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Special feature: Vehicle lights Daytime Running Lights have been a feature of cars for over a decade, but a general poor understanding of their role has left some drivers in the dark. Tom Harrington explains why their role in illuminating drivers is critical – and the factors that lead to trouble when wrongly used DRLs and the importance of visibility and conspicuity The golden rules of lighting up your car are to see and be seen. In accordance with EC Directive 2008/89/EC, dedicated Daytime Running Lights (DRL) were required on all new passenger cars and small vans from February 2011 onwards, and on all other new types of road vehicles (including trucks and buses but excluding motorcycles and agricultural tractors) from August 2012. Daytime Running Lights automatically activate when the engine is switched on. They are a fantastic asset to road safety and reduce the likelihood of a side or head-on collisions during the day. However, just as the darkness creeps in unnoticed so too has an over-reliance on modern technology and the fear is that too many rely solely on DRLs when darkness falls during the winter months. For a time, several US states banned the use of these lights entirely. In the 1990s, General Motors pushed to include DRLs on all vehicles in the US. This was met with resistance, but eventually government regulations allowed their use, and their popularity has increased ever since. But judging from recent press announcements, a growing number of motorists are mistaking their DRLs for dipped headlights. Now, no one is doing this intentionally. It’s an honest mistake as drivers start out in the daytime and fail to recognise the fading light. But unfortunately it poses a severe road safety risk. DRLs don’t produce enough light to illuminate the road in darkness. Some manufacturers choose to pair front daytime running lights with rear ones too, but it is not compulsory. This means there may be some motorists driving around at night in the mistaken belief that just because they have lights which switch on automatically at the front; they are also on at the rear. So, driving with DRLs at night not only seriously diminishes your view of the road, your visibility to others is minimal and you could run the risk of being rear ended. Therefore, if you have dedicated DRLs on your vehicle, make sure to switch to your headlights during ‘lighting up’ hours – legally defined as ‘the period commencing one half-hour after sunset on any day and expiring one half-hour before sunrise on the next day’. Visibility, Detection and DRL Why is the issue of DRLs so important? It all comes down to our ability to see clearly, levels of visibility and perception. ‘Visual perception’ is a concept which refers to all perceptual processes and results imaginable. As a result of its generalised nature, the literature often distinguishes between the various aspects of perception. Concepts such as detection, conspicuity and visibility are often mentioned in the perception literature. For the purposes of clarification, therefore, some of these concepts will now briefly be discussed. The concepts of visibility and detectability are often interchanged. Visibility can be defined as a 50% probability of detection (threshold of visibility). If an object becomes more visible it is generally implied that its detection improves in one way or another, so that the probability of detection becomes increasingly greater; this implies that, in general, an object can be detected at a greater distance, or that observers need less time to decide whether or not an object is present (reaction time). Visibility is subject to a human assessment component, as there is no equipment that can directly measure visibility: human intervention is always necessary to determine this parameter. Often, such factors are studied with the 22 NEWSLINK n OCTOBER <strong>2021</strong>
For all the latest news, see www.msagb.com aid of detection experiments. One important factor that determines whether an object is detected is the contrast between object and background. Although contrast is related to visibility, it is not the same thing. Di Laura offers a simple example of this phenomenon. Take an object that contrasts 50% with the background on a large stage in a theatre and illuminate it with a pocket flashlight: it will hardly be visible. That same object, lit by a large floodlight measuring 10,000 times the luminous intensity of the flashlight - the contrast remains the same, but the visibility differs markedly. Both luminance and contrast are important for visibility. Another factor is the size of objects; large objects are more visible than small ones. The degree to which the visual system is sensitive to contrast is therefore not the same under all circumstances. Blackwell (1946; 1968) has probably conducted the most extensive research into the sensitivity of the visual system. The lower the luminance level, the greater the contrast between an object and its background should be in order to ensure the same probability of detection. But given a particular luminance, the detectability of an object will improve if the contrast with the background is enhanced or if the object is larger, for example. Visibility and Conspicuity Sometimes, visibility means more than simply detecting something. One can detect something among other elements; in that case, one can speak of conspicuity. Conspicuity implies that a particular object must compete with other objects in order to attract attention while visibility implies the detection of the presence of a particular object against an empty background. Visibility does not necessarily imply conspicuity; a particular object may also be visible between similar objects (i.e. be detectable), but may not necessarily be conspicuous. There are many definitions that describe the term conspicuity. Wertheim (1986) and Theeuwes (1989) have offered an overview. The measurement and definition of conspicuity is performed in so many different ways that it is in fact impossible to speak of the conspicuity of an object. However, all definitions of conspicuity do share a reference to attention: a conspicuous object draws attention to itself. All definitions also state that external, physical factors determine the conspicuity of an object. According to Engel, visual conspicuity is defined as the object factor, or more precisely, as the set of object factors (physical properties) determining the probability that a visible object will be noticed against its background. Eccentricity, ie, the angle between the object and the direction of view, is an important factor in conspicuity. The contrast between object and background and the complexity of that background is also important. Nevertheless, factors other than external ones can influence conspicuity. Engel makes a specific distinction between visual conspicuity (bottom-up) and cognitive conspicuity (top-down). In more or less the same manner, Hughes & Cole have pointed out that conspicuity cannot only be regarded as characteristic of an object, precisely because it has to do with attracting attention. Whether an object will attract the attention of an observer is largely determined by that observer. Hughes & Cole (1984) therefore distinguish between two types of conspicuity: attention conspicuity and search conspicuity. The first type refers to the possibility that an object will attract the attention of an observer who is not specifically looking for such an object. The second type, search conspicuity, is defined as the characteristics of an object that allow it to be easily and quickly localised if the observer is looking for it. Hughes & Cole summarise a number of factors that also determine whether an object will be conspicuous or not. These include physical properties of the object and its background; the information that is supplied, including information concerning the unusual or unexpected nature of the object; the observer’s need for information (is the observer looking for particular object? etc); the perceptual strategy of the observer (road user), which is also determined by the information in his environment and his need for information. Continued on page 24 NEWSLINK n OCTOBER <strong>2021</strong> 23
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