General Knowledge: Speed Management

37 questions in the bank cover this topic. Everything below is drawn from the sources those questions cite.

Sources: 49 CFR 392.14, 49 CFR 393.52, TC 21-305-20 para 13-16, TC 21-305-20 para 13-18, TC 21-305-20 para 2-14, TC 21-305-20 para 3-13, TC 21-305-20 para 3-37, TC 21-305-20 para 3-38, TC 21-305-20 para 3-42, TC 21-305-20 para 3-43, TC 21-305-20 para 3-44, TC 21-305-20 para 3-52, TC 21-305-20 para 3-66, TC 21-305-20 para 3-67, TC 21-305-20 para 6-33, TC 21-305-20 para 6-34

What you need to know

392.14 states plainly that speed shall be reduced when hazardous conditions adversely affect visibility or traction.

Anti-lock brakes and carrier instruction do not change the requirement.

Extreme caution in the operation of a commercial motor vehicle shall be exercised when hazardous conditions, such as those caused by snow, ice, sleet, fog, mist, rain, dust, or smoke, adversely affect visibility or traction. Speed shall be reduced when such conditions exist. If conditions become sufficiently dangerous, the operation of the commercial motor vehicle shall be discontinued and shall not be resumed until the commercial motor vehicle can be safely operated.

49 CFR 392.14

392.14 requires operation to be discontinued if conditions become sufficiently dangerous, and not resumed until the vehicle can be operated safely.

The only qualification is where stopping would increase hazard to passengers, in which case the vehicle may proceed to the nearest point where passenger safety is assured.

Extreme caution in the operation of a commercial motor vehicle shall be exercised when hazardous conditions, such as those caused by snow, ice, sleet, fog, mist, rain, dust, or smoke, adversely affect visibility or traction. Speed shall be reduced when such conditions exist. If conditions become sufficiently dangerous, the operation of the commercial motor vehicle shall be discontinued and shall not be resumed until the commercial motor vehicle can be safely operated.

49 CFR 392.14

392.14 lists snow, ice, sleet, fog, mist, rain, dust and smoke.

High wind is not in that list, though it appears in 383.111(a)(12) as an extreme driving condition and 392.14's language is not exhaustive.

Extreme caution in the operation of a commercial motor vehicle shall be exercised when hazardous conditions, such as those caused by snow, ice, sleet, fog, mist, rain, dust, or smoke, adversely affect visibility or traction. Speed shall be reduced when such conditions exist. If conditions become sufficiently dangerous, the operation of the commercial motor vehicle shall be discontinued and shall not be resumed until the commercial motor vehicle can be safely operated.

49 CFR 392.14

393.52(a)(2) and (a)(3) both measure deceleration and stopping distance from 20 miles per hour against the table in 393.52(d).

Upon application of its service brakes, a motor vehicle or combination of motor vehicles must under any condition of loading in which it is found on a public highway, be capable of

Stopping from 20 miles per hour in a distance

49 CFR 393.52

393.52(c)(2) requires the vehicle to begin in the center of a 12-foot-wide lane and not deviate from it during the test.

The surface must also be hard, substantially level, dry, smooth and free of loose material.

Any test must be made with the vehicle on a hard surface that is substantially level, dry, smooth, and free of loose material

The vehicle must be in the center of a 12-foot-wide lane when the test begins and must not deviate from that lane during the test

49 CFR 393.52

The speed is chosen before you enter, not during.

Once you are in the curve at too high a speed, braking shifts load and can break traction, and steering harder only asks the tires for grip they do not have.

Driving slowly into a curve is the only way to prevent skidding.

TC 21-305-20 para 3-52

The curve's radius is fixed by the road and the weight is fixed by the load.

Speed is the only thing the driver can change in the moment, which is why it is the only real control over whether the vehicle holds the curve.

The magnitude of centrifugal force depends upon the degree of the curve and the weight and speed of the object. In driving, road curvature and vehicle weight are freed. The only variable is speed.

TC 21-305-20 para 3-37

Speed went up by half, from 20 to 30, but the force more than doubled.

That is the square relationship: 1.5 squared is 2.25, and 583 times 2.25 is about 1,312. It is why a small speed increase on a curve is far more dangerous than it feels.

An 11,000-pound vehicle, making a turn of 500-foot radius at 20 MPH has to overcome centrifugal force of only about 583 pounds. At 30 MPH, the force is increased to 1,312 pounds.

TC 21-305-20 para 3-37

Either the tires let go and the vehicle slides, or, in a tall heavy vehicle, the top keeps going outward while the tires hold and it rolls.

A loaded truck can reach the tipping point before the sliding point, so a curve taken too fast may roll it rather than slide it. Rollover can come before any slide in a tall, loaded vehicle.

Whenever centrifugal force is greater than friction, the vehicle will skid. Centrifugal force also tends to tip overlarge vehicles on curves.

TC 21-305-20 para 3-37

It works to straighten your path, which is why a curve taken too fast ends with you leaving it on the outside.

Centrifugal force is that force which, acting upon an object traveling in a curve, tends to force it from the curved path into a straight one.

TC 21-305-20 para 3-37

The curve is already built and your load is already on.

Speed is the only term left, which is why every curve warning is a speed instruction.

The magnitude of centrifugal force depends upon the degree of the curve and the weight and speed of the object. In driving, road curvature and vehicle weight are freed. The only variable is speed.

TC 21-305-20 para 3-37

From 583 to 1,312 pounds, so more than double for a 50 percent speed increase.

Force follows the SQUARE of speed, which is why a modest extra few mph has an immodest effect.

An 11,000-pound vehicle, making a turn of 500-foot radius at 20 MPH has to overcome centrifugal force of only about 583 pounds. At 30 MPH, the force is increased to 1,312 pounds.

TC 21-305-20 para 3-37

It is a contest between two forces.

You can lose it by raising one, with speed, or by lowering the other, with ice or water, and ice plus a safe speed can still lose.

Whenever centrifugal force is greater than friction, the vehicle will skid.

TC 21-305-20 para 3-37

Dry road is not unlimited grip, only more grip.

A high enough speed beats any amount of friction available, which is why curve advisory speeds apply in perfect weather too.

If the vehicle enters the curve at too great a speed, centrifugal force will overcome friction even if the road is dry and free of ice.

TC 21-305-20 para 3-37

Rollover.

A loaded trailer carries its weight high, so the force acting at that height can tip it before the tires ever slide, meaning you can roll without any warning skid first.

Centrifugal force also tends to tip overlarge vehicles on curves.

TC 21-305-20 para 3-37

Six seconds for 60 feet, plus one more for being above 40 mph, so seven.

The length term exists because your whole vehicle has to stop, not just the cab.

at least one second for each 10 feet of vehicle length at speeds below 40 MPH. At greater speeds, you must add one second for safety.

TC 21-305-20 para 3-67

One second per 10 feet, so four.

Above 40 mph you would add one more, making five.

One good rule to keep when determining how much space should be kept in front of the vehicle is that at least one second for each 10 feet of vehicle length at speeds below 40 MPH.

TC 21-305-20 para 3-67

Following too closely, and the reason it is so common is in the next sentence: the smaller vehicle ahead of you can stop faster than you can.

trucks and buses most often run into the vehicle in front of them as a result of following too closely. Remember, a smaller vehicle can stop faster than a larger vehicle.

TC 21-305-20 para 3-66

The gap between seeing and understanding.

It is the part of total stopping distance that happens entirely inside your head, before a muscle has moved. That part happens entirely inside your head.

Perception distance is that distance traveled between the time a dangerous situation is first seen by the driver and the time he/she actually recognizes it as being dangerous.

TC 21-305-20 para 3-42

Between decision and the brakes acting.

Perception, reaction, brake lag and braking are four separate distances, and only the last one is about your brakes.

Reaction distance is the distance traveled by a vehicle between the times the driver determines

TC 21-305-20 para 3-43

Speed, above every other infraction, and it is also the one likeliest to make the resulting accident fatal.

Both effects come from the same place: energy rises with the square of speed.

Driving at a high rate of speed is more likely to cause an accident than any other driving infraction.

TC 21-305-20 para 3-38

Reasonable and prudent for the conditions, which can be well below the posted limit.

The posted number is a ceiling for good conditions, not permission for bad ones.

NO PERSON SHALL DRIVE A VEHICLE AT A SPEED GREATER THAN IS REASONABLE AND PRUDENT UNDER THE EXISTING CONDITIONS

TC 21-305-20 para 3-38

More work means more heat, and heat is what makes brakes fade.

That is why a long downgrade is handled with gears and a retarder rather than with the brake pedal.

The heavier a vehicle, the more work the brakes must do to stop it and the more heat they absorb.

TC 21-305-20 para 3-13

Low enough that engine and retarder hold the speed by themselves.

If you need the service brakes to maintain speed, you are already in too high a gear and the brakes are heating with the whole grade still ahead.

Select a gear that allows the engine with the engine retarder applied to control the truck speed with the engine at or below the recommended RPM and service brakes not applied.

TC 21-305-20 para 13-18

Before you start down.

Once speed has built, a loaded vehicle may not let you shift down at all, and a missed shift on a grade leaves you in neutral with only the brakes.

As the vehicle operator approaches a downgrade, they should progressively select a gear that, when combined with the engine retarder, allows them to maintain an engine speed at the recommended RPM.

TC 21-305-20 para 13-18

The tires ride up on water instead of touching the road.

Steering and braking both stop working because both depend on contact you no longer have.

also cause hydroplaning, a condition in which directional control is partially or totally lost as the tires on the vehicle lose traction with the road

TC 21-305-20 para 6-33

Speed is chosen at the top, where you still have the option.

Continuous light braking is the classic error: it heats the brakes steadily and fades them exactly when the grade steepens. Then hold it with gear and retarder rather than the service brakes.

The engine retarder is most effective between

TC 21-305-20 para 13-16

Thin and soft, so the tires reach the road underneath.

Once the snow packs, the tires ride on snow rather than road and the grip that was there disappears. Once it packs, the tires ride on snow rather than on the road.

A careful operator can negotiate a thin layer of soft snow that allows tires to sink through to the road.

TC 21-305-20 para 6-34

Your stopping decision has to account for what hard braking will do.

In a car the choice is stop or go; loaded, there is a third possibility you have to rule out first.

prepare to stop unless braking will cause the vehicle to skid

TC 21-305-20 para 2-14

All three, and only the first is under your control at the moment of braking.

The other two were decided when you loaded and when the weather changed.

How hard and steadily the operator presses the brake pedal.

How efficient the brakes are compared to the weight of the vehicle and its load.

How slippery is the road surface.

TC 21-305-20 para 3-44

Dry.

Every legal stopping figure is a dry-road figure, so on a wet or icy road you are outside the assumption the number was built on.

Most vehicles can be stopped on a dry road surface within the distance required by state laws. However; snow, ice, rain, and gravel reduce the vehicle's stopping ability

TC 21-305-20 para 3-44

Entering slowly, and the manual says 'the only way'.

Once you are in the curve at too high a speed, every remaining option is damage control.

Driving slowly into a curve is the only way to prevent skidding.

TC 21-305-20 para 3-52

A contest the tires lose.

The vehicle wants to go straight and only friction turns it, so a skid begins the moment that friction is outbid.

Whenever the tendency of the vehicle to travel in a straight line becomes too powerful for the traction holding the vehicle on the curve, the tires slide on the road and the vehicle starts to skid.

TC 21-305-20 para 3-52

Four patches of rubber, each about the size of a hand.

Every force you ask for, braking, steering and accelerating, has to pass through them.

of rubber are the only connection between the road and the vehicle

TC 21-305-20 para 3-52

Your steering input against the vehicle's momentum.

When their difference exceeds what the tires can hold, control is lost regardless of how smoothly you steered.

The force represented by the natural tendency of the vehicle to follow a straight line is opposed by the force placed on the steering wheel to turn the vehicle. These opposing forces can cause the loss of control of the vehicle.

TC 21-305-20 para 3-52

Brakes are sized against weight, so the same system that stops an empty vehicle easily is working at its limit loaded.

Nothing about the brakes changed; the demand did.

How efficient the brakes are compared to the weight of the vehicle and its load.

TC 21-305-20 para 3-44

Hard and steady.

Stabbing at the pedal cycles the tires between gripping and sliding, which stops you less well than a firm steady application.

How hard and steadily the operator presses the brake pedal.

TC 21-305-20 para 3-44

Check yourself

• When rain reduces traction, your ______.

• How should you act when conditions become sufficiently dangerous?

• What is NOT listed in the rule as an example of a hazardous condition?

• What is the speed at which brake performance stopping distances are specified?

• What is the lane width your vehicle must stay within during a brake performance test?

• According to the manual, what is the only way to prevent skidding on a curve?

• Of the factors determining centrifugal force on a curve, which one is actually under your control?

• What does centrifugal force rising from 583 to 1,312 lb between 20 and 30 mph show?

• What two outcomes does centrifugal force exceeding friction produce on a curve?

• What is the effect of centrifugal force on your vehicle in a curve?

• Of curve radius, vehicle weight and speed, which can you actually change in the moment?

• Going from 20 to 30 mph in the same curve, roughly how much does centrifugal force grow?

• What exactly decides whether you skid in a curve?

• Can you skid in a curve on dry, clean pavement?

• Besides skidding, what else does centrifugal force threaten in a large vehicle?

• What following distance does a 60-foot combination need at 55 mph?

• Below 40 mph, how much following distance does a 40-foot vehicle need?

• What do accident reports give as the commonest way trucks and buses hit the vehicle ahead?

• Perception distance is the distance travelled ______.

• What is reaction distance?

• Which driving infraction does the manual call the likeliest to cause an accident?

• What is the basic speed rule the manual calls the most important?

• Why do heavier vehicles need more braking care?

• On a downgrade with good traction, what gear should you be in?

• When should you select your descent gear?

• What is hydroplaning?

• What should your speed policy be on a long downgrade?

• Which snow condition does the manual say a careful driver can negotiate?

• Why does a yellow light need a judgment rather than a reflex in a heavy vehicle?

• What does your ability to slow down depend on?

• Legal stopping distances assume what about the road?

• What does the manual call the ONLY way to prevent skidding in a curve?

• What actually starts a skid in a curve?

• The whole connection between your vehicle and the road is ______.

• What two forces oppose each other as you steer through a curve?

• Why does the same brake system stop you differently on different days?

• What is the manual's rule on HOW you press the brake pedal?

Answers are in the practice test for this topic.