Daytime Running Lights [DRL] commonly only illuminate the front lights of a vehicle, and not the rear lights. (See the photograph below.)
Usually, they also do not illuminate the dashboard lights for the speedometer and other instruments and controls. This is intended to alert drivers to the fact that only the DRLs are operating and therefore only the front lights are on, but as most drivers have never been adequately informed about this scenario, many just assume that there is a fault with the dashboard lights and drive on, unaware of the danger they are causing for themselves and others.
DRLs were brought into use quite a few years ago because of concern from environmentalists that using “unnecessary,” additional electrical power in vehicles does fractionally increase fuel consumption and therefore it also increases pollution. My response to this — as a person who has been an enthusiast conservationist and now environmentalist, for over 50 years — is the question: “When does it become worthwhile to risk human lives for the sake of preventing undeniably miniscule amounts of pollution?” There are many much more serious sources of pollution that would give far greater gains through their removal and create safer conditions rather than additional risk for people. In every context, they would achieve more.
What are the benefits of driving with headlights on at all times?
Through targeted research, it has been shown that using low beam headlights or DRLs — at all times of day and no matter how good the weather and natural light are — increases the conspicuity of your vehicle and thereby reduces your chance of having a vehicle or a person emerge into your path and trigger a collision. Depending on exactly how the results are measured, the reductions in risk vary between 14 percent and 28 percent. As having a vehicle or Vulnerable Road User [VRU] come into the path of your vehicle is clearly a very dangerous type of collision, it is equally clear that even a slight reduction of the risk level is a very good thing, and a cut of 14-28% is actually a major issue. So it is very wise indeed to drive with low-beam headlights or DRLs on at all times.
Indeed, in Sweden using headlights at all times of day, even in full sunshine, was recognized as giving important safety benefits as long ago as the late 1960s, and since then it has been the law there to do so.
To put this into further context, Sweden has long been one of the two most frequent leading nations in the world for having the lowest rates of road deaths (the other such country being Great Britain). So it is fair to assume that they know what they are doing.
What are the automakers doing wrong?
The ridiculous scenario of cars using DRLs being driven at night with no rear lights on has been going on for years, and the situation is a dangerous disgrace. (And even the majority of other drivers who see cars with no back lights on at night typically don’t know what the cause is… There’s that lack of adequate and accurate education again.)
The situation could be rectified in a heartbeat if car makers simply combined DRLs with the sensors used for “automatic headlights” (which come on only when it starts to get dark). Doing this would mean that in good light only the front DRLs would illuminate, thereby saving that very small amount of extra emissions for which they were intended. However, even this would require the sensors to identify poor light as opposed to imminent, proper darkness, because it’s the lack of rear lights during bad atmospheric visibility — such as heavy rain and spray, or fog, or falling snow, that is the most dangerous scenario which current DRLs typically fail to rectify.
What’s the safest thing for drivers to do?
The safest answer is extremely easy. Simply turn on your proper headlights every time you get into the car, to drive. Not DRLs, not automatic headlights — proper headlights, on low beam… at ALL times.
The conspicuity of your vehicle — again, at all times — is a major factor in your safety, so feel free to enhance it.
And those extra emissions? You will have to work out for yourself whether the tiny additional amount involved, by switching from DRLs to low-beam headlamps in your own vehicle, are worth anything even remotely close to the value of your own life and/or the lives of any other people in your vicinity.
Young people might say that the decision is “simples!”