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Braking Systems and Stopping Distances
1. The scale of the truck accident problem
Every year 14,500 - 16,000 large goods vehicles are involved in crashes
in the UK
2. Safety Advice
The advice provided in the Highway Code recommends that drivers give
large goods vehicles (LGV) more room in which to stop. The issue is
how much room do they need?
3. Experimental Demonstration
The braking distance of a range of vehicle types was compared both
at 30 and 45 mph. This illustrated that LGVs can need up to three
times the distance in which to stop when compared to a car.
The general principle of heavier vehicles needing more room to stop
was supported in subsequent test conducted at the Army's driver training
facility at Leconfield.
4. Braking theory & practice
Since the combined tyre footprint of a multi-axle LGV is proportionate
to its weight the overall braking performance between vehicles should
also be proportionate. But this is not so. If both the car and the
laden articulated combination had both braked from 30mph, the lorry
would still have been travelling at more than 20mph when the car had
stopped. The question was why truck braking systems were so relatively
5. Vehicle Design and Construction
All vehicle types have to at least satisfy European Standards. They
tend to be determined through compromise and trade-off between vested
interests and result in an adequate rather than optimum standard.
b) Braking performance
Type approval sets minimum standards of retardation rather than
required stopping distances. This enables manufacturers to design
braking systems that meet a common standard of adequacy rather than
a higher standard.
The braking systems on cars and motorcycles have improved in step
with the other performance factors. The braking performance of LGV's
is subject to the following factors that can eat away at their effectiveness.
c) Hydraulic vs air brake systems
Cars use hydraulic braking systems but the pneumatic systems on
LGV's creates an unavoidable time delay between brake pedal application
and the transfer of air pressure to the brake units.
d) Drum vs disc brake systems
The favoured drums on LGV's, while suitable at lower speeds tend
to fade and become less effective under sustained heavy braking.
e) Brake balancing
Truck braking force is also balanced between axle groups and between
the tractor and trailer unit by a series of valves, the settings
can be manually adjusted and may not create optimum performance
f) Electronic braking systems
Electronic braking systems, currently being fitted to some Volvo
and Mercedes Benz trucks, ensures optimum braking force in all situations
g) Tyres Different tyre compounds are chosen for different
purposes. A soft compound tyre reduces skid risk but increases wear.
The heat levels generated in compounds may increase pollution due
to rubber deposits and carbon black and, because of increased drag,
causes reduced fuel economy. A hard compound tyre will last longer,
enhance fuel economy, but provide less grip and causes more damage
to the road surface. The general-purpose tyre will optimise durability
and adhesion. There is also a cost penalty and issues of environmental
and operational effectiveness.
h) Driver behaviour Lorry drivers are often blamed for travelling
too close to other vehicles. Video footage of both motorway driving
behaviour and crashes makes it clear that drivers may:
Not be aware of the facts
Not believe them
Be convinced it won't ever happen to them
Simply not care
These issues remain a challenge for road safety experts to deal
There is a difference in braking efficiency between vehicle types.
Technology has enabled braking efficiencies in excess of 100%. The
stopping distance of poorly maintained trucks will undoubtedly be
far greater than the values found during the experimental demonstrations.
However, a higher level of retardation may not be welcomed by drivers
hauling 25 tonnes of steel rods positioned one metre away from their
back. They will always obey Newton's Laws and will want to keep going
at the pre braking speed. That in itself causes health and safety
There is though an issue of
Public awareness raising for all drivers of motor vehicles
To ensure that adequate and appropriate information is made available
to all. This may be through the media and better information in
The Highway Code.
b) Research, development and fitment of station sensing and
The present state of development of vehicle telematic, proximity
sensors and similar devices is such that vehicle, speed and situation-specific
data could be used to inform and warn drivers of the fact that they
were travelling too close to the vehicle ahead.
c) Targeted enforcement activity
In tailgating collisions the offending driver could be prosecuted
for dangerous or careless driving. Why not adopt an accident prevention
enforcement strategy and prosecute before the inevitable collision?
d) Improved vehicle brake system design and performance standards
Since there appears to be prima-facie evidence to support the case
for improved truck braking efficiency, UK Government should press
Europe for such improvement.
An allowance should be made for variations in the actual speed and
the point when the brakes were applied. Acknowledgement. Captain Bailey,
Defence Road Safety Officer and Staff at Leconfield.
Paper written by Kevin Birch - RoSPA Driver Services - June 2001
COMPARATIVE BRAKING DISTANCESCONDITIONS -
METALED RUNWAY SLIGHT INCLINE. GOOD REPAIRFINE AND DRY - 19°C
Crew Cab (Laden)
Bomb Disposal DAF
Leyland Drops (¼ Laden)
Leyland Drops (Unladen)
Tavern (Armoured Personnel Carrier)
Saxon Armoured Personnel Carrier
Volvo Self Loader FL12
Military Land Rover (TUM - LWB)
ATMP Super Cat
Honda Pan European Solo M/C
Iveco tractor and trailer Unladen*
Iveco tractor and trailer Laden*
Seat medium family saloon*
1.3 Tonnes (ABS)
1.4 tonnes (no ABS)
32 tonnes ABS
36 tonnes ABS
6.7 Tonnes ABS
6.9 tonnes ABS
42 tonnes (ABS)
25.36 (19.7 Skid)
Brakes: Cold 14.1
Brakes: Warm 13.58
Brakes: Warm +12.47
* MIRA - slight down slope, warm and dry good surface
© RoSPA June 2001 - Reproduced courtesy of