The Case for Driving Hydration – Methodology

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The Case for Driving Hydration – Methodology

In the previous blog ‘The Case for Driving Hydration – Hypothesis’ we began an in-depth look at Loughborough University’s peer-reviewed and ground-breaking 2015 study on the effects of dehydration on driving ability. Funded in part by the European Hydration Institute, a foundation which works with independent scientists, health and nutrition professionals to advance and distribute knowledge of human hydration and its impact on health, wellness and competence; the research focused specifically on the impact of mild dehydration (hypohydration) on driving performance during a prolonged monotonous task.

Past studies showed that even slight dehydration could result in impaired cognitive function; with changes in mood; and shifts in concentration, alertness and short-term memory.

Around 22 000 people are seriously injured each year in road traffic accidents, with driver error attributed to 68% of all vehicle crashes. With this in mind, the assumption was reasonable that ‘dehydrated drivers may be more susceptible to errors in judgement and/or the successful execution of motor skill’.

To test this hypothesis (the end results of which fully support the case for the Driving Hydration solution), the methodology can be broken down into the following elements:


The group consisted of eleven healthy males, all experienced drivers with more than two years on a full licence and all driving for more than two hours per week. The mean of their physical characteristics was age: 22 years; height 1.75m; and weight 77.4kg.

Experimental Design

Each volunteer visited the laboratory on three different occasions. The first visit was to allow each man a chance to familiarise himself with the environment and the task to be completed in the driving simulator – ‘a full-size, interactive, computer-generated road projection of a dull monotonous dual carriageway, each carriageway having two lanes’.  The remaining two visits or experimental trials (each trial took place over two days) were separated by at least seven days. The men were asked to keep a dietary and physical activity diary on the day before the second visit – as the aim was to mimic free-living conditions, the only restrictions put in place were to consume at least 2.5 litres of fluid spread evenly across the day; no strenuous exercise; and no alcohol consumption on the day before or during the trial. They were then asked to replicate the contents of the diary on the day before the third visit.

Experimental Protocol

All volunteers began their trial in the morning and were asked not to eat or drink anything for up to 10 hours before their morning visit. They were weighed, urine and blood samples were taken before and after the trials, and they were assessed on their subjective feelings related to thirst, hunger, concentration and alertness. Volunteers were separated into two groups: those well hydrated and those with fluid restriction.

During the hydrated trial, participants continued to consume at least 2.5 litres of fluid spread evenly across the day. During the fluid restriction trial, only 25% of the 2.5 litres of fluid (625ml of fluid) was permitted – the aim was to achieve a 1% drop in hydration levels over a 24hour period.

According to the Hospital Hydration Best Practice Toolkit , an online resource provided by the Royal College of Nursing, ‘dehydration is defined as a 1% or greater loss of body weight as a result of fluid loss.’ It goes on to say that we usually experience thirst when dehydration reaches 0.8-2%. Thirst, along with dry mouth and headaches (in addition to the other well-established physiological consequences of hypohydration) is thought to directly produce a negative effect on mood state, with some authors convinced that ‘dehydration-associated impairment of tasks with a large cognitive component is driven primarily by the discomfort and distraction associated with these symptoms’.

Not only that, but according to Dr Rekha Elaswarapu (Natural Hydration Council 2015), our ability to feel thirst reduces and continues to diminish after the age 50. If one considers that the average age of the participant was 22 years, whereas the average age of an HGV driver (according to Skills for Logistics) is in fact 53 years, the startling results of the Loughborough study with regards to driver error are actually conservative when compared to what they might have been had the control group been older. 

Driving Task

The task comprised a two hour continuous drive in the driving simulator. The road had two lanes separated by a barrier, with a hard shoulder on either side of the carriageway; there were long straight sections followed by gradual bends. Drivers were asked to remain within their lane unless overtaking slow moving vehicles.

Those who formed part of the hydrated trial were given 200ml of fluid every hour; while those in the fluid restriction trial were only given 25ml every hour.

Driving Related Measures

Driving incidents were grouped into minor or major groups.

‘Minor incidents’ of driver error were considered to be lane drifting, late breaking, and a car wheel touching (or crossing) the lane line. ‘Major incidents’ of driver error were classified as cases where the car left the lane, hit the barrier or hit another car.

Unobtrusive filming of the task enabled the researchers to determine the cause of the incidents. ‘[Incidents] due to sleepiness (e.g. excessive blinking, eye closure, eyes rolling upwards or vacant staring ahead) were logged as ‘sleep-related incidents’. Non-sleep related incidents (driver distraction, fidgeting or looking around) were also recorded.’

The far-reaching results of the study, which we examine in the final part of ‘The Case for Driving Hydration’ blog series, were startling and underpin our Driving Hydrations solution – a solution proving to be invaluable to transport managers and directors who constantly strive to reduce vehicle incidents related to ‘driver error’.

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