DRLs are not the bloody safety menace the anti-DRL types claim they are, but neither are they the next seatbelt or disc brake. The potential safety benefit from optimised DRLs is real but rather small, and non-optimised implementations can introduce safety-negative effects that can cancel or, in extreme cases, reverse the safety benefit. Most arguments against DRLs, whether or not their exponents realise or admit it, are arguments against the problems caused by particular implementations of DRLs, not against the concept itself.

Headlamp-based DRLs, both high and low beam, are very common in North America. They are also really the wrong way to do it, for numerous reasons:

Low beam DRLs present a bulb life issue. Cars with headlamp-based DRLs usually come equipped with Long Life bulbs. These are exceptionally warranty-friendly and conducive to use as DRLs due to their long life, but their output is low and the beam focus (and resultant beam reach) they produce is poor, due to the filament modifications made to get extra long life out of them. For example, the high-performing version of one particular kind of headlamp bulb produces 1700 lumens over a lifespan of 450 hours, and its filament coil is focused such that the low beam reach is approximately 85 metres. The long-life version of that same kind of headlamp bulb produces 1460 lumens over a lifespan of 1200 hours, and its filament coil focus is such that the low beam reach is approximately 70 metres. Even if we disregard bulb burnouts that still happen more frequently with DRL operation of headlamps than without, this is not a good tradeoff: A small potential increase in daytime safety for a very large and definite decrease in nighttime safety.

And, photometrically, one cannot have a lamp that serves both as a good low beam and as a good DRL. A good low-beam light pattern is asymmetrical, directing most of the light downward-rightward with only minimal light towards upward-leftward and eye-level leftward lateral angles (RH-traffic countries), which means it's an ineffective DRL. A proper DRL puts light in those upward & lateral leftward angles (more technically, a proper DRL has a symmetrical light pattern in the general shape of a horizontal oval or oblong centred about the DRL's axis). See for example any of the recent Mercedes models with projector halogen low beams. These low beam DRLs are practically invisible until ambient light levels are low enough that full headlamps (+ tails & markers) ought to be on.

High beam DRLs are quite common on North American roads, too. They produce a symmetrical beam pattern, but are essentially impossible to implement well: If the intensity is high enough that there is a useful amount of light directed towards lateral angles, there is far too much light on axis (straight ahead), causing genuine glare problems. If the axial intensity is reduced enough to control glare properly, the cone of illumination narrows so much that there is essentially no light from the DRL at angles off axis. Because most of the safety benefit from DRLs under North American conditions is in avoiding angular collisions rather than head-ons, this is significant. There is also the issue of high beam bulb envelope blackening due to deposited tungsten -- halogen headlamp bulbs operated significantly below their rated voltage don't self-clean as they do when operated at full power, so the bulb glass blackens. This condensed tungsten will clean up and redeposit with sufficiently long full-voltage operation of the high beams, but most drivers need and use their high beams only for short periods, so the diminution of their power due to envelope blackening is a significant safety performance concern. Both low and high beam headlamp-based DRLs are so inefficient given the power input and functional output that using them is very much akin to opening the fridge door, pulling up a chair and using the fridge light to read a book. System wattage in DRL mode ranges from 60 to 240 watts. The fuel consumption is substantial enough that General Motors petitioned the U.S. Environmental Protection Agency -- successfully -- for the DRLs to be disconnected during Federal new-vehicle fuel economy and emission certification testing. Dedicated DRLs (not headlamps in any operating mode) will be mandatory in Europe and all other countries applying ECE Regulation 48 (installation of lighting and light-signalling devices) starting in 2011; this kind of low-wattage, high-efficacy DRL is already appearing on European cars, even in North America.

Compounding the problems with headlamp DRLs is the way the regulations are written regarding illumination of a vehicle's parking, tail and marker bulbs with DRLs. Parkers, markers and tails _must_ be illuminated with full-power low-beam DRLs unless the vehicle is equipped with an ambient-light sensor that will turn on the parkers, markers and tails when ambient light levels drop low enough to require the vehicle's conspicuity lights. This presents a manifold conundrum:

• Having the parkers, markers and tails on during the day adds between 20 and 84 watts' power consumption to the 60 to 240 watts' power consumption of a headlamp-based DRL system. Now we're looking at between 80 and 324 watts...definitely not trivial in terms of fuel burned and pollution produced.

• Having the taillamps on during the day significantly reduces the visual contrast between the brake lamps being on and their being off. This has been shown to worsen the speed and accuracy with which following drivers react to a vehicle's brake lamps.

OK, so what do we do if we want to have a headlamp-based DRL system, but not run the parkers, markers or tails during the day, and we don't want to install an expensive and troublesome automatic light control? We've got three options permitted by the applicable regulations:

• Run the low beams at reduced intensity (which aggravates the already poor DRL performance of low beam headlamps as described above)

• Run the high beams at reduced intensity (which makes DRLs of limited effectiveness and high glare, as described above)

• Run the high and low beams in series with one another (which makes fairly effective DRLs on some kinds of headlamp systems, but is very fuel inefficient).

For retrofit purposes, the best implementation is the full-time operation of the front directional signals (except, of course, when they are operating as signals). There is some ambiguity introduced into the front turn signals per se with this implementation, but it does not appear to be of worrisome magnitude. Directional signals produce a light distribution with a wide conspicuity angle, are generally well located for DRL service at the outboard edges of the front of the vehicle, consume considerably less power than any headlamp-based DRL implementation, use light sources of generally much longer life than any headlamp bulb, do not encourage improper nighttime use of lights, and do not require additional lighting devices to be added. Most recent Cadillacs, Chevrolet/GMC large vans and minivans, some Toyota and Lexus models, certain new Lincolns and assorted other vehicles use this implementation.