This story is the second in a two-part series on tailgating. You can read the first installment here

So you’re in a hurry, and the guy driving in front of you on the Northway is going way too slow. You ease your car closer and closer until you’re ten feet off his bumper, certain he’ll get the message and speed up. But what happens if, at the same time you get closer, a deer runs in front of his car and he slams on the brakes?

The fact is, in that scenario you’re likely to be in an accident. And depending on how fast you’re driving, that accident could mean serious injury or even death.

And that’s not just a fact, it’s the law – the Laws of Physics.

Physics is the study of matter and its motion. And physics tells us that any matter in motion (such as a car) needs time and distance to come to a complete stop. But when you’re driving 60 miles an hour just ten feet off someone’s bumper, you don’t have much of either.

So here’s the math, thanks to Mary Odekon, the Physics Department chair at Skidmore College. The stopping distance for a car includes two things; “The distance you travel during your reaction time, and the distance you travel while braking.”

So let’s see how long it would take you to stop.

First is reaction time. Roger Ratcliff is a professor in the Cognitive Modeling Lab at The Ohio State University in Columbus. He and his colleagues study the reaction times of people by using computing simulators outfitted with a steering wheel and a brake pedal.

Ratcliff says reaction time is not as simple as seeing a stimulus, such as taillights coming on, and then responding. Rather, reaction time is quite complex.

Reaction time, he says, is variable – sometimes people react quickly, and sometimes slowly. There’s no real explanation for it; our reaction time just varies. “On top of that you might get distractions that might slow you down as well,” says Ratcliff. Distractions such as that cell phone you’re talking on, or the radio you’re adjusting, or the text you’re sending.

Even then, says Ratcliff, you might be okay – sometimes you have a faster reaction time while you’re distracted, or a slower reaction time when you’re really paying attention, so it balances out. The real problem though comes when those two things happen at the same time. “The killer is when you are distracted and make a slow response.”

Ratcliff says distraction behind the wheel is bad. So bad, he says, it’s very similar to the effect of sleep deprivation.

So what’s a good reaction time? “The average reaction time for young drivers – college students – is about 800 milliseconds.” In layman’s terms, that’s 8/10 of a second. A good reaction time, he says, is 700 milliseconds, or 7/10 of a second.

But what does that mean? In order to understand that figure, we need to look at the second factor in stopping distance – speed.

Say you’re going 60 miles an hour. That’s one mile a minute, or 5,280 feet. So when you’re driving 60 miles an hour, says Odekon, you’re traveling 88 feet each second. To put that in football terms, every second you’re traveling the distance from the goal line to the 30-yard line. In three and a half seconds, you’ve traveled more than the length of the entire football field.

Now, remember that reaction time? 8/10 of a second. That means at 60 miles an hour you’ve traveled 70 feet before your foot ever hits the brake.

So here it all is in practical terms. Many drivers on the Northway travel just one or two car lengths off the bumper of the car in front of them, or about 15 to 30 feet. So before your foot is halfway to the pedal, you’re in the other car’s trunk.

Of course, that’s not quite right. That would only happen if the other car has stopped without traveling any additional distance, but that car too has braking time before it comes to a complete stop. But the point is, says Ratcliff, that driving too closely can easily lead to a major accident. “I deliberately stay back when I’m driving because of knowing this.”

And all this supposes a reaction time of 800 milliseconds. But Ratcliff says it could take much longer, depending on distractions and other circumstances – as much as two seconds. “So shut your eyes on the road for two seconds, and if someone stops you’re completely…” Well, let’s just say out of luck.

“If it took two seconds rather than one second to react, there would be carnage everywhere.”

So what’s the answer? Sergeant Andrew Prestigiacomo of the Saratoga Springs Police Department says the old rule of thumb is still a good one. Drivers should keep at least “one car length per 10 miles an hour” between their car and the one in front of them. And keep the distractions to a minimum if you want to reduce your odds of being in an accident.