Staying hydrated behind the wheel is essential for driver concentration.
New research, reported by the Daily Telegraph, claims having as little as five sips of water an hour can have the same detrimental impact as being over the drink-drive limit, potentially putting road users at risk.
The study, carried out by a team at Loughborough University , suggests drivers make more than twice as many mistakes when they are just mildly dehydrated, compared to when they have a sufficient intake of water.
Male drivers had to do a series of tests using a laboratory-based driving simulator over two days.
This included a two-hour continuous drive on a dual carriageway, with bends, a hard shoulder and simulated auditory 'rumble strips', as well as slow-moving vehicles that had to be overtaken at particular points.
The men were provided with 200ml of fluid every hour on one of the days to keep them normally hydrated, while on the other they were only given 25ml of fluid every hour to look at the effects of dehydration.
Driver errors, such as lane drifting, late braking and touching or crossing the rumble strip or lane line, dramatically increased from 47 during the normal hydration test to 101 during the dehydration test.
Professor Ron Maughan, who led the study, says the findings highlight an unrecognised danger on the roads.
He recommends that drivers make sure they are properly hydrated, particularly when embarking on long journeys.
It is the first study into dehydration, driving errors and accident risk.
RAC spokesman Simon Williams said: "While the serious effects of drink and drug driving are well known, much less attention is paid to other factors that can impair driving performance - so studies such as this are to be welcomed.
"It appears that just as keeping vehicle fluids like oil topped up helps maintain the health of your vehicle, it's worth keeping an eye on your own water intake as it may have an effect on your driving."
The levels of driver errors found in the dehydration test were of a similar magnitude to those found in people with a blood alcohol content of 0.08% - the current legal drink-driving limit - stressing the need to stay hydrated.
Dehydration can result in impaired mental functioning, changes in mood, not to mention reductions in concentration, alertness and short-term memory. All this makes dehydrated drivers a potential risk to themselves and others.
The Physiology and Behavior journal published the findings.
Copyright Press Association 2015