Company car and van drivers that fail to keep themselves sufficiently hydrated make as many mistakes on the road as drink or drug-drivers, according to a new study.

The body loses and needs to replace approximately two to three litres of water daily. However, as every individual’s body responds differently and weather conditions and activity levels also influence dehydration rates, experts say there is no set amount of water that drivers should consume on a daily basis to keep suitably hydrated.

Professor Ron Maughan, emeritus professor of sport and exercise nutrition at Loughborough University, who led the study, told Fleet News: “They may need to consume more or less .

“Even in summer, the weather varies from day to day, so plans need to be flexible. The chauffeur might be very sedentary but the delivery driver can be very active, so there is no simple answer.”

However, he added: “The main point is that drinking should be sufficient to meet needs and deliberate restriction because of access to toilets should be avoided. If drivers are passing only very small volumes of dark coloured urine, they have not been drinking enough.”

The study of 11 male drivers over two days revealed 47 driving incidents during normal hydration tests. However, when the men were dehydrated the number of driving incidents during the two-hour simulator drive more than doubled to 101 – a similar number to what might be expected of someone driving under the influence of drugs or alcohol.

With fluid deficits of 4% and more, severe performance decrements may be observed as well as difficulties in concentration, headaches, irritability and sleepiness, and increases in body temperature and in respiratory rates. Dehydration that causes a loss of 10% or more of body weight can be fatal.

The level of dehydration induced for the tests was mild and could easily reflect that of individuals with limited access to fluid over a busy working day, said the Institute.

The study concluded: “The results suggest mild hypohydration produced a significant increase in minor driving errors during a prolonged, monotonous drive, compared to that observed while performing the same task in a hydrated condition.” The driving incidents recorded included lane drifting, late braking and touching or crossing the rumble strip or lane line.

As a result, one of the study conclusions was that “driver education programmes should also encourage appropriate hydration practices”.

Maughan, who is also chairman of the European Hydration Institute’s Science Advisory Board, said: “There is no question that driving while under the influence of drink or drugs increases the risk of accidents, but our findings highlight an unrecognised danger and suggest that drivers should be encouraged to make sure they are properly hydrated.

“To put our results into perspective, the levels of driver errors we found are of a similar magnitude to those found in people with a blood alcohol content of 0.08%, the current UK legal driving limit. In other words, drivers who are not properly hydrated make the same number of errors as people who are over the drink-drive limit.”

Driver errors account for 68% of all vehicle crashes in the UK. In light of the research, the European Hydration Institute has urged drivers to be cautious.

Institute director Jane Holdsworth said “Anecdotal evidence suggests that many drivers avoid drinking on long journeys to minimise bathroom stops, yet we know that even mild hydration can cause symptoms such as headache, tiredness and lethargy. Driver error is by far the largest cause of road traffic accidents and we wanted to test whether mild hydration had an impact on the incidence of common driver errors.”