According to a recent study published in the journal Physiology and Behavior, driving while dehydrated is just as dangerous as driving drunk.
In addition to drivers not bringing adequate beverages on long road trips, many do not stop intermittently to drink fluids when thirsty. In a haste to reach their destination, some drivers avoid all food and drink throughout their road journey. Doing so, however, may prove to be a neglectful habit that can endanger themselves, other occupants, as well as other drivers on the road.
“In the study, researchers from Britain’s Loughborough University carried out simulated tests on drivers when they were both hydrated and dehydrated. The tests included a two-hour monotonous drive with bends, a hard shoulder, rumble strips, and slow-moving vehicles that needed to be passed. On one day, participants were given nearly a cup of fluid to drink per hour and on the dehydration day, they were given just a few sips of liquid per hour.
When participants were hydrated, there were 47 driving incidents, but when they were dehydrated, that number was more than doubled at 101. The errors also increased during the two-hour period and were worse during the last leg of the drive. Those incidents included lane drifting, late braking, and touching or crossing the rumble strip or lane line.” 
The environmental conditions within an automobile or truck can also be quite conducive toward causing dehydration. Particularly during the hot and sunny summer months, the ambient air in a vehicle will naturally be drying. Turning up the air conditioner to compensate for the radiant heat emitted from the hot, black roadways only further contributes to the dehydration phenomenon. Windy days will even further exacerbate the situation.
Driving on today’s busy highways and byways, as well as congested city streets, requires a high level of concentration (and patience). The eyes and ears are especially engaged as are the hands glued to the steering wheel and the right foot on the pedal(s). Such focus of attention and energy demands a significant uptick in energy and response time. Interstate driving, in particular, can be especially exhausting because of the higher speeds which are now allowed by law.
All taken together, the task of driving long distances, on roads that are often in disrepair is not as easy as it once was. The stresses, both physical and psychological, can take a toll in such a way that low level dehydration can easily result. For these and other safety reasons, it is highly advisable that water or other beverages be brought on any short or long distance trip so that they can be sipped on when needed.
How is dehydration linked to poor driving?
“Lead researcher Ron Maughan, PhD, tells Yahoo Health that our brain function becomes conserved when we’re dehydrated, which can then impact how well we drive.
That brain conservation can create a whole range of issues, says nutritionist Kim Larson, a spokesperson for the Academy of Nutrition and Dietetics. Dehydration impacts our mental clarity, reaction time, focus, concentration, thinking, and even our mood, she says, and the impact isn’t just felt when we’re driving. “Even if you’re sitting at a computer, you can experience the same symptoms,” she says.
Dehydration can also affect your blood volume, Larson says, which can lead to headaches, lethargy, and an overall fuzzy feeling — not exactly ideal symptoms to experience while driving…or any other time.” 
What are the best drinks to bring on a long-distance road trip?
Lemon water, orange juice, and grapefruit juice are some of the more healthful alternatives that can keep you hydrated at the wheel. So are various herbal teas which do not contain any diuretics. Diluted fruit juices can also serve to quench one’s thirst and somewhat satisfy the appetite when food stops are not taken.
One of the quickest and best way to rehydrate is to drink coconut water while on the road. The fresher the coconut water, the better – as it will also help balance electrolytes as well as replace trace minerals. It will also stave off hunger attacks while driving.
Both caffeinated drinks and alcohol are highly discouraged for obvious reasons. Coffee is a diuretic as is black tea. Green tea would be the best choice in a glass-lined thermos. All forms of alcohol are not only diuretics, they also grossly impair judgment and slow down reaction time.
Of course, when significantly dehydrated behind the wheel, drinking practically any fluid (except alcohol) is preferable to not drinking at all.
Whenever water is kept in the car, it is important that all plastic containers be avoided. Glass bottles or steel thermoses do not have toxic petrochemical components that will leach into the water and are therefore recommended. These chemicals, such as BPA, can have some nasty effects.
Unless drinking juices while cold from a cooler, they should not be kept in plastic bottles either. Vehicles can get warm even in the cooler months on a sunny day and therefore all plastic bottles ought to be kept out of the sun and in a ice chest.
Endnotes and Sources
 Yahoo Health