There are a range of factors responsible for difficulties with night driving. Below are some common causes of poor night vision.
In low light conditions our pupils dilate (get bigger) to let more light into the eyes. An unintended consequence is that we become more susceptible to glare – from opposing headlights to street lamps. This can make driving more difficult.
Perhaps surprisingly, the optical performance of the eye is much worse when the pupils are dilated. As a consequence, if your glasses prescription is no longer accurate, your worsened eyesight will be more noticeable in low light conditions, as opposed to bright conditions.
Similarly, many people who don’t wear glasses find that their vision is adequate during the day but poor at night. Usually there is a low level of hyperopia, myopia or astigmatism behind this, which only becomes apparent with night driving.
At night, the retina, the light sensitive tissue inside the eye, also adjusts itself to become more sensitive, but it needs time to do this. To properly adapt, this can take minutes to hours.
As we get older, it takes longer for our retina to adjust to dark conditions, particularly if we have been exposed to bright conditions. A few hours in bright sunlight during the daytime can affect our ability to adapt to night conditions, even hours later.
The first step to improve your night driving vision is to determine the cause. Seeing an Optometrist for an eye test is an excellent first step.
There is a minimum standard of vision required to drive in New Zealand. Many people require glasses or contact lenses to meet this standard.
Having clear distance vision allows you to identify hazards ahead of you and react quickly. It is important to be able to recognise street signs, and road signs. Having a quick reaction time is critical to avoiding hazardous situations. Speed is an important factor as well. For example, it takes four times the distance to stop when driving at 80km/h compared to 40km/h. Therefore it is critical that your distance vision allows you to react as quickly as possible.
Having two eyes allows us to see the world In 3D and judge distances. We need good depth perception to make sure we have adequate following distances and can make manoeuvres at the appropriate time. None of us are perfect, and often one eye does not see as well naturally as our other eye. Wearing glasses or contact lenses to make both eyes as clear as possible can improve depth perception. This is useful for many people, even if they have “perfect” vision in one eye. There is a certain standard that each eye must achieve to hold a passenger or heavy vehicle licence. Having only one functional eye is not a prerequisite for a private vehicle licence, but caution is needed.
Drivers must be able to see 140 degrees field of view to hold a licence. Good peripheral vision allows us to locate hazards that are not immediately in our line of sight. Working and properly adjusted side and rear view mirrors are essential. Many eye conditions can affect peripheral vision including Glaucoma and genetic conditions such as retinitis pigmentosa.
When driving we must be able to change our focus from the road, far away, to things much closer, like the dashboard. A process called presbyopia develops in everyone from the age of 45 where the lens in the eye loses its ability to change focus. People who have enjoyed good long distance vision for most of their lives start to notice the dashboard getting harder to see. In this case, progressive or bifocal lenses can help to increase the range of vision.