Over at Freakonomics, Steven Dubner wonders about the ethics of dimming headlights in the face of oncoming traffic (e.g., why do drivers perform this everyday altruism in the face of seemingly small consequences for not doing so). He also asks: “What I’d like to know is whether the benefit of dimming your headlights — that is, the benefit of not blinding the oncoming driver — is indeed larger than the benefit of keeping your own brights burning?”
This is a question that people who study vision and lighting and driving have thought about a lot. To summarize a conversation I had with Michael Sivak, at UM-TRI, there’s three distances involved here: The legally required distance to dim one’s lights in the face of oncoming vehicles, the optimal distance for maintaining one’s own visibility (and, I suppose, not blinding the other), and then what drivers’ actually do. Readers of this blog will suspect the last factor does not often match up with the two previous factors (and, I should add, as with many things in driving, the scientific issues around night-time illumination are much more complex than the “average expert driver” — i.e., everyone — realizes).
From a pure visibility point of view, opposing drivers should never dim their lights, but should drive on high beam through the whole meeting process. There are, however, certainly other reasons for dimming the lights, such as discomfort glare and fatigue over a longer period with repeated high-beam meetings.
The study of Helmers and Rumar (1975) indicates that the improvements in the low beam since the fifties and sixties have been considerable. That is probably the main reason why the high-beam visibility curve and the low-beam visibility curve in later studies do cross each other—at least when the intensity differences between the two opposing high beams are not too large (about triple or less).
When the two opposing high beams differ substantially in intensity, the visibility differences between the two opposing drivers are quite pronounced (see Figure 3). In such situations, it is most probable that the driver with the weaker high beams will be the one who wants to initiate the dimming, because the driver with the weaker high beam experiences substantial disability and discomfort glare. On a straight, flat road, such a driver will want to dim the high beams at a very large distance between the vehicles.
An early dimming means that both drivers will have to drive on low beams for an extensive part of the meeting process. However, as stated above and illustrated in Figures 2 and 3, at larger separations low beams normally offer shorter visibility distances than high beams. This means that an early dimming leads to short visibility distances for a greater distance traveled, for both opposing drivers.