Night Driving

An image of a road with oncoming headlight glare.Many people complain of increasing difficulty when night driving, stating that it seems ‘worse than last year’. Night driving is difficult, especially on dark narrow roads without white lines or reflective studs.

Our eyes have evolved to use natural sources of light-sunlight and moonlight, and every thing we would see would be because the objects reflected some of the light. Bright objects reflect more light than dull objects, but obviously no object could reflect more light than hits it, also no object absorbs all of the light which hits it. Most objects will reflect somewhere between 80% and 20%. This ratio is important- the 60% difference is the maximum the eyes can cope with, and anything outwith this range will cause glare. There are numerous types of glare, but in this circumstance we are talking about discomfort and disability glare.

Light adaptation- the eye’s decreasing sensitivity to light, is a very quick process- the pupils constrict (which only makes a relatively small difference) and retinal sensitivity soon decreases. By contrast dark adaptation is a much slower process, which can take about an hour or more to achieve fully. In the natural world this isn’t an issue as the sun slowly rises and sets. Artificial lighting can cause problems however, and when night driving, with no other car on the road, our eyes are adjusted for about middle brightness vision- obviously not daylight, but we can tell colours (which can’t happen in full darkness adaptation). Everything we can see is by reflection from our headlights and that maximum 80% minimum 20% reflection holds true- except for perhaps highly reflective road signs, which may reflect more light and cause ‘glare’.

Then we meet a car, and immediately our eyes have to adapt to the new increased light levels- The pupils constrict, and these will only go so far before we feel that familiar discomfort. Light adaptation begins within the retina- immediately everything around the headlights seems darker (almost black), and then light sensitivity begins to reduce, rapidly. Soon the car has passed and the eye slowly begins to dark adapt once more, leaving our vision seemingly poorer for a number of seconds.

Basically the brightness of the headlights isn’t the problem, it is the difference between them and the surround -we need our eyes to work outwith that 60% difference, which they can’t do. Those same headlights would cause no difficulty during the day!

So what can we do? Basically not a lot, although minimising added causes of light scatter and reflection will help.

Ensure your windscreen is crystal clear-inside free of foggyness and smears, and ensure there is no traffic film on the outside and that wipers don’t smear. Clean spectacles help also, should you wear them!

Ensure headlamps are clean and both working- reduced output- say if only one light is working increases the difference between yours and theirs. Ensure you dip promptly (and hope they do also) and try to look along the verge- the further you look from the glare source the less troublesome it will be.

Spectacles may be helpful, but only if there is a refractive error to correct- even a small amount can make a difference, because in the dark, the pupils dilate which cause more blur than would be evident on a sunny day.

Many patients want to try a tint for night driving, which is definitely not recommended- it is advised that any tint darker than 80% transmission (a very light colour in the lens} would be unsafe at night, although some people do drive with darker (and swear by them), in theory because it doesn‘t affect the difference in brightness between the lights and their surround it shouldn’t be any benefit. (It makes the oncoming lights dimmer, but your’s too)

As we age, the natural lens in the eye does start to become yellowed and a little cloudy- this will have started in most eyes by about 60. Some will become bad enough to call a cataract (an opacity on or in the lens of the eye). Cataract certainly would impede night driving ability, but even if the clouding isn’t all that bad, it may still cause increasing difficulty through the years. Any optician can (and will) check for these during a routine eye exam, and can discuss their findings with you.

Finally most opticians would recommend an AntiReflection coating- these reduce reflections caused by the spectacle lenses, which can cause multiple reflections of oncoming lights, however, do be aware that some establishments recommend coatings even if a corrective lens is not required. The coating greatly eliminates reflections caused by the lenses, but if the lens isn’t required it will not help.

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