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Old rules

The television show Real Time, with Bill Maher always closes with a humorous segment called "New Rules". This brought to mind some "old rules" that I recall from my youth, all related in some way to driving.

When I was growing up in Great Neck, New York, school bus service was only provided for elementary school students who lived more than a mile from the school, and for junior high school or high school students who lived more than two miles from their school. It was presumed that all other students could walk. Great Neck had many streets that lacked sidewalks, so we received instruction on how to walk on such streets. If I'm remembering correctly, this subject was discussed at least once every year in an all-school assembly. Without a sidewalk, we were to walk on the left side of the road, facing traffic (in the United States, we drive on the right). This was drilled into our heads at least once annually.

Thus I was a little surprised to find that here in Wayland, Massachusetts, which is also a semi-rural town with many sidewalk-free roads, such instruction was never given to my children. It just never seemed to be anything that the schools thought about at all. And perhaps as a result of this, I frequently see people in Massachusetts walking along the right side of roads. This means, of course, that if they were to be tragically hit by a car, they wouldn't see it coming, and wouldn't have a chance to jump out of the way.

It seems odd to me that a piece of safety oriented advice that was considered to be of major importance in the State of New York seems to not even be mentioned in Massachusetts.

When people of my age were taught to drive, we were of course taught about safe following distances. How far should one be behind the car in front of you? The rule we were taught was to leave one car length for every 10 miles per hour ("mph") of speed. For example, if you're following the car ahead of you at 30 mph (about 50 km/hr), you should leave three car lengths (we were taught in English system units, miles per hour and feet). In my youth, a standard "full size" sedan was about 16 feet long (about 5 m), so at 30 mph we were being told to follow at a distance of about 48 feet (15 m).

Today, that rule of thumb is a bit problematic, as it's a little bit less clear what's meant by "one car length". There are quite a few cars on the road that are longer than 16 feet, and quite a few that are shorter. It's also not that easy to visually estimate the distance between you and the car in front of you.

A better rule is generally used these days: you should follow at least two seconds behind the car ahead of you. Specifying how to follow in terms of time instead of distance has the advantage of not being dependent on speed. You follow two seconds behind at 15 mph, two seconds behind at 30, two seconds behind at 60, and so on.

The distance corresponding to that time interval increases linearly with velocity. You're 44 feet behind the car in front of you at 15 mph, 88 feet at 30 mph, and 176 feet at 60. Note that this new rule recommends substantially greater distances than the rule I was taught in my youth. It recommends 88 feet at 30 mph as opposed to only 48 feet for three 16-foot car lengths. Note 1

The two second rule is also easier to follow than a rule based on distance. As I noted above, it's not that easy to estimate the distance to the car in front of you, but it's quite easy to estimate how far behind you are in time. Simply watch the back of the car in front of you pass some point on the road, and count off the seconds until the front of your car passes the same point ("a thousand one, a thousand two, a thousand three, ...").

Apart from not texting (which I hope would be obvious), leaving an adequate following distance is the greatest single safety factor in driving. It also seems to be among the most ignored good practices, judging by the behavior I see in other drivers. I sometimes get the impression that there are people who think that if they are following closely, they're going faster. I can't believe they're so stupid as to believe that's actually the case, at least if they would give it a moment's thought. If the car in front of me is going 30 mph, then if I am neither getting closer nor further away, I'm also going 30 mph, whether I'm one second, two seconds, or three seconds behind.

Here's another rule I learned as a young driver. I learned this one from a television program called "The National Drivers Test". The question asked was:

"You're waiting to make a left turn, but can't turn yet due to traffic in the opposite direction. Should you turn your wheels to the left to prepare for the turn, or not?"

Taking the test, my reaction was that it didn't make any difference. You could turn your wheels or not. Seeing your wheels turned might notify oncoming drivers that you intended to make a left turn, but your turn signal should have been blinking anyway. Since seeing your wheels turned to the left might be an additional indication of your intent, I figured, why not turn them? It might let you make the turn just a tiny bit more rapidly.

So I got it wrong, because you should never turn your wheels to the left in that situation. Violating this rule can cost you your life.

The reason, which was explained on The National Drivers Test, is that an extremely common type of accident is for a driver waiting to make a left turn to be struck from behind by another car. This is particularly likely to happen if there is no dedicated left turn lane, so that your car is stopped in a travel lane. If your wheels are turned to the left to prepare for your turn, then if you're struck from behind, your car will swing to the left, into the oncoming traffic. And there will be such traffic, because after all, that's what you were waiting for. Struck from behind in such a situation, you are apt to be involved in a head-on collision, possibly with a car that's moving very fast.

On the other hand, if your wheels are pointing straight ahead, your car will probably jump straight forward, and you won't be involved in a frontal collision. Hopefully, you will be protected from whiplash by your headrest, and won't suffer any injuries at all. I might add that although my daughters were not taught by the Commonwealth of Massachusetts to walk on the left, in their Massachusetts driver education training, they definitely were taught to never turn their wheels while waiting for a left turn.

As long as I'm giving driving advice, I'll talk about one more thing, although it really isn't an "old rule" that I learned in my youth. Modern cars, including my car and Margie's, often display fuel efficiency, to help the driver conserve fuel. In the United States, this is displayed in miles per gallon. I suspect modern cars are able to do this easily as a side-effect of having computers that manage their fuel injection systems. It would've been harder in the past to compute fuel usage when cars had carburetors (and for that matter, lacked computers). I think these displays have the desired effect of making the driver more aware of his or her fuel usage

Margie, in particular, pays attention to her mileage, and tries to drive so as to maximize it. I've thought about what that means, and discovered that a good way to think about it is to think in terms of energy. That leads to the conclusion that to get the best gas mileage, one of the things you want to do is to brake as little as possible.

Here's how to think about energy in the context of an automobile. The engine burns fuel (generally gasoline or diesel fuel) to accelerate the car, converting chemical energy into kinetic energy (the energy of motion). When you step on the brake, you're throwing away some of that kinetic energy, by converting it to heat in the brakes.

So in travelling from point A to point B, if you minimize the amount of braking, you will have discarded less energy than if you made the same trip without braking as much, and hence you will have used less energy for the trip (and hence burned less fuel). Note 2

This conclusion is a bit counterintuitive, because while you're stepping on the brake, you're not feeding any fuel. But you are throwing away kinetic energy. It cost you fuel to obtain that kinetic energy in the first place, and it will subsequently require more fuel to bring your speed back up to where it was before you braked.

Thus, getting good gas mileage is another argument for maintaining a reasonably long following distance. I tend to follow two and a half to three seconds behind the car in front of me. If the car in front of me brakes in order to make a turn, or for some other reason, I frequently don't need to brake at all.

It's always interesting to follow three seconds behind a car which is itself only one second behind the car in front of it. I watch the brake lights of the car I'm following go on-and-off and on-and-off and on-and-off, while I virtually never have to step on the brake.

There are plenty of other tips, which can be found all over the web, about how to drive for the best gas mileage. Don't make jackrabbit starts, and accelerate gradually. Remember that your best gas mileage is obtained at around 45 mph (about 70 km/hr), and goes down precipitously at higher speeds. But driving in a manner so as to minimize braking is also one of the rules that should be followed.

When approaching the left turn onto our street, Margie is always annoyed when oncoming traffic forces her to stop and wait. This is because she loses the kinetic energy that she could otherwise use to carry her up the slight hill at the entrance to our road.

There's one other thing that can cost Margie in gas mileage - that's when I drive her car. I'm just not as good as she at squeezing extra miles out of each gallon of gas.

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© 2014 Lawrence J. Krakauer   Click here to send me e-mail.
Originally posted April 17, 2014

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Footnotes (click [return to text] to go back to the footnote link)

Note 1:   To explain to non-American readers the units I was brought up with: 1 mile (about 1.6 km) is a bizarre 5,280 feet (a foot is about 30.5 cm). 60 miles per hour (about 100 km/hr) is a typical highway speed, equal to 88 feet per second.  [return to text]

Note 2:   The techies among you will recall that kinetic energy is defined as mv2/2, where m is the mass of the car, and v is its velocity (speed). Thus, kinetic energy goes up as the square of the velocity. It takes almost as much energy to accelerate from 50 mph to 70 mph as it took to accelerate from 0 to 50 (since 50 squared is 2500, and 70 squared is 4900, almost double).

Since the damage done in a collision is roughly proportional to the energy dissipated by that collision, the above also means that an accident at 70 mph is twice as bad as an accident at 50.

Your kinetic energy also determines the height of a hill which that energy will carry you up. In the absence of friction, a speed of 30 mph gives you sufficient energy to climb a 30 foot hill. But double your speed to 60 mph, and you have enough energy to climb four times as high, to a height of 120 feet (well, 121 feet, to be precise).

The formula for the height h to which you can climb is determined by setting the potential energy you'll have at the top of the hill equal to the kinetic energy you start with at the bottom. Thus mgh = mv2/2, resulting in h = v2/2g (g is the acceleration of gravity, which in the English system is 32 ft/s2). Turning this around, to get the velocity needed to climb a hill of height h, v = SQRT(2gh).

This is of more than academic interest in New England winters. In order to drive up an icy hill, one needs to hit the bottom of the hill with sufficient velocity to carry you to the top, in the absence of traction. The formula above gives you a pretty good estimate, if you can estimate the height of the hill. Usually you do have some traction, and the traction you have generally makes up for the fact that energy is lost to friction.

When one of my daughters visited a friend at the nearby Sunderland house in the winter, under icy conditions, it could be difficult to get up the steep hill at the end of their long, straight driveway. There was a parking area at the top, at the other side of which was their garage door. I discovered by trial and error that I needed to hit the bottom of the hilly portion of the driveway at around 40 mph in order to successfully get all the way to the top. But no faster - I needed to reach the top at a low speed, so as not to skid across the parking area and crash into the garage door.

By the above formula, 40 mph = 58 2/3 feet/second carries you up nearly 54 feet, which seems like a pretty good approximation for the height of the Sunderland hill. Of course, the ability to do this on an icy roadway depended on the fact that the driveway was perfectly straight. It's not possible to deal with a curve at 40 mph on ice.

Getting to the top successfully also depended on knowing the proper technique to control a nascent skid on ice at that speed, a skill that I, along with a lot of other New England drivers, have developed. And you really don't want to have to brake, if you can avoid it. Not, in this case, to get the best gas mileage, but because braking is apt to initiate a skid, and if a skid begins, you'd need to release the brake immediately (and then quickly maintain your front wheels aimed in the direction of your forward motion).  [return to text]

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