48 topics · Updated 2026-09-17
The distance a commercial vehicle needs to stop is affected by brake condition, traction, weight (GVW) and speed. Poorly aligned or maintained brakes force remaining brakes to generate more friction, lengthening stopping distance.
A heavy vehicle, though it may have better traction, needs more time and distance to stop. Doubling weight doubles the force needed. Doubling speed requires four times the braking force. Doubling both weight and speed requires eight times the braking force.
Total stopping distance is the sum of perception distance, reaction distance, lag time distance and braking distance. It is measured from when the driver realizes the need to brake until the vehicle fully stops.
Perception time increases if not paying attention or unwell. Reaction time is slower if the driver is tired or has consumed alcohol or drugs. Lag time is the delay for compressed air to flow and apply the brakes. Braking time depends on brake force, lining/drum condition, traction and vehicle weight and speed.
To stop a moving vehicle, a driver must See a hazard, Think and decide to stop, and Do by placing foot on the brake pedal until the vehicle stops. Heavy commercial vehicles take more time and distance to stop than smaller vehicles.
Traction is the friction between the road surface and the tire contact area. More traction means less time and distance to stop. There is the most traction just before all wheels lock up, and less traction when wheels are skidding.
The slope or grade of a road affects stopping distance. A vehicle travelling downhill needs a longer stopping distance due to gravity. A vehicle travelling uphill stops in a shorter distance.
Driving while exhausted makes you a road hazard. Drowsy driving is as dangerous as impaired driving because it slows reaction time, decreases awareness, and can impair judgment like alcohol or drugs.
Recognize fatigue warning signs: yawning; inability to keep eyes focused or head up; wandering, disconnected thoughts; driving the past few kilometres without remembering them; drifting between lanes, tailgating, or missing traffic signs; and noticing a vehicle in your mirror that seemed to appear out of nowhere.
Short-term aids (radio, open window, coffee, gum, eating) only help briefly. Prevention: know your biological clock and avoid driving during down time; stop when sleepy; sleep well before a long trip; avoid working all day then driving all night; schedule a break every two hours or 160 km; take a 20-40 minute mid-afternoon nap; and travel with an awake, alert passenger.
Lack of sleep is the most common cause of drowsy driving. Contributing factors include driving alone, long distances without breaks, driving overnight, medication that increases sleepiness, and alcohol. Most at risk: shift workers, commercial drivers, people with untreated sleep disorders, teenagers, and young adults.
Most fatigue-related collisions occur between 1-4 p.m. and early morning between 2-6 a.m. These collisions typically occur at higher speeds and can result in running off the road or head-on collisions with vehicles or stationary objects.
All vehicles have driver perception and reaction time (about 1½ seconds) plus actual braking time and distance. Air brake vehicles additionally have air pressure lag time—the time it takes for air pressure to pass through the hoses to the brakes.
Air lines to the service brakes are empty when the brakes are released. When brakes are applied, air pressure must move through the air lines, creating a delay before the service brakes apply. The longer a vehicle, the greater the brake lag. Air line layout and fittings affect air pressure flow. A body job has the least lag; a triple trailer combination has the most, with control air passing through five pairs of glad hand couplers over up to 35 metres.
Hydraulic brake fluid is always in the lines so hydraulic brakes apply immediately. Brake fluid is held in reservoirs and lines so the system is constantly full. Brake fluid cannot be compressed and transfers pressure rather than flows, making force applied to the brake immediate.
Brakes convert the energy of momentum into heat, which is dissipated to the atmosphere by brake drums or disc rotors. Brake failure (fade) arises when more heat is absorbed by the drums or rotors than can be dissipated, caused by driving too fast for conditions.
All vehicles have driver perception and reaction time (1½ seconds) plus actual braking time and distance. Air brake vehicles additionally have air pressure lag time — the time for air pressure to pass through hoses to the brakes. Hydraulic brakes apply immediately because fluid is always in the lines.
Required brake power increases 2 times for doubling weight, 4 times for doubling speed, and 8 times for doubling both weight and speed. Heavy vehicles take longer to stop, requiring more space and time for manoeuvres. Drivers must maintain appropriate following distances and speeds.
The longer a vehicle, the greater the brake lag. Air line layout and fittings affect air pressure flow. A body job has the least lag; a triple trailer combination experiences the most lag, with control air passing through five pairs of glad hand couplers, two 90-degree bends per set, over distances up to 35 metres (114.8 feet).
Travelling up a grade slows the vehicle down, while travelling down a grade will dramatically increase stopping distance. Drivers can quickly get into trouble on steep grades.
Brakes convert the energy of momentum into the energy of heat, which is dissipated to the atmosphere by the brake drums or disc rotors. Brake failure (fade) occurs when more heat is absorbed by the drums or rotors than can be dissipated, caused by driving too fast for conditions.
Air brakes experience brake lag because air must travel through the lines before brakes apply, unlike hydraulic brakes where fluid is always in the lines and applies immediately. The longer the vehicle, the greater the brake lag. Air line layout and fittings affect flow. A body job has the least lag; triple trailers have the most, with control air passing through five pairs of glad hand couplers over distances up to 35 metres.
Increasing vehicle mass increases braking requirements. Increasing speed also increases braking requirements. Large vehicles take longer to stop than passenger vehicles and need more space and time for manoeuvres. Doubling both weight and speed increases the complexity of the braking relationship. Steep grades can quickly get a driver into trouble.
Brakes convert the energy of momentum into the energy of heat, which is dissipated to the atmosphere by the brake drums or disc rotors. Brake failure arises when more heat is absorbed by the drums or rotors than can be dissipated, which is caused by driving too fast for conditions.
All vehicles have a driver's perception and reaction time (approximately 1½ seconds) plus actual braking time and distance. Air brake vehicles additionally have air pressure lag time - the time for air pressure to pass through the hoses to the brakes. Hydraulic brakes apply immediately because fluid is always in the lines and cannot be compressed.
Increasing vehicle mass increases braking requirements; doubling weight requires 2 times the brake power. Increasing speed increases braking requirements; doubling speed requires 4 times the brake power. Doubling both weight and speed requires 8 times the brake power. Drivers must maintain appropriate following distances and speeds for stopping requirements.
The longer a vehicle is, the greater the brake lag. Air line layout and types of fittings also affect air pressure flow. A body job has the least brake lag while triples (three trailers) have the most, with control air passing through five pairs of glad hand couplers and distances up to 35 metres.
The park component rides on the back of the service chamber and applies the foundation brakes with mechanical spring force rather than air pressure. Air pressure from the service tanks is used to release the park brake, compressing the spring.
The park control valve (yellow button) is the driver's park circuit control. It sits on a spring-loaded spool requiring air pressure to hold in. When air pressure drops, the spring pops the button out, typically 20-45 PSI (138-310 kPa), or as low as 10 PSI in new trucks. Some motor coaches have an isolated park tank not shown on gauges.
Caging bolts (release tools) pull the park spring off the application so the vehicle can be towed or repaired. They are not used to disassemble park chambers. When caging all park brakes, chock blocks must be placed on both sides of the wheels. Do not drive with park springs caged.
Air pressure for the park circuit is delivered from the higher pressure of either the primary or secondary tank. This keeps the spring park brake released during a system failure in either service circuit, allowing a safe and controlled stop.
When air pressure drops to 82-84 PSI (566-580 kPa), the park springs begin to expand. Brake lining starts to drag around 55-60 PSI (380-414 kPa). Park springs are applied firmly by 45-50 PSI (310-345 kPa).
Other than the Class 1 licence, the air brake 'Q' endorsement is independent of the class of licence held. A Class 5 licence endorsed for air brakes is for a single axle vehicle. The Class 1 licence includes the air brake endorsement and allows the holder to drive everything except motorcycles (Class 6), but the holder must still pass the Alberta Air Brake Program.
Drivers of two axle single motor vehicles registered as farm vehicles do not require an air brake 'Q' endorsement. Every jurisdiction in Canada, except Nunavut, requires drivers of air brake equipped vehicles to have an air brake endorsement.
Unlike service brakes that depend on air pressure to apply, the park component rides on the back of the service chamber and uses mechanical spring force to apply the foundation brakes. Air pressure from service tanks is used to release the park brake.
The yellow park control valve sits on a spring-loaded spool requiring air pressure to hold in (open/released). When air drops, the spring pops the button out, applying park brakes. The valve must default to park when air is depleted.
As air drops, the park springs progressively apply. Understanding these thresholds is critical for emergency braking situations where park springs become the stopping mechanism.
Air pressure for the park circuit is delivered from the higher pressure of either the primary or secondary tank. This keeps the spring park brake released during a single circuit failure, allowing a controlled stop.
Caging bolts (release tools) pull the park spring off the brakes so the vehicle can be towed or repaired. They are not for taking chambers apart. Vehicles must not be driven after caging.
Descending mountain grades is a challenging skill. The vehicle should be placed in the proper (lower) gear before descending, which avoids the need for corrective procedures during the descent. Large vehicles often require slower descent speeds than other traffic - drivers must drive slowly and not be pressured by other traffic. Runaway lanes exist as a safety feature for vehicles that lose braking control.
The biggest challenge for combination units is loss of brake balance from uneven load distribution. A heavy load at the front of a trailer gives drive axles good traction, but the brakes under an empty rear of the trailer receive the same application air pressure without load to hold them down, causing the rear to slide and swing. This problem is magnified in multiple-trailer combinations. ABS can help maintain brake balance.
ABS uses sensors, modulators and a computer. Axle-end sensors relay a voltage signal interpreted as wheel speed. The computer detects differences in wheel speed and directs a modulator to release air pressure in the service chamber of slower-rotating (locking) wheels. Modulators can be mounted separately near the chambers or wing-mounted to the relay/quick release valve. ABS applies to both air and hydraulic brake vehicles. The dash ABS warning light does a bulb test at ignition then stays unlit; a continuously flashing light means ABS is actively controlling the brakes, while a steady light indicates a system problem.
A 'bobtail' is a highway tractor operating with no trailer. Highway tractors are designed to carry heavy weight, so driving a bobtail with no weight on the axles can be dangerous even in good conditions. Tractors equipped with bobtail relays cut primary service circuit applications by as much as 80 per cent when the trailer air lines are empty, to help balance braking.
Auxiliary retarders help control speed but are NOT intended for emergency braking and do not meet sudden or large braking demands. Engine retarders open the exhaust valve at the top of the compression stroke, turning the engine into an air compressor. Exhaust retarders increase exhaust back pressure via a butterfly or sliding gate valve and must be adjusted so back pressure stays within the engine's capacity (excessive back pressure floats the exhaust valves open, hitting piston tops). Hydraulic driveline retarders pump oil to create driveline resistance - watch oil temperature and turn off when too hot. Electric driveline retarders use an electromagnet, are common on transit buses/emergency/waste vehicles, and are activated by service brake application (unlike the other three).
Braking technique on icy roads differs depending on whether the vehicle has ABS. Drivers must exercise safety installing tire chains and drive with extra caution on icy roads and unfamiliar grades. This topic is expanded in the ABS section.
Trailers with ABS have a warning light at the rear of the left side. If power comes only through the stop light circuit, the light comes on when the service brake is applied then goes out to indicate normal function. Most modern tractors supply steady power through the blue wire (centre pin) of the trailer cord when the ignition is on, so the light comes on at ignition then goes out for normal function. In either case, if the light stays on, there is a system fault. Look down the left side of the trailer when starting the ignition and occasionally when braking.
ATC is a variation of ABS using the same wheel-end sensors and computer, but the computer is enhanced to monitor wheel spin and locking. Unlike ABS modulators (which only release pressure), ATC valves have a separate air pressure delivery that can apply brakes to a spinning wheel, redirecting driveline power to axle ends with better traction. ATC valves act like electrically controlled relays. A separate ATC dash warning light shows when ATC is working and when it needs repair.
Stability Control Systems are more advanced than Traction Control Systems, using the same wheel speed sensors and ATC valves but monitoring rollover and yaw (jackknife) with a computer capable of handling multiple programmed situations. Stability Control replaces and incorporates the functions of Traction Control, and its warning light replaces the ATC warning light. Drivers should not rely on brake management systems and should drive as if they do not have them.
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