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NIGHTTIME VISIBILITY

It’s a story that we hear all too often. In the dark of night in an area where there are no streetlights, a pedestrian wearing dark clothing attempts to cross a road. The pedestrian looks left and sees nothing coming, looks right and sees a vehicle in the distance but decides that there is plenty of time to cross. The pedestrian crosses the first traffic lane and then steps across the centerline, but his judgement of the speed of the approaching vehicle is inaccurate and the pedestrian is struck and seriously injured, or even killed. The driver says he didn’t see him until it was too late.

 

As we all know, our vehicle headlight systems offer us high-beam and low-beam settings. The high beams are utilized when there is no oncoming traffic, it the driver thinks to use them. Most times, we drive with the low-beam setting engaged, which offers adequate illumination for most conditions at or near the legal speed limit. The low-beam setting tends to provide more illumination for the right side of the road than it does for the left as the beam setting is aimed so as not to cause glare problems for oncoming vehicles.

 

The pedestrian in the above example came from the left side of the road (with respect to the driver) and was not adequately illuminated until he crossed the centerline. Our driver stated that he didn’t see him until it was too late. For the driver to be able to detect the pedestrian, perceive it as a problem and then react by braking or steering to avoid, time passes before any evasive action actually occurs. During daytime driving, studies have shown that this time, termed the Perception Reaction Time (PRT), under normal driving conditions for a normally alert driver can be around 1.5 to 1.75 seconds. During nighttime driving, our perception time can be increased due to many contributing factors, resulting in a PRT of up to 2.5 seconds or more, depending on conditions. At 100 kph, this 1.0 second difference from 1.5 to 2.5 seconds equates to almost 28 metres or 90 feet.

 

Some things are easier to see while driving at night than others. A reflectorized highway sign may be visible (not necessarily readable) a kilometre away, but a dark clad pedestrian, or a black dog, may suddenly appear out of the darkness when it is very close to the vehicle. Detection requires contrast between the object and its background. The object must be brighter or darker than its background. If the background is completely snow covered, a darkly clad person will show up better than a person clad in white. Conversely, a person clad in white against a background of trees (which can look black at night) will show up much better than a darkly clad person. Size really has no bearing on an object’s visibility. It doesn’t matter if the object is a brick or a boxcar, it will not be detected without adequate contrast.

 

Referring back to our above example, could our driver have detected the pedestrian sooner if he was walking along the right edge of the road? The answer is yes for two reasons. Firstly, the headlight system would have better illuminated the pedestrian at the right edge of the road than it would at the center of the road, as that is where more light is directed due to the aim of the headlights. This would provide the driver with more distance and time to avoid. Secondly, the driver’s expectancy is different for the two scenarios. It is a normal occurrence to see a person walking on the right side of the road, and although a driver may be startled by the sudden appearance of the darkly clad person, he is quick to react and avoid any unfortunate incidents. However, the pedestrian crossing the road from the left side suddenly appears to the driver in the middle of the road, a location that we wouldn’t normally expect to see a pedestrian.

 

Studies have shown that people tend to overestimate the distance at which an approaching vehicle can see them. Allen et al. (1970) had subjects stand along the side of the road and estimate the distance that an approaching driver could see them. At the same time, the driver indicated the distance at which he/she could see each pedestrian. On average, the pedestrians’ estimates were approximately twice the distance that they actually could be seen. Of course, the drivers expected to see the pedestrian so they were alert and ready for the appearance of the pedestrian. It has also been shown through testing that on average, when a driver knows what is ahead and about where it will be encountered, it will be detected at twice the distance that it will when the driver does not have these advantages. Lets put some numbers to this to make it clearer.

• A darkly clad pedestrian who is volunteering in a nighttime visibility test estimates te distance at which the approaching driver can see him at 100 metres.

• The volunteer driver aware of the testing procedures indicates that he can see the pedestrian at a distance of 50 metres.

• Another volunteer driver, unaware of the testing procedure, nor aware of the possibility of an object of any kind in the road, indicates that he sees the pedestrian at a distance of 25 metres.

 

This hypothetical example illustrates (backed by actual testing) that a pedestrian can overestimate the distance that they can be seen by up to four times.

 

It would be helpful to improve the understanding of both drivers and pedestrians concerning the limitations in nighttime visibility, and encourage pedestrians who do venture out in the streets at night to wear light coloured clothing, or better yet, reflective clothing.

 
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Walters Forensic Engineering | 277 Wellington Street West, Suite 800 | Toronto, ON M5V 3H2
Information contact: engineering@waltersforensic.com