It's robustly demonstrated in all the world's DRL studies that most of the safety benefit from DRLs is in reduction of *angular* collisions with pedestrians, cyclists and other vehicles -- not in head-on or near-head-on collisions. It is essentially impossible to produce a lamp that gives good high beam performance at full voltage, AND can be run at reduced intensity such that it produces a wide enough cone of illumination to give significant improvements in angular conspicuity without producing far too much glare on axis. High-beam DRLs tend to illuminate at the maximum allowable intensities on axis (= excessive glare) but at or near the minimum allowable intensities laterally off-axis (= insufficient angular conspicuity, therefore minimal actual safety performance benefit). In addition, high-beam DRLs share the disadvantages of all headlamp-based DRLs: They consume so much power that their use is akin to opening the refrigerator door, pulling up a chair and using the fridge light to read a book, and they are too often improperly used instead of full-voltage headlamps after dark, because they create the appearance of a light beam in front of the car -- drivers and cops often can't tell the difference, or don't care. Come up to Canada sometime and see for yourself! This use of headlamp-based DRLs after dark creates various unsafe situations: Cars unlit from the sides and rear, cars producing much too much glare for other road users and too much backdazzle in bad weather, etc.

Low-beam DRLs have the energy-inefficiency problem, as well as the conundrum that a good low-beam light distribution is opposite what is needed for a good DRL light distribution.

And, there is the bulb life problem with all headlamp-based DRLs. The effective decrease in lifespan pushes makers to use long-life bulbs, which give reduced luminance and poorer beam focus, resulting in diminished headlamp performance after dark.

High beam headlamps aren't frequently used, so there's less impact on the effective useful life of a device a driver might be unhappy with if it were to last a shorter time than he expected (ask any VW Beetle owner how he likes having to replace low beam headlamp bulbs every other month). Also, with high beam DRLs, there's no need to worry about turn signal intensity.

High beam DRLs don't work very well (too much intensity straight ahead, not enough intensity out to the sides), but fog lamps are not legal as DRLs in the US, only in Canada. Low beams don't make very good DRLs at all. If one is constrained to using only the lighting equipment on a non-DRL vehicle, without adding any new lighting devices, then the front turn signal DRL is definitely the best pick, all things considered.

The best DRLs are functionally-specific ones. The second-best ones are the front turn signals burned full time.

- Daniel Stern