By Daniel Stern, DVN Chief Editor
Last week, we reported on a headlight glare report published by TRL, a private transport consultancy in the UK. Their study is unusual in that it looked for (and purports to have found) a link between luminance and headlight glare, rather than quantifying glare exposure by looking at illuminance or intensity. Luminance surely plays a valid part in a discussion of discomfort glare, particularly at close distances between the glare source and the observer, and it seems this study, with its wide range of recorded luminance levels, might help identify a threshold of headlamp luminance above which it becomes uncomfortably glaring. This could potentially enhance the understanding brought about by existing research – not cited or noted in the TRL report – finding little or no correlation between a range of headlamp lit-area sizes and discomfort glare when intensity is held constant (i.e., between headlamp luminance and discomfort glare).
Nevertheless, the TRL study reads as if the authors have reversed into a spot without realizing it’s a valid parking space or connecting the dots between what they did with the car and where they now find it. It contains numerous small errors of fact, stating “LED bulbs produce directional light [and have] no filament, which in halogen and Xenon bulbs tends to burn out over time”, and “Regulations contain maximum luminous intensity […] at various test points in relation to the driver” (emphasis added). It puts forth misdefinitions and misabbreviations, using “matrix LED headlamps” as a synonym for ADB, and referring to AFS as “AFLS”. They’re not big enough by themselves to derail the study, but the number and nature of errors suggest a dismaying unfamiliarity with the subject matter. So does the authors’ puzzling note of experiencing insurmountable “challenges” in converting luminance camera readings to headlamp intensities (a thing which cannot be done).
The study rests upon pillars which appear structurally unsound. It parlays layer upon layer of subjective impressions and guesses into what is treated as data. Half of this comes from an RAC study which asked leading questions, made more so by the content and tone of the accompanying press releases and outreach, inviting respondents to vent their exasperation over perceived glare. And the on-road research which was this study’s other input stream involved observers sitting in the back seat of a car, pressing a button whenever they thought they saw glare they thought they might object to if they were driving. There are pages and pages of photographs vaguely showing oncoming headlamps or arrays of traffic lights, and some of the images are described as if a photograph can accurately show glare.
Throughout the report, there is an apparent assumption of truth of an unexamined position that discomfort glare is a dire safety threat – a fact neither in evidence nor quantified in any other manner.
Headlight glare is real. There is an arguable case to be made that it ought to be better controlled than it is, even in countries like the UK where relatively stringent glare-control measures are already in effect (versus North America, for example, where that is not at all the case). There very well might be a link between discomfort glare and traffic safety – probably a circuitous, difficult-to-discern link involving the likes of distraction and rage, both of which are already known to degrade traffic safety. The matter ought to be systematically studied with thoughtful, appropriately informed scientific rigour. But this study doesn’t appear to do that. The UK College of Optometrists’ applauding call for UK regulators to use it as grounds to mitigate the effects of disability glare is difficult to reconcile with the content of this study, which does not examine disability glare; are the optometrists conflating disability glare and discomfort glare, or…?
All in all, it is difficult to read this study without questioning its scientific merit and value. It seems a costly, pointless pity, at best, if public policy were to be changed based on this work. Perhaps it can be regarded as a first, faltering step toward sturdier, more objectively-based research, but there is danger that it will be taken as authoritative, when in fact it is deeply flawed.
There is one relatively meaty bit: the authors say, “Lighting regulations are currently based on testing the output of headlamps in relation to luminous intensity at various test points defined in relation to the headlamp itself, not the potential observer. Existing requirements in vehicle lighting regulations may therefore not be sufficient to address issues of glare from vehicle lights”. That’s an idea worth examining, with appropriate scientific rigour.
