Driver Knowledge Tests

The dangers of driving while dehydrated

We are well and truly into the hot weather and as the summer holidays approach you are likely to be having a break, taking a road trip, spending more time outside in the sun and so on. If you are driving for a living, you’ll be spending more time in a hot vehicle, or in hot protective clothing if on a motorbike. In general, the warmer you are the quicker you lose water from your body. If you don’t replace the water you lose through sweating and breathing, you get dehydrated and that can have some quite serious effects on you and your passengers. They include effects that make it much more difficult and dangerous for you to drive.

The types of drivers that have the most risk of dehydration are:

  • Truck drivers who only want to stop during scheduled breaks so that they don’t miss deadlines or lose time
  • Taxi drivers who don’t want to give up their place in a rank
  • Bus drivers who can’t leave their buses during a shift
  • Driving instructors who are with clients in the car all day
  • Holidaymakers on long journeys
  • Motorbike and scooter riders wearing full protective clothing
  • Farm workers on open vehicles (quad bikes, motorbikes and open tractors) in the hot sun, or in older tractors that don’t have air conditioning.

There is also a risk for passengers in vehicles if the sun is shining directly on them. Remember that babies cannot tell you if they are thirsty other than by crying.

So, if you are planning some long driving missions this summer, here’s what you need to look out for with dehydration and what you can do to counteract it.

What happens as you get dehydrated when driving?

You are constantly breathing and sweating, both of which cause you to lose water. Usually we start to feel thirsty as the first sign of dehydration but in older people this sensation is much less, and other people just ignore it (young children might not know how to express it). Parents are sometimes reluctant to drink much or give their children something to drink because it means more stops when trying to make progress on a journey.

A typical amount of liquid needed for an adult male in temperate weather with light activity is two litres a day (other than that derived from food). If the weather is hot and the activity more intense, more water will be needed to replace that sweated out. Therefore, if you’ve driven to the beach and laid around in the sun, then gone for a swim, and thrown a ball around a bit, you will have lost quite a bit of water. You then need to drive home, but you could have the first signs of dehydration.


The easiest way to tell if you are dehydrated is to look at the colour of your urine. If it’s clear or almost clear, you are hydrated; if it’s darker yellow or orange (and it’s not as a result of a vitamin tablet) you are dehydrated. The initial symptom of dehydration is thirst – when you are thirsty you are already slightly dehydrated. ¬†If you ignore this, one or more of the following symptoms will arise:

  • Tiredness
  • Dry mouth
  • Bad breath
  • Dizziness
  • Headache

A headache, dizziness and tiredness can all affect your safety behind the wheel. At this point, though, you can simply drink or eat water-rich foods and your body should be able to absorb enough liquid. Remember that at the first sign of thirst you could be as much as one full glass of water in deficit – that’s about 200-250ml. More severe dehydration can need immediate medical treatment because the body starts to shut down, gradually becoming less and less able to react quickly.

Driving advice for staving off dehydration

Remember to carry liquid with you when driving. If you get caught in a bad traffic jam because of an accident, you could be stranded in your hot car for some time (sometimes even hours longer). The liquid should be easy to access by you and your passengers – a water bottle is best.

Eating water-rich foods can be a better way of maintaining hydration because the food replaces¬†important minerals that are lost when we sweat and urinate, and the water is released over a longer period of time, meaning that it doesn’t just get quickly processed by the kidneys and expelled as urine. Cucumber contains 96% water, strawberries are 92% water, apricots are 86% water and a banana is 74% water. For a list of water content in fruit and vegetables click here. Foods with a lot of protein and fat take more water to digest.

If you start to notice any of the symptoms, particularly a headache and feeling thirsty, drink plenty of liquid – not just a sip. Then wait 15 minutes to see if it subsides. Driving dehydrated affects your concentration and coordination, so at all times you should be aware of what your hydration levels are.

Darren is an expert on driving and transport, and is a member of the Institute of Advanced Motorists

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  1. […] Cars can heat up quickly. If your car doesn’t have air conditioning and you are sitting in the hot sun, the temperatures inside the car can reach 60 degrees Celsius in just a few minutes if you don’t open the windows. Even with the windows open the temperature can be much hotter than the outside temperature. For this reason you should never leave pets or children in your vehicle – always take them with you. Some cars have sun blinds built into the windows which can help, but still won’t prevent the inside of a car heating up if left in the sun. Read our full article about driving dehydrated. […]