showed that early spring and fall days had the highest glare exposures and thus proved susceptible periods for glare-caused collisions at intersections.
Crash analysis
When it comes to looking at a motor vehicle accident with a potential glare issue, the accident reconstruction analysis often requires the automotive engineer to employ more traditional investigative methods like on-site recreations of the accident sequence rather than relying on technologies like crash data retrieval.
Critical to the site visit is capturing the exact time of day and the weather at the accident scene from the driver’s perspective. Achieving that timing precision can require multiple attempts because the sun’s angle and other conditions do not always coincide exactly as they did at the moment of the crash.
There are a number of tools at the investigating engineer’s disposal to aid the data gathering and confirm the accuracy of the test-drive parameters. The azimuth angle, or the compass direction from which the sunlight comes at any given day, time or location, is trackable via online sources like Suncalc.org. The engineer can determine the position of the sun in the sky as seen from a particular location at a specific time by using the sun’s azimuth angle and the elevation at the crash site. Pinpointing the sun’s angle at that scene is a crucial data point that feeds into ascertaining the degree of visual impairment a driver experienced prior to a crash.
Canny use of Street View on Google Maps can deliver another perspective of the crash site with a high degree of geospatial accuracy that could potentially track solar exposure. In fact, MIT researchers built a model system using Street View to determine sun glare values in streets adjacent to the university’s Cambridge, Massachusetts neighborhood. Municipal traffic cameras might also be tapped for data about broader driver behaviors on the roadway in the accident’s vicinity during peak sun-glare periods.
The driver’s physical point of view is also critical to determine for the crash investigation and includes:
- His/her height behind the wheel and the resulting line of sight;
- Obstacles on the roadway that impact visual field;
- Weather conditions at the time;
- The condition of the windshield; and
- Driver actions to reduce glare issues like using the sun visor or wearing polarized sunglasses.
Also contributing to the analysis is gauging the age and visual health of the driver. Glare is harder on elderly drivers. Dazzling sunlight reduces contrast and imperils visual acuity. For the young, the impairment is quick to resolve, but older drivers can take as much as 50% longer to recover from a glaring assault on their eyesight. Input from a human factors specialist may also provide perspective to the driver’s reactions to visual impairment.
Glare is an ever-present challenge for motor vehicle drivers, whether it derives from a blast of bright sunlight in the driver’s line of sight or a piercing ray of light reflected off an adjacent vehicle’s shiny chrome trim. While its impact on crash statistics may be underrepresented for now, glare is a driving challenge that forces alert drivers into defensive driving behaviors, especially as they travel roadways with east/west exposures during vulnerable time windows.
Robert A. Siegel (engineering@prtassoc.com) is a geographer and the client services director at Peter R. Thom and Associates Inc., a national firm of consulting automotive engineers.
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