WHY DO SOME CARS’ REAR LIGHTS NOT WORK?

QWhy is it some cars do not display any rear lights when they apparently have sidelights on at the front? This is dangerous in inclement weather. I’ve lost count of the number of cars I have come across and cursed the driver for not switching on the lights, only to pass and notice the vehicle is partially lit at the front. How does that happen? – NS

A

What you are seeing on the front of these cars are not sidelights but daytime running lights (DRLs). These come on with the ignition to show the car is running and make it more visible when moving – say, to differentiate it from a row of parked cars so you see it better when pulling out of a junction.

The trouble is, the laws that mandate DRLs only stipulate that they must show at the front, not at the rear. While some manufacturers wire the rear lights to come on with DRLs, they are not required to. The cars you are seeing, therefore, are not those whose drivers have switched on only the sidelights but ones where the drivers have not turned on the lights at all.

It may be because many drivers have become accustomed to using the dashboard illumination as a way of reminding them to turn on their lights or for checking they are on. Unfortunately, most new cars’ dashboard illumination comes on with the ignition and is on permanently. As a result, many drivers think their lights are on (and indeed, may even see a glow from the front of the car, caused by the DRLs) when in fact they are not.

This problem is compounded by the proliferation of automatic headlights, which many drivers treat as “set and forget” – without realising they may not activate in fog or rain. I’ve also heard of drivers who share cars only realising well into their journey that another of the car’s users has flicked the switch from “automatic” to “off ” – this can happen after a service or MOT test, too.

I always advise people to double check the lights are on at the switch or by using the green dashboard light rather than going by the dash illumination. It’s also good practice to switch from “auto” to dipped beams in poor weather, just to be certain.