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Effects of dehydration on driving

Now we’re starting to get into the warm weather you are more likely to be travelling places either on holiday or for day trips, and you will be losing water from your body more rapidly. If you don’t replace your fluids then you become dehydrated and this can have a profound effect on your body – enough to serious affect your driving or riding. The types of drivers most at risk are:

  • truck drivers who may not want to stop because they’re on a schedule
  • holiday makers on long journeys (including their passengers, if the sun shines directly on them)
  • taxi and bus drivers who may not be able to leave their vehicles frequently
  • driving instructors who are travelling to and from appointments and are with clients during much of the day
  • motorcycle and scooter riders in full protective gear
  • forklift drivers in busy warehouses
  • farmers and agricultural workers working in fields on open tractors, on quad bikes or motorbikes, or in tractors without air conditioning

This article is important reading for those of you planning a road trip this summer, those who drive with children, motorbike riders (riding is more physically strenuous than driving) and those who drive long distances for a living.

Recognising dehydration while you are driving

If you are older you are less likely to feel thirsty even if you are dehydrated. If you ignore your thirst for long periods of time, your body will be less likely to warn you of it. Children will often ignore thirst, and babies can’t express their thirst, so they are at risk of dehydration, too, when they are a passenger on long distance journeys. Passengers may feel the pressure to not drink if you are on a tight schedule and don’t want to stop.

A normal intake of liquid for an adult is around two litres a day. If you are doing physical exertion in hot weather you might need three or more litres. If you have spent time at the beach and not had much to drink, by the time you get in your car to drive home you are most likely slightly dehydrated. Thirst is an indicator that you are already dehydrated – in fact, once you feel thirsty you are approximately one glass of liquid dehydrated. The simplest test you can do is to check the colour of your urine: if it’s clear or light-coloured, you are hydrated; if it’s dark yellow or amber, you are dehydrated. You can download a urine colour chart to help.


Once you go past the initial thirst you are likely to get one of more of the following, any of which can happen even with mild dehydration:

  • Headache
  • Tiredness
  • Dry mouth, lips and eyes
  • Halitosis (bad breath)
  • Dizziness or light-headedness

As you can see, some of these have the capacity to affect your driving. If you are feeling tired and have a headache from being dehydrated then you will have less concentration when driving. The cure for this is to stop, find a shady place and drink more fluids. It can help to eat food with high fluid content, too, because these contain minerals that your body will need to replace through sweating. Dehydration symptoms can often manifest as hunger symptoms because the body is looking for any source of liquid – bear in mind that if you find yourself snacking at night, you might just be lacking water rather than be actually hungry.

As your dehydration progresses, your ability to control a vehicle decreases. If you don’t treat your dehydration at this more advanced stage, the next stage could see low blood pressure, rapid heartbeat and breathing, fever symptoms, irritability and confusion and, in the worst cases, delirium and unconsciousness. For any of these symptoms medical intervention is likely to be required as simply drinking liquids may not be enough and you may need an intravenous drip. In the worst cases, the blood gets so thick that it won’t flow, and it can cause death. There’s a more detailed explanation of the effects of dehydration here.

What to do when driving

If you are going on a long journey (more than an hour or two), make sure that everyone in the car, particularly children, has liquid in a format that’s easy to access, for example a water bottle or small carton with straws. Plan to stop every couple of hours for a break and toilet stop. If you are on a motorbike remember that you are likely to sweat more, and if you stop or are riding slow in the hot sun, you can overheat quite quickly. Plan some stops.

If you drink a lot of strong coffee the caffeine can act as a diuretic, but you would need to be drinking several cups. A diuretic causes you to lose more water, therefore they are not good to drink to stave off dehydration.

Eating foods rich in water can help maintain your hydration over longer periods as the water content is released more slowly and they contain other nutrients. Cucumber is 96% water and many fruits such as apricots, pears, apples, melon, oranges and strawberries are more than 85% water. Fruit will have the effect of making you feel fuller, too.

If you start feeling thirsty, have a drink before other symptoms arrive and start to affect your driving.

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Darren has written over 3000 articles about driving and vehicles, plus almost 500 vehicle reviews and numerous driving courses. Connect with him on LinkedIn by clicking the name above

Posted in Advice, Car, Driver health, Driving Instructors, Heavy Vehicle, Motorbike