Dehydrated driving can be just as dangerous as drunk driving. A recent study found that mildly dehydrated drivers made as many errors — like lane drifting, late braking, or crossing over a rumble strip — as people who get behind the wheel with a blood alcohol concentration of 0.08 percent.
Using driving simulators, researchers at Loughborough University in the U.K. put a group of men to the test, once when they were sufficiently hydrated and again when they were slightly dehydrated. To prep for the hydrated test-drives, the guys drank 85 ounces of fluid throughout the previous day and 16 ounces of water the morning of the test. Then, leading up to the dehydrated tests, they consumed just 25 percent of that. "The actual level of dehydration we produced in these men was very mild," says lead study author Phil Watson, PhD. "We wanted to simulate real-world conditions, such as when you've had a particularly busy day and were unable to take breaks for drinks."
Mild, maybe, but it was enough to significantly skewer these guys' driving abilities. They made twice as many gaffes when dehydrated as when hydrated. This sharp spike in errors that could lead to driving accidents matched what Watson observed in his previous studies, when participants used the same simulator after downing 2.5 ounces of vodka.
Just like alcohol, "mild dehydration has been shown to reduce concentration, slow reaction times, impair memory recall, and produce negative effects on mood," says Watson. "All of these factors can impact our ability to safely drive a motor vehicle."
The lesson is simple: Hydrate before you drive and don't avoid drinking up before a long trip to avoid bathroom stops. Plain water is always a solid choice, but other options like seltzer, juice, tea, and milk can do the trick too. Even coffee can give you the fluids you need, along with a caffeine jolt to help your focus. Watson says the idea that coffee is dehydrating is a common misconception. That would mean coffee causes your body to lose more fluids than it provides, but that's generally not the case. "There's some evidence that if a beverage contains very large quantities of caffeine — say, more than 400 mg, which might be found in some very strong coffees — then the body will retain less of the fluid in the beverage." But that still doesn't make it dehydrating.