Rules of Defensive Driving

Cod wrote:

I suspect the honk rule is very culturally dependent. Some places it's perceived as always aggressive, while others perceive it as expected to let people know of your presence.

Wikipedia (Spanish profanity)[/url]](in Mexico) The seven-note musical flourish known as a shave and a haircut (two bits), commonly played on car horns, is associated with the seven-syllable phrase ¡Ch*nga tu madre, cabrón! (f*ck your mother, asshole!). Playing the jingle on a car horn can result in a hefty fine for traffic violation if done in the presence of police, or road rage if aimed at another driver or a pedestrian.

LarryC wrote:

Defensive driving rule: Honk proactively and cooperatively, not for punishment nor chastisement, nor for aggression. That is, honk well before the collision occurs, not after, not very shortly before. Honk to let people know you're there, not to assert your dominance. Honk to say, "Hi!" not to say, "Get out of my way!"

Disagree. The car horn is a horrible communication tool. It cannot be aimed, has a wide circumference of effect, and the only message it can send is: Look at me! That is the only way it should be used. " Hey! Your eyes obviously fail to detect me, let's try one of your other senses."
Edit
To be more clear... My only disagreement is with the proactive, pre collision phrasing. Honk as a last resort. The last thing you want is to startle someone trying to maneuver around a situation you aren't even aware of yet.
Edit2
The more I reread your phrasing the harder it is for me to specifically object. My main point is that the less often everyone honks the more effective it will be when we have to.

When I took drivers training in the 90s, the video and the teacher were adamant that we all use our horns in the exact way Larry is describing. The honk was a required part of urban driving; a sort of audible running lights.

Needless to say, that cultural impetus has shifted drastically. Even in the 90s, most of the 15 year old students snickered at the idea of using the horn for anything other than an emergency warning or a tool for annoyance.

Of course. Honking is just a tool. It's still better to position yourself properly when driving so that you're seen. This is not always possible, though; and drivers can be distracted, unfortunately. Reminding them of your presence well before a possible collision in a friendly manner avoids mishap. In an era with noisier cars and less airconditioning, this was less necessary. Arguably, you won't need to do it for convertibles with their roofs down; they can hear your engine just fine. Being heard is a vital tool in communicating position and presence. It's one of the reasons why motorcycle riders like a nice hefty rumble. Keeps 'em safe.

EDIT: It is important to NOT be passive-aggressive about your honking. Reminding other drivers of your presence doesn't mean you shouldn't be driving defensively. If they look like they're avoiding something, then you drive according - maneuver well ahead of time to avoid them as well! Give them room and tell them you're there.

I'll honk when someone is ahead of me and daydreaming (or texting, who really knows) and the light changes to green. But it's more of a tap then a long honk, just to get their attention back to where it should be.

Strewth wrote:

I'll honk when someone is ahead of me and daydreaming (or texting, who really knows) and the light changes to green. But it's more of a tap then a long honk, just to get their attention back to where it should be.

Same.

I wish I had more control over the horn in my car; it requires a lot of pressure to activate, and thus it's very difficult to just do a quick little 'toot' or 'chirp'. I almost never want to seriously honk at someone, but sometimes I'd like the car equivalent of 'ahem', or maybe just a little verbal nudge. "the light's green!"

As is, though, everything comes out HONNNNNK, so I never use it.

Shoal07 wrote:
Strewth wrote:

I'll honk when someone is ahead of me and daydreaming (or texting, who really knows) and the light changes to green. But it's more of a tap then a long honk, just to get their attention back to where it should be.

Same.

This is also the only reason I use the horn.

I've driven mostly Japanese cars, and have had people make fun of the way the horn sounds, because it's a higher pitched toot, not a big honk, which I guess is what American cars tend to have. But the way it sounds suits the reason I use it.

mudbunny wrote:
Baredil wrote:

Not necessarily. All other things being equal (road surface, tire compounds, brake capability), a heavy vehicle stops in the same distance as a light vehicle.

Your formulae don't cancel out because the direction that the gravity is acting on the vehicles is orthogonal to the direction that the vehicles are travelling. The significantly higher mass of the semi truck means that for the same force applied by the brakes (using F=ma) means that the acceleration of the semi will me much lower, resulting in a longer distance to stop.

Incorrect. The force decelerating the vehicle is friction; it is working directly against the kinetic energy. Force due to gravity determines how much friction is available.

A semi vs a car may be a bad comparison, as all things are not equal, the brakes on a semi may not be able to maintain the same level of energy conversion.

If you look at the graphs Shoal provide, however, you'll note while the semi does take longer to stop, it's less than twice as long. For a vehicle that masses more than 10 times as much. Mass is a proportional factor in both kinetic energy and friction - if it didn't cancel out then the difference should be significantly more

For a better comparison: http://www.msgroup.org/forums/mtt/images/motorcycleBrakingTests.pdf

The two motorcycles used for these experiments were of different types: a 2001 sport model Honda
CBR929RR and a 1999 custom style Honda GL1500 Valkyrie. In terms of braking performance there was no sig-
nificant difference between them. The mean stopping distance in 214 tests of the sport model was 41.67 metres
from 100 km/h compared with a mean of 41.83 metres in 84 passes of the custom machine. These results con-
vince us that a standard emergency braking procedure is possible and valuable despite the weight differences
among motorcycles.

A 379 lb sport bike and a 681 lb cruiser stop in the same average distance.

Additional discussion on the study http://www.msgroup.org/forums/mtt/topic.asp?TOPIC_ID=894&SearchTerms=stopping,valkyrie

Another way of putting that: those big trucks have a heck of a lot more weight per surface area of tire meeting pavement, aka, where the rubber hits the road. That contact area imposes the hard limit on acceleration and braking power. As long as the brakes are better than that, they're good enough.

If they were on, like, tank treads or something, they could stop a lot faster, but then their mileage and run cost would be too high to be economical.

edit to add: I think, with motorcycles, the limit is more the braking system; most motorcycle brakes are manual, no hydraulic assist. Further, I suspect motorcycle brakes are probably tuned so as not to exceed the available friction of the tires, at least not by very much, because a skid on a motorcycle so often ends up being a crash. (I drove a motorcycle for several years in my 20s, and I remember it as being quite difficult to induce a skid.)

That, and braking too hard with a motorcycle will very quickly cause it to flip. There's a finesse required to controlling the front brake.

Baredil - are you arguing that a semi truck and a buick have equivalent stopping distances at the same speeds?

clover wrote:
Cod wrote:

I suspect the honk rule is very culturally dependent. Some places it's perceived as always aggressive, while others perceive it as expected to let people know of your presence.

Wikipedia (Spanish profanity)[/url]](in Mexico) The seven-note musical flourish known as a shave and a haircut (two bits), commonly played on car horns, is associated with the seven-syllable phrase ¡Ch*nga tu madre, cabrón! (f*ck your mother, asshole!). Playing the jingle on a car horn can result in a hefty fine for traffic violation if done in the presence of police, or road rage if aimed at another driver or a pedestrian.

How am I never not going to think of that when I hear "shave and a haircut"?

"La Cucaracha" has 17 syllables by my count. Is there a Borges quote that fits in there or something?

I'll never hear the Smothers Brothers the same way again.

Malor wrote:

Another way of putting that: those big trucks have a heck of a lot more weight per surface area of tire meeting pavement, aka, where the rubber hits the road. That contact area imposes the hard limit on acceleration and braking power. As long as the brakes are better than that, they're good enough.

If they were on, like, tank treads or something, they could stop a lot faster, but then their mileage and run cost would be too high to be economical.

edit to add: I think, with motorcycles, the limit is more the braking system; most motorcycle brakes are manual, no hydraulic assist. Further, I suspect motorcycle brakes are probably tuned so as not to exceed the available friction of the tires, at least not by very much, because a skid on a motorcycle so often ends up being a crash. (I drove a motorcycle for several years in my 20s, and I remember it as being quite difficult to induce a skid.)

Any modern motorcycle's brakes have more than sufficient power to lock the tires and cause a skid.

That, and braking too hard with a motorcycle will very quickly cause it to flip. There's a finesse required to controlling the front brake.

A standard or sport motorcycle, yes. Big a** cruisers will skid before they'll lift the rear tire (low centre of gravity and longer rake in the forks). In any case, yes proper braking of a motorcycle is a critical skill.

Seth wrote:

Baredil - are you arguing that a semi truck and a buick have equivalent stopping distances at the same speeds?

If the brakes and tires are sufficient, yes, they should stop in the same distance. Simple physics. As the mass increases, the available friction increases to provide greater available stopping power.

I suspect the reason a truck can't stop in the same distance has to do with the tires and brakes not being able to maintain maximum braking, either due to heat fade of the brakes or a tendency towards the tires locking - iirc air brakes are harder to modulate/slower to react than hydraulic.

I think it's just more weight per contact area, Baredil. Braking is just friction of the rubber against the road, and the size of the contact area determines how much of it you have. Loaded semis weigh more (much more) per square inch, so they physically cannot slow down as quickly.

Baredil wrote:
Seth wrote:

Baredil - are you arguing that a semi truck and a buick have equivalent stopping distances at the same speeds?

If the brakes and tires are sufficient, yes, they should stop in the same distance. Simple physics. As the mass increases, the available friction increases to provide greater available stopping power.

That's like saying that as long as my muscles and bones are sufficient, I could throw that same truck into orbit. Simple physics, right?

While technically correct, ignores the fact that the brakes and, specifically, the tires, as Malor points out, are not sufficient. The limiting factor on braking is not the brakes themselves, its the area of rubber that is contacting the road. The more rubber, the more force you apply under braking before a skid occurs (which braking force drops off precipitously).

Simple physics.

Jonman wrote:

the area of rubber that is contacting the road. The more rubber, the more force you apply under braking before a skid occurs (which braking force drops off precipitously).

Nope - friction is (mostly) independent of surface area.

http://www.mouatonline.com/Teachers/BHutchinson/Physics/Phys11/Unit%203/Surface%20Area%20&%20Friction.htm

Question

As an engineer, I know that friction does not depend upon surface area. As a car nut, I know that wider tires have better traction. How do you explain this contradiction?

Answer

This is a good question and one which is commonly asked by students when friction is discussed. It is true that wider tires commonly have better traction. The main reason why this is so does not relate to contact patch, however, but to composition. Soft compound tires are required to be wider in order for the side-wall to support the weight of the car. softer tires have a larger coefficient of friction, therefore better traction. A narrow, soft tire would not be strong enough, nor would it last very long. Wear in a tire is related to contact patch. Harder compound tires wear much longer, and can be narrower. They do, however have a lower coefficient of friction, therefore less traction. Among tires of the same type and composition, here is no appreciable difference in 'traction' with different widths. Wider tires, assuming all other factors are equal, commonly have stiffer side-walls and experience less roll. This gives better cornering performance.

This explains the difference in stopping distance between a car and a semi. Truck tires are generally taller and thinner and designed for longer wear with a harder compound, and thus a lower coefficient of friction.

That squares with what I experience in cycling. A 28c tire doesn't necessarily have better stopping power or traction than a 20c one. It's all about the compound. 20c tires can have fantastic traction and stop power, but they'll wear out super fast.

I believe an understanding of the physics surrounding driving can make us better drivers, buy understanding what actually is happening rather than what we think is happening. Physics can often be counter-intuitive. Especially at the speeds/masses/energies involved in driving a modern vehicle; we're not adapted to understand them fully (yet).

I challenged the idea that a semi takes longer to stop than a car on the basis of mass not a factor in stopping distance, all other things being equal. It turns out not all other things are equal; the tires on a semi cannot create as much friction due to a harder compound/lower CoF.

It is still true, however, that increasing the weight of a vehicle does not increase its stopping distance. Believing that it does can be dangerous - "I had to run that red light because I couldn't stop" dangerous. Don't take my word for it, look it up, I've provided some examples.

Agreed. Also agrees with my experiences in driving, oddly enough. A heavily laden car accelerates slower, but the stopping power feels relatively untouched. This is in contrast to the same car in the wet, where the stopping power is significantly and palpably reduced.

I'm still having a hard time grasping this. Are you saying an empty uhaul truck and a fully loaded uhaul truck stop in the same distance? Because the answer is most assuredly "no it does not."

Seth wrote:

I'm still having a hard time grasping this. Are you saying an empty uhaul truck and a fully loaded uhaul truck stop in the same distance? Because the answer is most assuredly "no it does not."

That could be from factors other than mass. If weight were the only variable, then they ought to the stop in the same distance. In real world terms, this may not be true. An easy factor is the brakes - a heavier load may be beyond the stopping power of the brakes, resulting in a longer stop distance - that is, the brakes can maximize the tire traction when the truck is unloaded, but cannot once it is fully loaded, because of the higher values involved.

Another way to say this is provided that the brakes can reliably lock the wheels on a bike, the weight of the rider doesn't affect the bike's stopping distance.

Seth wrote:

I'm still having a hard time grasping this. Are you saying an empty uhaul truck and a fully loaded uhaul truck stop in the same distance? Because the answer is most assuredly "no it does not."

I think what we are coming across here is the difference between "In a perfect world" and "the real world." In a perfect mathematical world, it may be true (I have my doubts, but don't know the physics/math enough to check), but in the real world, a full tractor trailer take a lot longer to stop than my Matrix.

Baredil wrote:
Jonman wrote:

the area of rubber that is contacting the road. The more rubber, the more force you apply under braking before a skid occurs (which braking force drops off precipitously).

Nope - friction is (mostly) independent of surface area.

http://www.mouatonline.com/Teachers/BHutchinson/Physics/Phys11/Unit%203/Surface%20Area%20&%20Friction.htm

I don't buy it.

If I drag a 1 square inch piece of rubber across a surface, I only need to apply a tiny amount of force.

What you're saying is that I can move a 1 square *mile* piece of rubber across the same surface for the same amount of force.

Which is clearly bullsh*t.

That's not what he's saying. Moving and stopping something are not the same things. Even with a perfectly smooth surface, it would take a greater amount of force to move the large rubber tile on account of its having more mass. The mass doesn't cancel out in that particular situation.

Nothing like a new season of Don't Drive Here to make me appreciate the everyday bad driving here at home—or at least that I don't have to deal with the conditions in Nairobi. You guys want to talk peds in traffic? Go to Nairobi.

Speaking of the Discovery Channel, this mass-versus-stopping-distance sounds ideal for the Mythbusters.

I skimmed this, but re: real world vs perfect world- if I'm doing my job right, I'm never slamming my brakes. Whatever you're arguing about, so rarely do you see the full braking power applied in the real world.

Plus f*cking up your load. Which is generally worse. If I screw up, of course it depends on the particular scenario, but it's going to be cheaper and easier all around if I bump the car in front. It's like hitting a deer. Bump the car in front or fall off the shoulder and then it's all up in the air. Keep in mind this is highly hypothetical.

A braking vehicle only becomes a "simple" physics equation if all the tires are locked. That's the only time it becomes a simple formula of velocity, pressure, and coefficient of friction. Until the tires lock, it's an interplay of friction forces involving the road, the tires, the brake pad, and the disk or drum.

Jonman wrote:
Baredil wrote:
Jonman wrote:

the area of rubber that is contacting the road. The more rubber, the more force you apply under braking before a skid occurs (which braking force drops off precipitously).

Nope - friction is (mostly) independent of surface area.

http://www.mouatonline.com/Teachers/BHutchinson/Physics/Phys11/Unit%203/Surface%20Area%20&%20Friction.htm

I don't buy it.

If I drag a 1 square inch piece of rubber across a surface, I only need to apply a tiny amount of force.

What you're saying is that I can move a 1 square *mile* piece of rubber across the same surface for the same amount of force.

Which is clearly bullsh*t.

The force needed to overcome static or kinetic friction is the coefficient of friction (either static or kinetic) times the force perpendicular to the surfaces. In your example, a square mile of rubber would have a force 4,014,489,600 times that of the square inch because of the weight of all that rubber.