LGV Safe Braking Distances 4 — Questions and Answers
Question 1: What is a 'retarder' on a modern LGV?
- An auxiliary braking device (hydraulic or electromagnetic) that supplements the main brakes, especially on long descents (Correct answer)
- A device that reduces engine power
- An automatic speed limiter
- A braking system that replaces ABS
Correct answer: An auxiliary braking device (hydraulic or electromagnetic) that supplements the main brakes, especially on long descents
A retarder is an auxiliary braking system fitted to many modern LGVs. Hydraulic retarders use fluid resistance and electromagnetic retarders use electromagnetic drag. Both slow the vehicle without using the main friction brakes, preserving them for emergency use.
Question 2: At 40 mph, how far does a vehicle travel during a 1-second reaction time?
- Approximately 18 metres (Correct answer)
- Approximately 11 metres
- Approximately 24 metres
- Approximately 30 metres
Correct answer: Approximately 18 metres
At 40 mph, a vehicle travels approximately 17.9 metres per second (40 mph ≈ 17.9 m/s). During a 1-second reaction time, the vehicle travels approximately 18 metres before braking begins.
Question 3: How does driving with a fully loaded trailer affect the stopping characteristics compared to the tractor unit alone?
- The combination has greater mass and momentum, significantly increasing stopping distance and requiring anticipatory braking (Correct answer)
- The trailer brakes compensate, making stopping similar to the tractor alone
- Only the tractor unit's brakes matter — trailer brakes are supplementary
- A fully loaded trailer actually improves stopping by increasing rear traction
Correct answer: The combination has greater mass and momentum, significantly increasing stopping distance and requiring anticipatory braking
A fully loaded articulated combination has vastly more mass than the tractor alone. The increase in kinetic energy means stopping distances are proportionally much longer. Both tractor and trailer brakes are needed, and brake balance (EBS) is critical to prevent jack-knife.
Question 4: What is 'progressive braking' and why is it important for LGV drivers?
- Gradually increasing brake pressure to slow smoothly — prevents wheel lock-up, jack-knife, and cargo shift (Correct answer)
- Maximum brake application from the beginning
- Pulsing the brakes to prevent lock-up (same as pumping)
- Using only the engine brake and avoiding the footbrake
Correct answer: Gradually increasing brake pressure to slow smoothly — prevents wheel lock-up, jack-knife, and cargo shift
Progressive braking involves gradually increasing brake pressure from light to heavier application. This gives time for brake balance systems (EBS) to adjust, prevents cargo from shifting, and reduces jack-knife risk. It is a fundamental professional driving technique.
Question 5: What is the Highway Code's stopping distance at 30 mph?
- 23 metres (75 feet) (Correct answer)
- 14 metres
- 30 metres
- 18 metres
Correct answer: 23 metres (75 feet)
At 30 mph, the Highway Code gives a total stopping distance of 23 metres (75 feet): 9 metres thinking distance plus 14 metres braking distance. This is for a car in ideal conditions — a loaded LGV would need considerably more.
Question 6: What effect does a downhill gradient have on an LGV's braking distance?
- It increases braking distance as gravity adds to the vehicle's kinetic energy and resists deceleration (Correct answer)
- It reduces braking distance as gravity helps slow the vehicle
- It has no significant effect on a modern air-braked vehicle
- It only affects braking distance when the road is also wet
Correct answer: It increases braking distance as gravity adds to the vehicle's kinetic energy and resists deceleration
When braking on a downhill gradient, gravity acts in the direction of travel, opposing the deceleration force from the brakes. This increases stopping distance compared to level ground. The steeper the gradient, the greater the additional stopping distance.
LGV Safe Braking Distances 4
What is a 'retarder' on a modern LGV?