I find it harder and harder these days to ride as a passenger when other people drive. Some of that is just part of
getting old. But it's not just that; as I've noted before, learning to ride a motorcycle has
helped make me a better
driver.
One tactic I've worked on is how much space I leave between me and the driver in front of me. To my mind, most people
follow too close. You really don't want to do that. Here's why.
Note: If you want the tl;dr, skip to So what do I do?
Speed and distance
Let's start by examining what sort of distance you're covering when you drive, because it might be more than you
think. At 10 mph, in 1 second you cover about 15 feet. At 70 mph, in that same 1 second you cover 103 feet. Here's a
graphic that illustrates this ratio.
An accepted measure of a car length is 10 feet. This means that if you're going 60 mph, in 1 second you will travel 9
car lengths. Even small differences in speed result in significant differences in how far you travel—the difference
between driving at 60 and at 70 is that in 1 second at 70 you will travel an additional 15 feet (88 vs 103).
More than one car length.
Braking distance
Let's imagine that for some reason you have to slam on the brakes. Alas, physics tells us that you cannot stop on the
proverbial dime. The faster you're going, the longer it takes to get to zero mph.
There's no standard chart for stopping distance for cars. Cars weigh different amounts and they have different
brakes; for example, newer cars have anti-lock brakes a.k.a. ABS. (Jeremy Clarkson of the TV show "Top
Gear" also makes the case [video] that
cars that are engineered to go fast are also engineered to stop quickly.) External factors also come into play, such
as the road surface (wet? gravelly?) and the tires on the car. Also whether you're going uphill or downhill.
Anyway, it's complex. The best that people can offer is a minimum braking distance based on a formula
that takes into account the initial speed, the car's mass (weight), and the friction coefficient of brakes and road.
I'll spare you that here, but I'll suggest some ballpark numbers, as shown in the chart. (You'll see a variety of
numbers for this measure if you look around; these are actually on the low end.)
Notice that braking distance at 60 is 180 feet—18 car lengths. That's about 1 city block. Going 70, which of
course is only 10 mph faster, the braking distance goes up to almost 250 feet, or an additional 7 car lengths.[1]
Reaction ("thinking") distance
Braking distance is measured from the time the brakes are engaged. But everyone will remind you that there's a delay
between the time you see that you have to brake and when you finally get your foot onto the pedal. Remember from the
earlier bit that if you delay one second before you hit the brakes, your car might already have traveled many car
lengths before you even start slowing down.
The combination of reaction distance and braking distance is referred to as the total stopping distance.
Since I'm all about the pretty charts today, here's one from the UK that shows the total stopping distance as
"thinking distance" plus braking distance (click to embiggen):
It's the total stopping distance that matters when you're driving behind someone: how far will you keep going before
you can stop if something happens ahead of you? Or stated another way, is there enough room between you and the car
ahead so that if they slammed on their brakes, you could avoid slamming into them?
As with braking distance, there's no standard measurement for reaction distance. Some people have faster reflexes
than others. But even lightning-quick reflexes result in some delay between stimulus and reaction.
And much more importantly, some people pay closer attention to traffic than others. People text while they drive,
they yack with their passengers, they watch their GPS, they fool with their radio, they search the passenger
footwell for their dropped phone, they watch TV while driving. There have been
accidents where the driver behind never hit the
brakes at all. In our little car bubble, we sometimes forget that we're piloting 1 ton or more of iron at 88
feet per second, something that's not scary only because we're used to doing it.
And it's your fault
The law generally puts the responsibility on you to leave enough distance. If you rear-end someone, it will almost be
always considered your fault. There are a few circumstances when you can make a case that the
driver in front contributed to the accident, but the default assumption will be that you were not driving in a safe
manner. Not to mention, I suppose, that if you have an accident, you will have had an accident, with all the hassle
that that entails.
Think of the traffic
Even if you have superhuman reflexes and a physics-defying vehicle that can stop on a dime, leaving a gap between you
and the car in front can have benefits for overall traffic flow. By leaving a gap, you can actually minimize the
accelerate-then-brake cycle that characterizes most heavy traffic.
Imagine that you’re behind someone and they tap their brakes to slow down by 5 mph. If you’re close, then you, too,
have to tap your brakes to slow down. Even this small slowdown, and even if both of you immediately speed up again,
creates a kind of wave that moves backward through traffic. In fact, this is sometimes the source of “phantom”
slowdowns on the freeway, where everyone slows down for no apparent reason.
However, if you’re far enough behind someone, when they tap their brakes, you might not need to tap yours at all, or
you can slow down enough just by easing off the accelerator. And if you don’t have to, the person behind you doesn’t
either, and so on backwards through traffic. Result: possible traffic jam averted.
The engineer William Beatty refers to this as
"eating" traffic waves. He explains:
By driving at the average speed of traffic, my car had been "eating"
the traffic waves. Everyone ahead of me was caught in the stop/go cycle, while everyone behind me was forced to
go at a nice smooth 35MPH or so. My single tiny car had erased miles and miles of stop-and-go traffic. Just one
single "lubricant atom" had a profound effect on the turbulent particle flow within miles of "tube."
Obviously, there are limitations to this; if traffic is completely stopped, it’s just completely
stopped. But if you can help reduce traffic jams by leaving a little extra space between you and the next person,
isn’t that a worthy goal?
So what do I do?
When I got my license many decades ago, the rule was to leave 1 car length ahead of you for every 10 mph. If you're
going 60 mph, 6 car lengths. As you can see from the various distances listed above, that's probably not enough. And
that's even assuming that you can estimate car lengths correctly, which I think a lot of (most?) people cannot.
(Little-known fact: the dashed white lines between lanes on the freeway are 10 feet long.)
When I got my motorcycle license, I learned a far more useful guideline: the three-second rule (PDF). The idea is that
when the car ahead of you passes some landmark (a streetlight, a sign, anything static), you
count "one-thousand-one, one-thousand-two, one-thousand-three." If you reach the landmark before you finish
counting, you're following too close.
The rule is useful because it's independent of speed—the faster
you're going, the longer the distance is that you go in 3 seconds, so it (kind of) works out. Naturally, this is
just a rule of thumb, which has to take into account all the factors that go into how quickly you can stop. But
it has the advantage that if you routinely practice counting off your 3 seconds as you drive on the highway, it
means you're paying attention, and that's a very big factor in how safely you're driving.
Event horizon
As if it isn't hard enough just to pay attention to the car in front of, you really should be aware of what's going
on in front of them. The same motorcycle manual that recommends the three-second rule suggests that you try
to see what's happening 12 seconds ahead of you. This is sound advice, because the driver
in front of you might not be paying close attention. If you see that the driver ahead will have to slow down even
before they're aware of it, you can adjust your own driving accordingly. (This is captured in the concept of assured clear distance
ahead, which you can read about in a particularly poorly written Wikipedia article.)
By the way, all of this applies also to cars behind you, and to your side. Driving safely is hard work.
If you can't see ahead of the car in front of you, it's best to leave even more room than you normally
would. I actually have this problem a lot. The motorcycle isn’t very tall, of course. And my normal car is a MINI,
which sits pretty low to the ground. In both cases, if I'm behind an SUV or pickup or anything larger than that, I
have no clue what's going on up front.
Parting notes
People have pointed out that if you leave gaps ahead of you, other drivers will swoop in. That's true. If you have
completely mastered the zen of good following practice, you just let them, and you slow down to again open up a
suitable gap ahead of you. I will acknowledge that this is a state of driving enlightenment that most of us can only
aspire to. Still, it has helped me to think holistically about safety and about traffic, and if I’m in just the
right frame of mind (like, not late to an appointment), I can do a decent impression of someone who actually has
mastered all this. And of course, if I'm on the motorcycle, I am ever mindful that even a small miscalculation in
how closely I follow can have grave consequences.
For the most part, driving is uneventful. Even if we speed, even if we speed and follow too closely, mostly things
don't happen. But it's this very uneventfulness that can make us complacent and lead us to stop paying close enough
attention to an activity that can wreak mayhem in the "unlikely event of" a crash.[2]
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