When scaled by the number of miles driven, the pedestrian fatality rate is three times higher at night.
Part of the reason is greater chance of driver drinking and fatigue, but the critical factor is lower visibility.
People have contrast sensitivity in dim light. Moreover, the eye exhibits “night myopia” focusing too near and causing distant objects to blur. Rain and on coming head lamps make things much worse.
Increased lighting, is often impractical due to high energy costs.
However LED street lights use can use 50 percent less energy and are more color rendering than sodium lamps. Sodium lamps make yellow flags look gray.
Moreover, there would also be a great outcry against the resulting “light pollution.” Since road lighting cannot be made sufficiently high for daytime safety levels, drivers typically rely on headlamp illumination to detect pedestrians.
However, normal low-beams make pedestrians visible only at relatively short distances. The key concept is “assured-clear-distance,” which refers to the distance ahead that a driver can see a pedestrian on the road. Most drivers are taught to drive slowly enough that they could stop for a pedestrian who just falls within their assured-clear-distance, otherwise they would be “overdriving” their headlamps.
Some states have even made this a matter of law; anyone who overdrives his/her headlamps and has an accident is automatically guilty
However, automobile headlamps provide such a short assured-clear-distance that even drivers who obey the speed limit are often overdriving their headlamps.
People fail to slow sufficiently at night because they are unaware how poor their vision has become.
Humans have two distinctly different types of vision, focal and ambient. They differ in the visual tasks that they perform, the parts of the visual field they examine and their pathways through the brain. Roughly speaking, focal vision tells us “what is there” while ambient vision tells us “where we are.”
Focal vision is used for detecting and recognizing objects, such as pedestrians. It is centered along the line of sight, so when we want to recognize an object, we turn our eyes to look directly at it. Focal vision declines rapidly in dim light. In fact, many people with impaired vision who are unable to receive driver’s license still have vision superior to that of a normal person at night.
Ambient vision is used for determining location in space and orientation in the environment and to perform tasks such as steering a car.
It operates out in the visual periphery and needs only detect faint large shapes. Most significantly, ambient vision is not greatly impaired when light level declines.
Drivers can steer the car just as well at night as during the day and feel little need to slow. They do not realize that their ability to see pedestrians has been greatly reduced.
Many studies have investigated the exact distance at which normal headlamps permit pedestrian detection. There is no single estimate because the distance is a function of many factors, including pedestrian clothing and location, and driver age and expectation.
Philip Skoog is a resident of Kirkland.
