Whole-Body Vibrations When Riding on Rough Roads

Publication 2000:31E

Whole-body vibration when
riding on rough roads
A study

2000-05

Date Document designation
Head Office 2000-05-15 (2002-03-08) Publ 2000:31E

Author
Road Engineering Division.
Contact person: Johan Granlund.
Title
Whole-body vibration when riding on rough roads.
Main content
At rural highway speeds, road roughness is a source of undesirable dynamic forces and displacement in
the interaction between road, vehicle and human. These vibrations can cause a sense of discomfort, and
it cannot be ruled out that they could impair the health and performance ability of both drivers and
passengers alike.

A study has therefore been conducted on National Highway 90 and County Road 950, aimed at ascer-
taining the seriousness of the problem of whole-body vibration during travel. The roughness index on
the test stretches varied from very good (IRI 20 = 0.43 mm/m) to extremely poor (IRI 20 = 22.78
mm/m). Vibrations that affect vehicle occupants were measured in different configurations of moving
timber lorries and ambulances. A separate report published by Ingemansson Technology AB presents a
detailed account of how the measurements were carried out and how the data was stored and analysed.
Another separate report published by the National Institute for Working Life presents the findings
from an analysis of the effect on the human body of the vibrations recorded.

This report is a summary of the study. It also contains an interpretation of the findings from collating
the vibration measurement data with the data collected in connection with the routine annual road co n-
dition surveys. There are three main causes of vibration: road roughness, vehicle properties and driver
behaviour (including the choice of speed). The results of this study support the opinion that, within
reasonable variations in these factors, road roughness has a far greater impact than the other two vari-
ables. Further, the study substantiates that the higher frequency of injury, especially in commercial driv-
ers’ locomotor systems (as been found in earlier studies), is related to rough roads. This correlation is
probably strongest in geographical areas where long stretches on a large percentage of the roads have a
high IRI, i.e. in the so-called ”forest counties” of Norrland, Värmland and Dalarna in Sweden. Riding
the roughest road stretches, peak values were registered on the ambulance stretchers with vibration levels
that are considerably above levels that completely healthy people are considered to experience as ”ex-
tremely uncomfortable”, as per an international standard on evaluation of human exposure to whole-
body vibration.
Publisher
Environmental Department.

ISSN 1401-9612
Vägverket printers in Borlänge 2002.

Picture of the ambulance on the cover is published with the permission of Anders Wiman AB, ambulance
manufacturer.

Publisher
National Road Management Division.

Key words
Roads, pavement, roughness, texture, ride, vibrations, shock, dampening, natural frequency, resonance,
dynamic forces, displacement, fracture mechanics, road grip, ride quality, stress, discomfort, performance
ability, health, motion sickness, living environment, working environment, road maintenance, surfacing

Distributor (name, postal address, telephone, telefax)
Swedish National Road Administration, Butiken, Internal Services Division, SE 781 87 BORLÄNGE,
Sweden+ 46 243-755 00, fax +46 243-755 50

Head Office
Postal address Telephone Telefax

SE 781 87 BORLÄNGE + 46 243 - 750 00 +46 243 - 758 25

Whole-body vibration when riding on rough roads
SNRA Publ. no. 2000:31E

Preface

This is a report of a study that was co-financed by the Västernorrland County Council,
SCA Forest and Timber AB, Själanders Åkeri AB (haulier) and the Environmental De-
partment at the Swedish National Road Administration (SNRA). The project was initiated
by the SNRA subsequent to a survey of the problems associated with driving on rough
roads. Further, the trend towards ”smoother roads” revealed in the SNRA’s annual IRI
(International Roughness Index) measurements seemed questionable in view of the intense
dissatisfaction revealed in road user opinion surveys. Particularly perplexing was the acute
dissatisfaction with the ride quality amongst commercial drivers, primarily in the north of
Sweden. Our interest was stimulated even more when interviewing hauliers and transport
purchasers in Västernorrland County. After having studied reams of literature containing
the key word ”vibration”, including reports on the impact of road roughness on driver per-
formance, driver fatigue, reports on incubators in ambulances being badly shaken during
transport, and the high frequency of health problems amongst commercial drivers, particu-
larly in their locomotor systems, sufficient research material had been collected to warrant
investment in this project.

Kjell Ahlin, Licentiate in Engineering and employed at Ingemansson Technology AB was
responsible for the surveys and analyses. Professor Ronnie Lundström of the National In-
stitute for Working Life was in charge of examining the impact on the human body of ex-
posure to those vibrations measured. The vibration data was collated with the SNRA’s
existing road surface condition data (collected through laser/inertial technology) by the
undersigned. The ambulances were driven by Leif Leding, medical orderly, and the trucks
by Hans Selin and Bengt Själander. Vibration measurements were conducted on non-
frozen roads, to comply with the SNRA routine road surface condition surveys. It is im-
portant to keep in mind that the vibration problem is considerably greater during the spring
thaw, when roads are still partially frozen and roughness even more pronounced.

I would like to take this opportunity to express my sincere appreciation to those who pro-
vided the financial backing for this project, as well as the persons mentioned above and
their colleagues, as well as to my own fellow colleagues throughout the Swedish National
Road Administration.

Finally, I would especially like to thank Kathleen Olsson at the SNRA International Secre-
tariat, for making the English translation possible.

Borlänge 15 May 20001

Johan Granlund, MSc (Civil Engineering)
Project Manager2

1Translation finished on 8 March 2002.
2Translation comments: Johan Granlund is now leading road roughness profilometry operations within SNRA Consulting Services. Kjell
Ahlin is now Professor at Blekinge Institute of Technology. Professor Ronnie Lundström is now head of the Biomedical Engineering
and Informatics Department at the University Hospital of Northern Sweden.

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Contents
1 SUMMARY ........................................................................................................................................................ 4
2 READER INSTRUCTIONS ........................................................................................................................ 6
3 DEFINITION OF TERMS .......................................................................................................................... 7
4 BACKGROUND..............................................................................................................................................18
4.1 FROM PAST TO PRESENT.............................................................................................................................. 18
4.2 ROAD ROUGHNESS IN BRIEF ....................................................................................................................... 21
4.3 MONITORING OF THE ROAD CONDITION AT THE SNRA....................................................................... 23
4.3.1 Road roughness measurements........................................................................................................ 23
4.3.2 Road user opinion polls.................................................................................................................... 25
4.4 ANALYSIS OF ROAD ROUGHNESS ............................................................................................................... 28
4.5 TRANSMISSION OF VIBRATIONS THROUGH THE VEHICLE....................................................................... 29
4.6 W HOLE-BODY VIBRATION.......................................................................................................................... 31
4.6.1 Natural frequencies and resonance in the human body............................................................... 32
4.6.2 Examples of the effect of whole-body vibration in the 0.5-80 Hz range..................................... 33
4.6.3 Examples of the effect of extremely low frequency whole-body vibrations ................................ 33
4.6.4 Origin of whole-body vibration...................................................................................................... 34
4.6.5 Measurement of whole-body vibration........................................................................................... 36
5 METHOD ........................................................................................................................................................ 38
5.1 TEST STRETCHES .......................................................................................................................................... 38
5.1.1 National Highway No. 90............................................................................................................... 38
5.1.2 County Road 950.............................................................................................................................. 39
5.2 VEHICLES ...................................................................................................................................................... 40
5.2.1 Ambulances ....................................................................................................................................... 40
5.2.2 Heavy trucks...................................................................................................................................... 41
5.3 MEASUREMENT AND ANALYSIS OF WHOLE-BODY VIBRATIONS............................................................. 44
5.3.1 Variables ............................................................................................................................................ 44
5.4 E XPERT ANALYSIS OF THE EFFECT OF VIBRATION ON THE HUMAN BODY........................................... 46
5.5 COLLATION BETWEEN THE VIBRATION DATA AND THE DATA FROM THE ROAD CONDITION
SURVEYS ................................................................................................................................................................... 46
5.5.1 Effect of emergency action, ”the devil’s choice”, on National Highway 90............................... 46
6 RESULTS ......................................................................................................................................................... 49
6.1 ROAD SURFACE CONDITION AS PER THE SNRA’S ”PMS” DATABASE ................................................... 50
6.1.1 Roughness expressed as International Roughness Index............................................................... 50
6.1.2 Crossfall.............................................................................................................................................. 52
6.1.3 Lane cross-sections............................................................................................................................. 53
6.1.4 Seasonal variation in road roughness, County Road 950 ........................................................... 54
6.2 CAB ACCELERATION MODEL AS A FUNCTION OF ROAD ROUGHNESS (IRI).......................................... 55
7 DISCUSSION.................................................................................................................................................. 58
7.1 ROAD STRETCHES WHERE THE ROUGHNESS PRESENTS A HEALTH HAZARD........................................ 59
7.2 VARIATIONS IN THE ROAD CROSSFALL ARE PARTICULARLY HAZARDOUS ............................................ 64
7.3 METHODS TO REDUCE WHOLE-BODY VIBRATION IN CONNECTION WITH ROAD TRANSPORT .......... 67
7.3.1 Changed travel speeds....................................................................................................................... 67
7.3.2 Changes in vehicles ........................................................................................................................... 69
7.3.3 Road maintenance............................................................................................................................ 70
7.3.4 Does the choice of road maintenance strategy matter? ................................................................ 71
7.4 CONCLUSIONS .............................................................................................................................................. 72
7.4.1 Evaluation of impact on humans of vibrations related to road roughness ............................... 72
7.4.2 Assessment of the need to take action on the road network, etc.................................................. 72
7.4.3 Need for further research and development................................................................................... 74
8 REFERENCE LIST...................................................................................................................................... 75

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Introduction
Apart from the direct impact road roughness and texture has on vehicles and the environ-
ment, these road characteristics are also an indirect source of the noise, infrasonic sound
and whole-body vibration that cause stress on road users. The effect of this stress/load can
be divided into the following three categories, for which criteria can be stipulated and well-
motivated limits set:
1. discomfort
2. performance ability
3. health impact

The effect on the human body depends on the type of load. It varies from individual to
individual, depending on the person’s own particular situation. Reactions can be acute (like
speech impairment), gradually increase during travel (like motion sickness) or steadily de-
velop over time (like spinal injury). The effects can be transient, as in temporary visual im-
pairment, or chronic as in kidney damage. Temporary exposure can cause stress reactions,
like a faster pulse or higher blood pressure, which in turn entails a greater stress on the
heart. Sustained exposure can tire the brain a nd produce drowsiness. Daily exposure can, in
the long run, impair health and result in long periods of sick leave or even early retirement.
Sometimes these ailments can require medical treatment, which in turn can have side ef-
fects that can substantially impair quality of life. Musculo-skeletal injury is by far the great-
est working environment problem in the Western world today.

In the mid 1970’s, the exposure of truck drivers to vibration was an issue raised at the fed-
eral government level in the USA, formulated as ”Do vibrations (as well as noise, toxic
fumes and other factors that contribute to truck “ride quality”) have a negative effect on
driver health and on highway safety?” A research programme that extended over several
years, ”Ride Quality of Commercial Motor Vehicles and the Impact on Truck Driver Per-
formance” was initiated in 1977 to answer this question. The findings were summarised in
a report published in 1982 entitled ”Truck Cab Vibrations and Highway Safety” [66]. This
report was jointly produced by leading researchers, road authorities, vehicle manufacturers,
hauliers and commercial drivers. It shows that the answer to the key question as to whether
there is any correlation between cab vibrations and road safety is YES, that there is good
reason to believe that vibrations affect drivers’ health, and that vibrations must be elimi-
nated at source through effective road maintenance rather than merely dampened. The
report concludes that if the deterioration of the road network is allowed to continue, the
result will be serious health and road safety problems.

Today, further on down the road, we can see how the American road network has been
upgraded. According to the FHWA report Life-Cycle Cost Analysis in Pavement Design,
action is nowadays initiated on federal roads before the condition reaches a level corre-
sponding to IRI 2.7 mm/m [67 ]. In the study conducted during summer on Swedish Na-
tional Highway 90, IRI1 values close to 100 mm/m have been measured3, 37 times above
the American limit. Hw 90 is known to be much rougher during the spring thaw.

At the time of writing, an EC directive stipulating limits for exposure to whole-body vibra-
tion based on health and safety criteria is in the process of being drawn up.

3 Laser/inertial Profilometers have limitated laser measuring range (MR). On the Profilometers used in Sweden MR for vertical distance

is +/ - 100 mm. Since the distance from the laser beam to the front axle of the Profilometer vehicle is close to 1 m, profile slopes (used
when calculating IRI) will begin to be underestimated when they exceed about 100 mm / 1 m = 100 mm/m in static theory case. In
practise, Profilometer pitch and roll dynamic motion reduces this range of use further.

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1 Summary
The overall aim of this study was to ascertain the seriousness of the problem of whole-
body vibration when driving on roads; ”Is the road roughness such that it entails a health
hazard and/or a road safety hazard through its impact on drivers?”. Other objectives were
to estimate the scope of the problem during non-frozen ground conditions, to examine the
problems and potential related to measurement techniques and to point out the necessity
of further research in this field.

The measurement data was collected when driving on 37 kilometres of National Highway
No. 90 (Hw 90) and 21 kilometres of County Road 950 (Lv 950) in Västernorrland County.
The road condition on the test stretches covered the entire range from very smooth (IRI20
= 0.43 mm/m) to very rough (IRI20 = 22.78 mm/m). Whole-body vibration was measured
in compliance with the ISO 2631-1 (1997) standard “Evaluation of human exposure to
whole-body vibration”. This was done on stretchers with patients in different types of
ambulance and at different speeds, and on the floor and driver and passenger seats for
seated occupants in some different truck configurations.

There are three main sources of vibration: road roughness, vehicle properties and driver
behaviour (including choice of speed). The interpretation of the results supports the opin-
ion that within reasonable variations in these factors, road roughness plays a considerably
greater part than the other two. High-energy, multi-directional vibrations at many natural
body part frequencies were found at the seats in trucks. This is serious due to the risk of
resonance, meaning a greater reproduction of vibration in the parts of the body afflicted
than at the surface4 from which the vibrations are transferred. Further, the study substanti-
ates findings from earlier studies; i.e., that the high frequency of occupational diseases
among commercial drivers, especially in the locomotor systems, is related to rough roads.
This relationship is probably strongest in geographic areas where the road roughness level
is high on a large percentage of the roads. Where the roughness was greatest, peak values
were registered on ambulance stretchers that considerably exceed the level that completely
healthy people are assumed to experience as ”extremely uncomfortable” by international
standards.

4 seat, seat back, floor, stretcher

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Vibration
2
(weighted acceleration [m/s ])
on stretchers, and in the case
of the bulldozer, at the
operator’s seat. Some studies indicate that
exposure to vibrations of
1.6
2
1.30-1.35 m/s for 10
1.4 minutes a day can be
harmful even for healthy
1.2
people. The journey by
1 ambulance on the rough
stretch of highway took a
0.8
little more than 15 minutes.
0.6
Clearing forest for new road construction, bulldozer
0.4 CASE 1150 C

0.2 Mobile Intensive Care Unit Ambulance

0
Emergency Ambulance

Rough road, IRI
average = 4.0 mm/m Smooth road, IRI
average = 1.2 mm/m

During a 15-minute ride on a stretch of National Highway 90, the vibration level in one
type of ambulance was high enough to pose a potential health ha zard had a healthy person
been exposed to it for as little as 10 minutes a day. It was shown that the vibration on the
ambulance stretchers was as great as at the drivers’ seat in wheel loaders loading blasted
rock, bulldozers clearing way in forests for new road construction, etc. See the figure
above. Vibration problems are even greater in the spring due to seasonal frost damage re-
lated a dditional roughness.

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2 Reader instructions
The project is presented in three separate reports:

1. The first is a technical analysis of the whole-body vibrations measured in trucks and
ambulances. This report was compiled by Kjell Ahlin, Licentiate in Engineering at In-
gemansson Technology AB [64], and may be of interest for researchers etc.

2. The second analyses the impact on the human body of the vibrations measured. This
report was compiled by Professor Ronnie Lundström at the National Institute for
Working Life [65]. A summary of the conclusions is presented in Chapter 7. The report
is available (in Swedish) at SNRA as well as NIWL websites, using the following links:
http://www.vv.se/aktuellt/pressmed/2000/VVRapport.pdf or http://umetech.niwl.se/Published/.Publ.html

3. The third is the report at hand, compiled by Johan Granlund, MSc (Civil Engineering),
of the Swedish National Road Administration. This report presents the results from
collating the data collected in the annual road condition surveys with the whole-body
vibrations measured on the test stretches. It also compares the results with the ISO
limits for whole-body vibrations, and assesses the magnitude of the problem on the
state network. This report is available on the SNRA website, using the following link
for the Swedish version http://www.vv.se/publ_blank/bokhylla/miljo/2000_31/intro.htm and this link for
the English version http://www.vv.se/for_lang/english/publications/index.htm

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3 Definition of terms
The definitions of the following terms are based on those in the Swedish National Enc yclopaedia [1] with its
appurtenant dictionaries [34], the Swedish Centre for Technical Terminology glossaries [9], Engineering
Mechanics [40], Handbook of Human Vibrations [11], Vägtrafikteknisk nomenklatur (Highway Engineer-
ing Terminology) [16] published by the Transport Research Institute and ASTM´s Terminology Relating to
Vehicle-Pavement Systems [20]

Accident frequency
Number of accidents at a certain intersection, stretch or unit of distance.

Differences in the accident ratio between two road networks show that one
is ”more dangerous” for an individual than the other. Differences in acci-
dent frequency between two road networks depends partially on the differ-
ence in the accident ratio, and partially on the difference in the number of
vehicles using the road networks. A simple way to reduce the accident fre-
quency on a road with heavy traffic volume is to divert certain parts of traf-
fic to other smaller roads. However, as the accident ratio is generally higher
on smaller roads, this would increase the total number of accidents. From
this perspective, the accident ratio is better than the accident frequency for
assessing how dangerous roads are. The road network in Jämtland County
(known to have low traffic volumes but poor roads) has the highest accident
ratio in Sweden.

Accident ratio
Number of accidents related to units of measure in traffic; i.e., the term vehicle kilometres
is the unit commonly used on road stretches. At junctions the unit of measure is the num-
ber of vehicles entering the intersection.

Accuracy
The ability of the measurement instrument to give results close to the true value for the
parameter measured. The greater the accuracy, the less the error.

Alignment
The design of the road profile in space.

Amplitude
Amplitude is the maximum deviation from the mean of a signal (e.g., road roughness, or
vibration), see Figure 10.

Comfort
A subjective state of well-being or absence of mechanical disturbance in relation to the
induced environment (mechanical vibration or repetitive shock).

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Agreeable and practical convenience {relaxed conditions under which to live and work}.

Comfort connotes the absence of significantly disturbing or intrusive physi-
cal factors. It is a complex subjective entity depending upon the effective
summation all the physical factors present in the induced environment, as
well as upon individual sensitivity to those factors and their summation,
and such psychological factors as expectation. (For these reasons, for example,
the same values of vibration that might be judged by most riders to be un-
comfortable in a limousine may be judged acceptably comfortable in a bus.)

The main factors behind comfort reduction (discomfort) are shown in Figure 9.

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Crest factor
The ratio between the frequency-weighted peak value and the frequency-weighted root
mean square for the parameter studied. See Figure 3.

Criteria
A criterion is a verbal description of the effect, e.g. discomfort, reduced performance ability
or physical injury that is of interest. Limits (threshold values, target values, etc.) are set to
ensure an acceptably low probability of the effect that the criterion defines. In other words,
the criteria explain the reasons for the different limits.

Crossfall
The angle between the horizontal plane and the surface of the roadway, carriageway or
shoulder, measured at a right angle to the longitudinal direction of the road.

Ergonomics
Study of the relationship between people and their work environment, especially the
equipment they use. See also [52].

Estimated vibration dose value, eVDV)
An estimation of a cumulative measure of the vibrations and shocks that a person is ex-
posed to during the period under study, based on the frequency-weighted root mean square
for the vibration. See Formula 1.

If the vibration level varies or contains shock elements, the vibration dose value
must be determined directly from the complete measurement series. This is usually
the case when the crest factor exceeds 6 – 9, which makes eVDV less useful for ride
quality assessment on the rougher roads.

eVDV = 1.4 * a rms * T1/4
Formula 1 Estimated vibration dose value during exposure time T
Fracture mechanics
The science of how solid material breaks. This is often characterised by one or more cracks
spreading throughout the mass of a structure, ultimately resulting in its splitting into two or
more parts. Cracks can increase through different mechanisms, like fatigue. An increase in
fatigue occurs in structures exposed to repeated load. The increase can be very little at any
individual load. However, major cracks can form in a very short time through exposure to
vibration. The research that laid the foundation for fracture mechanics was carried out dur-
ing the Second World War. Since the 1950’s, fracture mechanics has developed into an
important element in the mechanics of materials. Most research has been conducted in the
USA and has been motivated by safety demands, primarily within the nuclear power and
aviation industries. Fracture mechanics can be used to answer the question ”how quickly
does a small crack grow through fatigue at the load spectrum to which the structure is ex-
posed?”.

Health
A condition of complete physical, psychological and social well-being, and not only the
absence of illness or disability [World Health Organisation (WHO), 1946].

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Horizontal curve
The curve indicating the direction of the road alignment in the horizontal plane.

International Roughness Index
The IRI value is a substitute measure for the vertical vibration that occurs in the suspen-
sion of a model (”the Golden Car”) of a quarter of a standard passenger car during a hypo-
thetical journey at the speed of 80 km/h on the road stretch studied. The value describes
the accumulated vertical displacement between the car body and the non-suspended mass
of the wheel, divided by the distance travelled. The unit of measure for the IRI is [mm/m],
which is low when the road is smooth along the wheel track5 in which the roughness pro-
file is measured. The IRI is currently the preferred unit of roughness measure used in Swe-
den and many other countries around the world that conduct objective surveys of the road
condition.

Index notation such as IRI 400, IRI 20, IRI 1 etc is used when explaining the
length of report/averaging interval, such as 400 metre, 20 metre and 1 me-
tre. Up until now, the basic report storage interval in the SNRA PMS has
been 20 metre. (As a comparison; the sampling spatial frequency used by ve-
hicle manufacturers fatigue researchers typically must be no longer than
about 1 decimetre, not to lose information about shock that causes damage).

Jerk
The first time-derivate of acceleration. Jerk is thus a measure of how fast the magnitude of
the acceleration changes.

When assessing damage potential, the relation between load and bearing ca-
pacity is studied. The “bearing capacity” of the human body depends strongly
of the state of muscular brace, comparable to the case where a small child is
learning to stand and walk. When exposed to unexpected occasional shock,
an intensive jerk may reduce the chance for the body to suddenly increase its
“bearing capacity” through instinctive brace. This implies that among differ-
ent motions with a similar peak acceleration, motions having a more inten-
sive jerk may be more serious than those with a less intensive jerk.

5 In Sweden, the IRI value is measured in the outer wheel track as seen from the centre of the road. In some countries, it is measured
from a mean profile of the outer and inner wheel tracks instead.

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Limits
The value stating the maximum permitted limit for a source of discomfort or injury.

A limit is generally only set for activities that are planned and governed by
directives issued by public authorities. The general trend in most countries is
towards reducing limits. It is usually the authority responsible for a specific
field of expertise that sets these limits. The health and hygienics limits are
particularly important in the work environment. A health and hygienics
limit is not a sharp line between harmful and non- harmful exposure. In
Sweden health and hygienics limits have a legal status. See also the Swedish
Environmental Code (SFS 1998:808) and the Health and Safety at Work
Act (SFS 1977:1160).

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Macrotexture
Term for those aberrations in the road surface (compared to an ideal plane) that have cha r-
acteristic wavelength and amplitude dimensions from 0.5 mm and upwards to those that
have no effect on the interaction between tyre and roadway.

Measurement error
Difference between the measurement value a nd the true value.

Measurement results
The product of the measurement value and the unit of measure. The measurement value
can have been corrected in connection with this through calibration in order to take known
systematic errors into consideration.

Measurement value
The value for the parameter compared to the unit of measure. Can be identical with the
measurement result.

Motion sickness
A physiological reaction in people induced by vibration, where the central nervous system
is incapable of co-ordinating information obtained visually, from the balance organ in the
ear and from joints and muscles. The reaction can cause drowsiness and affect perform-
ance ability. Symptoms include greater salivation, perspiration, depression, apathy, pallor,
nausea, dizziness and vomiting. Motion sickness seldom occurs in connection with vibra-
tions with a higher frequency than 0.5 Hz. When the reaction occurs in a moving vehicle, it
is usually called ”travel sickness”.

The most renowned hypothesis for a qualitative explanation for the origin of
motion sickness is called ”the sensory conflict hypothesis”[36]. A schematic dia-
gram of this hypothesis is shown in Figure 1. Several other conflict hypotheses
are discussed in Griffins ”Handbook of Human Vibrations” [11].

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Stimuli Receptors Central Nervous System Responses

Active Motor
control
movement Volitional
system and reflex
move-
ment
Eyes Updates internal
Internal model
model (adaption)
Semicir- neural store of
Motion cular expected signals
stimuli canals

Otoliths Leaky Neural centre Motion
and other Compa- integ- mediating signs sickness
gravi- rator & symptoms of symptom
ration
receptors motion sickness
Passive
movement Mismatch signal Threshold

Figure 1 Schematic diagram of the sensory conflict hypothesis. This figure has
been modified by Förstberg [36], originally developed by Benson (1988).

Natural frequency
The most fundamental property of an oscillating system. Natural frequency constitutes the
free oscillation frequency of a system after having been disturbed. Every real system has
several natural frequencies, and each of these has a given pattern of movement. When a
system is subjected to an external disruptive (driving) force whose frequency is equal to a
natural frequency in the system, resonance occurs and the magnitude of the vibration in-
creases. See Figure 2.

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Figure 2 Mechanical model of the human body specifying natural frequencies
for a few parts of the body [51]. Observe that the body lacks the female
bosom. The natural frequencies refer to vibrations in the axial direction of the body
parts (e.g. the spinal column)

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Parameter
A characteristic that is the object of measurement.

Pavement Management Systems, PMS
A system that in an organised, co-ordinated way manages the road administration process.

Peak value
The maximum deviation from the mean of a parameter during a given interval. See Figure
3. The peak value is used especially when assessing the risk of mechanical damage from
motion/force sequences of short duration - shock.

Precision
The degree of agreement between a number of values measured, determined through re-
peated measurements. Precision has nothing to do with the deviation of the values ob-
tained from the true values for the parameter. Precision is sub-divided into repeatability
and reproducibility.

Repeatability
The precision of the values measured for a given parameter, determined in a uniform way
and under similar conditions.

Reproducibility
The precision of the values measured for a given parameter, determined in a uniform way
but under different conditions, such as another measurement method, another operator,
another instrument or another point in time.

Resonance
General phenomenon in oscillating systems implying that even a weak intermittent external
disruption (driving force) within a narrow frequency range can result in a large increase in
the oscillation amplitudes, accelerations and energy content of the system. This increase
depends on the frequency and becomes maximal when the frequency is largely equal to the
free natural frequency of the system. Through resonance, large amounts of energy can be
transferred by the driving force to the oscillating system, in connection with which damage
or disruptions in operation often occur. This phenomenon is of key importance from a
safety perspective, etc.

Road alignment
The (imagined) large scale vertical and horizontal curvature of a road.

Road roughness
Term used for deviations in a road surface compared to a real plane, which affect vehicle
movement, ride quality, dynamic loads, drainage and winter maintenance.

Roadway
Carriageway including the shoulders.

Roll
Movement of rotation around the x-axis. See Figure 24.

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Root mean square, rms
The root mean square of a variable during the period studied. See Formula 2 and Figure 3.

When assessing ride quality, effects of occasional shock are often of great in-
terest. The running root mean square of the weighted acceleration (using in-
tegration time 1 s) is then often the preferred definition of vibration, since
this definition has been proven to correlate very closely with perceived an-
noyance [75]. To assess the risk of mechanical damage to the spine, the
weighted positive (compression phase) peak acceleration is the preferred defi-
nition.

t2

∫ a (t ) 2 dt
a rms =
t1

t 2 − t1
Formula 2 Root mean square for acceleration

Root sum of square, r.s.s.
A summation procedure for vectors in different directions. For the root mean
square, the square root is taken from the sum of the vectors squared root mean
squares.

Running rms
A filtering procedure that smoothens a very transient measurement series (as where occa-
sional shocks have occurred) that have a high crest factor.

Second Law of Newton
The acceleration of a particle is proportional to the force acting upon the particle and oc-
curs in the direction of that force. Normally expressed in dynamic analysis as F = m*a.

Stress
The physiological/hormonal reactions in the organs of the body that are triggered by
physical and mental ”stress factors”. Threatening or strenuous situations stimulate in-
creased secretions of adrenaline and noradrenaline. These hormones function such that
they increase the heart rate, blood pressure and circulation of blood to the skeletal muscles,
while decreasing the circulation of blood to other organs. Further, breathing is stimulated,
the trachea expand and the level of sugar and fatty acids in the blood increases. When peo-
ple are unable to control their own situation, the cortisol level also increases substantially.
Cortisol increases the amount of glucose in the blood, as well as the turnover of fats and
proteins. These and some 1400 other reactions to stress mean that the body, through all its
endeavours to adapt to the situation, is prepared to destroy itself after being subjected to an
all too extended or strenuous load.

Survey
A series of measures to determine the value of a parameter.

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Unit of measure
Reference value for a parameter; e.g., in the case of distance, a metre could be used as a
unit of measure.

Vibration
Oscillation in mechanical systems, where parts of the human body and human organs can
be included. (See Figure 2). This is governed by different kinds of force: mass, restoration,
calming and disruptive (driving, instigating) forces.

Vibration can be measured in terms of displacement, speed or acceleration.
The unit of measure used for acceleration is [m/s2]. Results are usually pre-
sented as peak values (mechanical spinal damage etc) or as a root-mean-square
or running root-mean-square (perceived ride quality etc). See Figure 3.

Peak value
Root mean square
Mean value

Mean value

Figure 3 Peak value, root mean square and mean value for a signal

Vibration dose value, VDV
A cumulative measure of the vibrations and shock elements to which a person was exposed
during the period under study. See Formula 3.

t2

VDV = 4 ∫ a w (t ) 4 dt
t1

Formula 3 Vibration dose value for acceleration

Wavelength
The shortest distance between two of the signal’s points with an equal phase. See Figure
10.

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4 Background
To most people, a roadway is merely a charcoal grey surface of infinite length. They expect
it to provide safe driving conditions, a smooth and quiet ride, minimal splash and spray
when it is raining, good visibility during poor conditions and that it will last a long time (to
avoid disturbance from road works).

A closer examination of the road reveals that it has several important characteristics, such
as surface texture. Texture is needed to provide road grip, minimise spray and mist when it
is raining, and reduce the glare from high beams at night. But the texture can cause more
noise as well as reduce the life span of both roads and tyres. As the road surface ages and is
worn down by studded tyres, heavy vehicle loads and climate, road damage begins to a p-
pear. Deformation (or road roughness), which is one type of damage, can limit both
shorten the life span of a road as well as reduce the quality of the ride. Roughness, primar-
ily longitudinal, can also be built into the road from the outset due to poor geometric de-
sign/construction.

Roughness is the source of many kinds of irritation that road users encounter; flickering
headlight reflections, deep pools of water, the dynamic forces that increase pavement stress
and damage to vehicles and cargo, and poor ride quality. Most road users are very sensitive
to ride quality, making this a prime criterion when setting road maintenance priorities [68].

Road roughness can mean reduced travel speeds. This has led many to believe that rough
roads are safer than smooth ones, since speed is generally acknowledged as dangerous.
However, after collating databases with information on accidents, road surface condition,
climate, road geometry, speed, etc at VTI (Swedish Road & Transport Research Institute),
it was concluded that ”the accident ratio increases with an increase in the roughness” and
”roughness has a major impact” [18]. A strong correlation between road roughness and the
accident ratio on the paved part of the state road network in Sweden has thus been ascer-
tained, implying that the idea of rough roads being safer is probably a serious misconcep-
tion.

However, such a statistical correlation is not clearly tantamount to the accident risk actually
being caused by roughness. For instance, it is likely that the vehicles on the roads in rural
areas, where roughness is worst, are older and less roadworthy than those found on the
smoother roads in and between the larger cities. It is therefore necessary to verify a statisti-
cal correlation through experiments that provide information about possible mechanisms
for an actual cause and effect relationship. This could include the effect of roughness
through mechanical interference on the steering and braking properties of road vehicles,
the effect on winter road maintenance and on drivers’ performance ability.

4.1 From past to present
The mechanisation of human transport on roads has taken place in a very short time. Peo-
ple were still basically travelling on the backs of animals or on foot in the 18th century,
despite the fact that the invention of the wheel 3 500 years before Christ had made the
development of animal-drawn carts possible. The reason was that roads were often almost
entirely impassable, which explains why the carts were primarily used to transport goods,
and even then only at average speeds up to about 10 km/h. This meant that travellers usu-

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ally experienced more discomfort from dirt than from vibrations. On the narrow streets of
Stockholm, the upper class avoided the dirt by being carried on palanquins. A tariff for this
was laid down in 1726 [58].

Through Carl Snoilsky’s classical poem on Stenbock’s courier, generations of Swedish stu-
dents have learned about how Captain Henrik Hammarberg, who was sent off by General
Magnus Stenbock from Helsingborg with a message to the king and government in Stock-
holm on February 28th 1710, rode so hard that ”a horse collapsed behind him at every
station”. Modern historians maintain, however, that the courier, who covered the journey
of 900 kilometres at an average speed of 18 km/h, probably did not ride on horseback at
all. Verification ”Folio 2282” in the 1710 treasury records (preserved in the National Ar-
chives) for Hammarberg’s travelling-expenses account clearly shows that he travelled by a
carriage drawn by a team of horses from station to station. It is believed that he suffered
from travel sickness during the journey; ”this coach is swaying so frightfully on these terri-
ble roads” [59].

The bicycle could enter the scene at the turn of this century, as a result of the soft non-
bituminous roads, which could be evened out by simple means as needed. Bicycles were
crowded further and further out to the periphery as motoring became more widespread. In
the past 130 years, mobility for people in Sweden has grown a thousand fold. See Figur 4.

Mobility trend in Sweden, 1850 - 1990
50000

45000

40000

Decade Travelled distance [m] 35000

1850 40 30000

1870 200 25000

1890 350 20000

1910 900 15000

1930 3000 10000

1960 20000 5000

1980 40000 0
1850 1870 1890 1910 1930 1950 1970 1990

Decade

Figur 4 The average daily distance travelled by vehicle by adults in Sweden
from the 1850’s to the 1990’s. Data in the tables extracted from [61], supple-
mented in the figure with data from [62].

Roads became steadily harder throughout the years, necessitating more sophisticated care
and maintenance routines. The long distances covered at the high speeds that characterise
modern road traffic, mean an exposure to vibration and shock that, in the presence of sig-
nificant road roughness, can mean people being exposed to mechanical energy that is sub-
stantially higher than at any other time in history. According to the second law of Newton,
the magnitude of this mechanical load can be estimated through measuring the acceleration
of whole-body vibration.

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The mental stress is more difficult to measure. At the macro level, it has been shown that
the risk for certain types of cardio-vascular disease in Sweden is more than three times
higher for commercial drivers than for the average worker. Mental stress under certain
driving conditions is considered to explain the raised level of stress hormones found in
commercial drivers, and is believed to cause the problem [69, 70].

Amongst older commercial drivers, musculo-skeletal problems and cardiovascular diseases
are the primary reasons for changing their occupation [71].

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4.2 Road roughness in brief
Road roughness can result from faulty basecourse adjustment (usually through insufficient
repair of crossfall variations or longwave deformations); incorrect initial construction; post-
compaction of added layers; subgrade settlement; material abrasion -- primarily by studded
tyres -- or through uneven frost heave during the spring thaw.

A general rule, based on laboratory tests, is that drained road structures can stand being
driven on by trucks six to seven times more than those without drainage before unaccept-
able deformation occurs in the unbound layers. Ditching is thus a very effective mainte-
nance measure for preventing roughness, if it is executed so that the gradient of the inner
embankment is not steeper than 1:3 (otherwise there is the risk of edge deformations due
to insufficient lateral counterstay). A drained road structure is also a prerequisite for avoid-
ing frost-related roughness in winter.

The binder stiffness affects the ability of the asphalt to distribute the load and thus the risk
of deformation in underlying layers. Temperature is a key factor for this stiffness, which
means that dark asphalt roads become rough faster than those with a light surface.

The mechanical properties of vehicles can also increase roughness. As early as in the
1930’s, a large-scale experiment showed a substantial increase in roughness on gravel roads
when the test vehicles had high-pressure tyres, while there was not even enough roughness
to measure when low-pressure tyres were used (the roadway had actually been smoothed
out). Speed and suspension were also shown to be major factors affecting roughness [56].
These conclusions could even be valid today for roads with a thin surface, like single sur-
face treatment (Y1G), which is very similar to the dust abatement measures undertaken on
the old gravel road.
8,00

7,00

6,00

5,00
IRI [mm/m]

4,00

3,00

2,00

1,00

0,00
01

01

-01
-01

-01

-01
-01

-01
-01

-01

-01

-01
1-

0-

-12
-03

-06

-09
-04

-07
-02

-05

-08

-11
-0

-1
00

00

00
00

00

00
00

00
00

00

00

00

Tid på året

Figure 5 Conceivable variation in roughness per month, on a specific road sec-
tion

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Road roughness varies throughout the year, which often is more noticeable than the annual
deterioration in the road condition. An estimation of what the variation might be like over
the year on a road that is not treated with salt de-icer and that is slightly frost-damaged is
shown in Figure 5. Variable weather conditions in addition to winter maintenance measures
accounts for the greater variation shown in the graph for the winter months, while the ex-
tremes in the spring are explained by the thaw. While there is a high rate of steadily increas-
ing deterioration on roads with deficient bearing capacity and/or problems related to fro-
zen ground conditions, the deterioration on well-constructed roads is minor and disappears
in time (except during the late stage when surface abrasion occurs if no preventive meas-
ures are undertaken). Roughness can be eliminated through appropriate periodic mainte-
nance. Road strengthening serves to reduce roughness immediately, while also retarding its
future speed of increase. Needless to say, this applies regardless of whether the improved
bearing capacity has meant a change in the administrative bearing capacity class of the road.

For microtexture, as well as that part of the macrotexture with wavelengths shorter than ca
25 mm, it is important that the road roughness amplitudes are neither too large nor too
small. To a certain extent, this kind of roughness produces desirable effects; like friction,
noise reduction, a certain amount of drainage, etc. Some effects are undesirable, like greater
wear and tear on tyres [2].

All roughness with wavelengths above ca 25 mm increases transport costs [2]. It is possible
to correct roughness with amplitudes under ca 15 - 30 mm and with wavelengths up to
about 10 metres simply through a new wearing course. Roughness with larger amplitudes,
or of a more longwave nature, is remedied through milling or more fill works. Frost-related
roughness normally demands highly extensive and expensive measures, such as deep drain-
age and extensive material replacement. The maintenance and repair budget (per square
metre road surface) must therefore be several times higher for roads damaged by frost than
for roads damaged by traffic.

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4.3 Monitoring of the road condition at the SNRA

Roads with an IRI > 5 mm/m

14%

12%

10%

Northern Region
Central Region
8%
Stockholm Region
Western Region
6% Mälardalen Region
South-Eastern Region
Skåne Region
4%

2%

0%
95 96 97 98 99
Year

Figure 6 Development of severe roughness on the paved state road network,
expressed as time series of IRI 20 > 5 mm/m per road management re-
gion. A lower percentage indicates fewer very rough stretches on the road network.

4.3.1 Road roughness measurements
The SNRA regularly measures roughness on paved state roads using high technology sur-
vey vehicles. European and National Highways are surveyed annually, and other roads at
least every third year. Up until now, the parameters that have been of greatest interest are
ruts, crossfall and roughness. The IRI value [mm/m] is the most important measure of
roughness, and is calculated from the road roughness profile measured. The IRI value can
be said to describe the vertical vibrations in the suspension of a mathematically simulated
passenger car driving at a speed of 80 km/h, and is affected primarily by roughness with
wavelengths between about 1 and 30 metres. IRI is very similar to the measures of rough-
ness used in the USA as early as the 1920’s when roughness began to be measured using
simple vehicles. These roughness measures were successfully used to stimulate competition
among civil engineers and contractors to achieve better ride quality through their being
officially published as objective comparisons of different road projects [54].

Today’s survey results are analysed and interpreted as the basis for budget discussions, set-
ting priorities, research projects, evaluating performance contracts, etc [8]. Figure 6 shows
the percentage of roads with excessive roughness (very high IRI values) in all road man-
agement regions. Signs of improvement can be seen, particularly up to 1999.

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However, an entirely different, negative trend was found when reviewing road users’ opin-
ions on ride quality. See Section 4.3.2. The discrepancy between the results from the road
condition and road user surveys can perhaps be attributed to the fact that people are travel-
ling more (which can increase the exposure to vibration even if the road roughness is un-
changed) and that the annual road condition surveys are only performed when there is no
ground frost, for reasons of measurement precision. Roughness on frozen roads can be
much worse than on non-frozen roads, and ground frost conditions vary substantially from
year to year. Much higher local IRI values have been measured on frost-damaged roads,
than what has been registered in the routine surveys in the summer months.

During the quality a ssurance of road condition surveys, it was observed how the vehicle
operators found it much more difficult on rough roads to follow the driving instruction
requirements. In other words, roughness has a strong adverse effect on driver performance
[private comment made by Kerstin Svartling, administrator for the SNRA’s road condition
surveys].

The significance of different types of roughness and different speeds can be studied in
Figure 7.

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Figure 7 Typical vertical motions at the rear axle and in the car body when driv-
ing at different speeds on different road roughness [12]

4.3.2 Road user opinion polls
The road user opinion polls conducted by the SNRA between 1995 and 1998 included 30
000 people. The questions cover new roads, care and maintenance. On the whole, the find-
ings were not too negative. The majority of the interviewees were satisfied in most respects,
with one major exception being road roughness.

The smoothest roads in Sweden are found in Skåne Region (southern Sweden). Despite
this, the percentage of commercial drivers in Skåne who are satisfied with ride quality on
the national road network is as low as 30 – 35 percent. The percentage of satisfied road
users is much lower in other parts of the country and with respect to other types of road.

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Dissatisfaction is greatest and growing most rapidly in the northern half of the country. For
example, it is expected that 0% (none) of the commercial truck drivers in the counties of
Västernorrland, Jämtland, Gävleborg and Dalarna (the Central Road Management Region,
being part of “northern Sweden”) will be satisfied with the ride quality on regional thor-
oughfares in the winter of 1999. See Figure 8.

SNRA Central Region
(Västernorrlands, Jämtlands, Gävleborgs och Dalarnas counties)

Marks given by commercial drivers for the ride quality on regional roads

Results 1996 Results 1997

8%

18%

82%

92%

Percentage dissatisfied
Results 1998 Percentage satisfied Prognosis 1999

0%
5%

95%
100%

Figure 8 Commercial drivers’ marks for ride quality on regional roads. Väs-
ternorrland, Jämtland, Gävleborg and Dalarna Counties [6].

At the national level, the percentage of commercial truck drivers who are satisfied with the
quality of the ride is about half that of passenger car drivers. However, even the percentage
of passenger car drivers who are satisfied in this respect is low [6][37].

Factors that are known to influence people’s sense of discomfort are shown in Figure 9.

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Texture Vibrations Vibration-
Roughness related discomfort

Alignment Jerky ride Vision disor-
ders

Megatexture Shock Difficulties in
Roughness handling

Noise and Sleeping disor-
Roughness Infrasound ders

Texture Visually Variance between Noisiness
Roughness
individuals

Road signs Information Speech diffi-
Road markings culties
The individual Ride quality
Rest areas Food / beverages Sweating / (sum of discomfort)
freezing

? Odours Air quality
Variance for
the individual

? Temperature Glaring
lights

? Body posture Uncomfortable
posture

? Privacy Social discom-
fort

Disruptive Other Other
road works

Figure 9 Factors associated with road management that produce discomfort in
connection with road transport. The figure has been modified on the basis of
[11]. The figure also helps us understand for example that faulty, irregular crossfall,
unsuitable texture and major road roughness cause many different kinds of discom-
fort. Road damage, along with recurrent disruptive road works, thus results in poor
ride quality.

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4.4 Analysis of road roughness
The deviation of the road surface from a horizontal plane can be described by the wave-
lengths and amplitudes of the roughness, see Figure 10. The very shortest roughness wave-
lengths are classed as microtexture, which is determined by the properties of the aggregate
and binder in the surface. Somewhat longer wavelengths are classed as macrotexture,
which is determined by such things as the shape of the aggregate and the particle size dis-
tribution. Longwave deviations are quite simply designated as roughness [2], often caused
by more or less extensive settlement, frost heave or ice lenses in or under the road struc-
ture in the winter.

λ A

λ/2

A
Figure 10 Wavelength (λ) and amplitude (A). Above at corrugation, below at a
pothole.

The basic relationship between travel speed (velocity) v [m/s], wavelength λ [m] and verti-
cal vibration frequency f [s-1] is shown in Formula 5. Depending on the travel speed and
type of vehicle, vehicle properties are a key factor where the wavelengths are up to 25 - 50
m. Where the wavelengths are longer (or more to the point, at lower frequencies) the
dampening property of the vehicle is insignificant [11]. The equation should therefore pro-
vide a reasonable estimation of the vibration frequencies where the roughness is of longer
wavelength. Vibrations with a frequency of 0.1 Hz are caused by roughness (unevenness)
with wavelengths of about 85 m at a travel speed of 30 km/h (8.3 m/s) and wavelengths of
about 360 m at 130 km/h (36.1 m/s). A vibration frequency of 0.5 Hz is caused by rough-
ness with wavelengths of some 15 - 20 m at a speed of 30 km/h and 70 m at 130 km/h.

λ = v/f

Formula 5 The basic relationship between roughness wavelength, travel speed
and vertical vibration frequency (one wheel, no suspension).

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4.5 Transmission of vibrations through the vehicle
The magnitude of the vibration transmitted to vehicle occupants through the vehicle de-
pends on the road roughness and the speed of travel, as shown clearly in Figure 7. This is
accentuated by the mechanical properties of the vehicle. For instance, those riding in
higher vehicles are exposed to a greater amount of pitch and roll than those in low vehicles
[13].

Two-axle cars, are said to have three natural frequencies for vertical vibrations: one that is
related to the car body bouncing on its suspension, one that is connected to the wheel axle
hop between the body suspension and the tyre suspension, and one that originates from
the rocking of the car seat. The car body has a natural frequency of about 1 Hz, and vibra-
tions close to this frequency are amplified by a factor of 1.5 – 3.0. The wheel axles of a car
have a natural frequency of 10 – 15 Hz, which means that at this frequency they tend to
vibrate more than what the car body and tyres together with the road surface would di-
rectly cause [13][14].

Formula 5 indicates that a vibration frequency of 1 Hz when travelling at 90 km/h is
caused by roughness with wavelengths of about 25 metres. Vibration frequencies of 10 –
15 Hz at 90 km/h seem to be caused by roughness with wavelengths of about 2 - 3 metres.
Multi-axle vehicles that are both heavy and long may have considerably different mechani-
cal behaviour than normal cars, particularly if they are towing heavy trailers. Moreover, the
properties of heavy vehicles are changed substantially by the actual weight of the payload.
Some types of heavy-duty vehicles lack suspension altogether.

The natural frequency of the roll of heavy vehicles is less than 3 Hz. Since roll motions at
frequencies under 5 Hz are not common when driving on roads with ”normal”(?) rough-
ness and at normal speeds, it is not usually considered to be of any greater significance.
[55]. This item will be under further discussion later in the report.

The current European trend towards fewer and more specialised hospitals is resulting in a
greater percentage6 of ambulance transports having to cover longer distances while simul-
taneously administering intensive care. To manage this, more -- and heavier -- medical
equipment is required on board. Ambulances must then have a greater load capacity than
before, which means that large vehicles (”container ambulances”) designed similarly to
trucks are needed. See Figure 11. An effective load capacity of more than a tonne is not
unusual.

In many cases it has been shown how even slight road roughness can, through vibration
and dynamic weight transfer, cause the wheel load to temporarily exceed twice the static
load and then revert just as suddenly to 0 (zero!) during the ride. See Figure 12. A feeling
for how dynamic loads can originate can be created by bouncing a little on the bathroom
scales. That the road grip varies between the wheels – and moreover is occasionally non-
existent – involves a major risk of skidding when hitting the brakes in an emergency. [3].
Dynamic loads have been proven to be a large problem when weighing vehicles in motion,
even on very smooth road stretches [48].

6Today, there already are some 850 000 ambulance transports per year in Sweden. Of these, about 200 000
are emergencies [29, 17].

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Figure 11 Well-equipped “Mobile Intensive Care Unit” type of ambulance. The
effective load capacity of the vehicle in the picture is 1.44 tonnes. Notice the heavy-
duty wheels, which are even mounted in pairs at the rear.

Wheel load

Wheel axle hop

Road roughness profile

Figure 12 Dynamic change in the wheel load when driving on a rough road [57].
The static wheel load is designated as ”p” in the figure. As seen here, the actual
wheel load -- which determines the road grip and thereby the risk of skidding when
braking -- varies between 0 (zero!) and twice the static load as roughness in the road
profile causes vibrations and weight transfer in the vehicle.

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4.6 Whole-body vibration
Describing the consequences of shaking the whole human body -- a complex, active, intel-
ligent structure -- is not a completely simple matter. The National Board of Occupational
Safety and Health has compiled the known effects of humans being over-exposed to
whole-body vibration. A few of the conclusions were: ”It can be assumed without a doubt
that the human being is negatively affected by whole-body vibration, from both a subjec-
tive and objective perspective” and ”It would obviously be desirable from everyone’s point
of view if vibrations could be totally eliminated” [51].

Needless to say, there are also vibrations that are positive. For instance, vibrations that
inform drivers about the movement of their vehicle, [11, 66], that they are driving over a
zebra crossing or that the right front wheel ha s a puncture. For safety reasons, such vibra-
tions -- ”a sense of the road” -- should not be dampened.

Like auditory stimuli, sensory impulses impart strong impressions. These should therefore
be used sparingly, since they partially block or suppress other sources of information. [57].
For instance, unlike operators of forestry machinery working out in the woods, those driv-
ing on public roads find it completely reasonable to expect the underlying surface to be
smooth enough that any vibration generated would be insignificant. In an upcoming EC
directive for limiting exposure to whole-body vibration, it is stipulated that ”the risks aris-
ing from exposure to mechanical vibration shall be eliminated at source or reduced to a
minimum [with the aim of reducing exposure to below the threshold level].”

The survey conducted by the National Board of Occupational Safety and Health on the
effects of overexposure to whole-body vibration showed that although this primarily causes
fatigue, it also gives rise to visual acuity disorders, motion sickness, dizziness,
back/abdomen/face pain, headaches and a frequent need to urinate [51].

That very extreme acceleration causes bodily injury is a factor that has set limits on the
manoeuvrability of manned fighter aircraft. Based on fracture mechanics, it is not unlikely
that even the substantially less intensive forces (but at higher frequencies) that cause more
”normal” whole-body vibrations can cause physical injury in connection with long-term
exposure.

When being subjected to vibration, human body reflexes try to protect organs that are sen-
sitive to resonance through a tightening of the muscles (this is only successful for very
short periods - seconds) [38]. Lengthy exposure to vibration therefore often results in high
muscular tonicity [15], which is dangerous to health on many accounts.

A governmental working committee on public health has estimated that the cost to society
for back problems, which is the primary reason for people reporting in sick and for early
disability retirement in Sweden, exceeds SEK 20 billion per year (1991). In its report, the
committee also ascertained that whole-body vibrations are of ”key importance” as a source
of back problems [53]. However, in the general health statistics, the concept of whole-body
vibration is lacking. ”Vibration injuries” primarily refers to hand/arm vibrations [33]. In
England a direct relationship has been found between the frequency of back problems and
the distance travelled per year [60].

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The findings of a recent review of epidemiologic studies conducted between 1986 and 1997
on the relationship between exposure to vibration and problems in the lumbar part of the
back provided ”clear evidence for an increased risk for LBP disorders in occupations with
exposure to WBV. Biodynamic and physiological experiments have shown that seated
WBV exposure can affect the spine by mechanical overloading and excessive muscular
fatigue, supporting the epidemiologic findings of a possible causal role of WBV in the de-
velopment of (low) back troubles”. The review also mentions that it is estimated that 4-7%
of the working population in the EU is exposed to potentially harmful whole-body vibra-
tion [74].

Surveys have shown that truck drivers are exposed to considerably greater vibrations than
most other categories of road user. The exposure often exceeds the recommendations and
limits in the International Vibration Standards [10, 7], as well as the limits proposed by the
EU.

Sensitivity to vibration differs substantially between men and women. Women (and the
foetus) are particularly sensitive during pregnancy [30].

Those who are most sensitive to vibration are injured, sick or disabled people who often
require ambulance transportation. The National Swedish Institute for Working Life has
compared the noise and vibration properties in traditional ambulances and the increasingly
more common larger MICU container ambulances. A major difference was found. The rms
for vertical vibration (0,5 – 80 Hz) at the driver’s seat in a large ambulance amounted to as
much as 1.44 m/s2. Interpretations of the findings indicate that levels above 0.5 m/s2 entail
an excessive risk for any normally healthy person sitting behind the wheel 6 hours a day.
The surveys also showed that the vibrations in an infant incubator on board are often even
greater than at the driver’s seat. On one occasion, the rms for the vertical vibration in the
incubator was ranked as a 5 (very uncomfortable) on a six-grade scale of discomfort in the
ISO 2631-1 “Evaluation of human exposure to whole-body vibration” standard. [17, 29].
According to ambulance orderlies, acute motion sickness is a common problem for staff
and patients alike.

More can be learned about the effects of whole-body vibration in the report by Prof.
Ronnie Lundström [65]. Additional information – including the effect of such loads as low
frequency noise and infrasound – can be found in other reference literature compiled in
Chapter 8 as well as from such sources as the National Board of Occupational Safety and
Health, the National Swedish Institute for Working Life’s Vibration Committee [31], Upp-
sala Academic Hospital [15] and the Swedish Road and Transport Research Institute.

4.6.1 Natural frequencies and resonance in the human body
All material bodies have a natural frequency, which to some extent can be compared to the
natural frequency of a swinging pendulum. When a body is exposed to a frequency vibra-
tion that coincides with its own natural frequency, it will vibrate strongly.

The various parts and organs of the human body have different natural frequencies. This
means that the body does not vibrate uniformly, but rather that the different parts behave
like individual, albeit interlinked, material bodies in this respect. (See Figure 2). External
vibrations with frequencies of about 6 Hz are amplified through resonance in the abdomen
by up to 200%. Certain vibrations are amplified in the spine by up to 240%. The head has a

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natural frequency of about 25 Hz, which means that vibrations with frequencies around
this are amplified by up to 350% [41][42]. The resonance phenomenon leads to a greater
load on the body and thereby a greater risk of injury.

4.6.2 Examples of the effect of whole-body vibration in the 0.5-80 Hz range
Shortwave road roughness produces vibrations with a frequency content that among other
things includes the 0.5 - 80 Hz band.

According to the ISO 2631-1 standard, vibrations with frequencies between 0.5 - 80 Hz
could probably cause a greater risk of injury to the vertebrae in the lumbar region and the
nerves connected to these segments. Exaggerated mechanical strain can be a factor in the
deterioration of the lumbar segments. Vibrations are reported as affecting the body
through causing deformation of the spine (spondylosis deformans), damaging the cartilage
between the vertebrae (osteochondrosis intervertebralis), and by producing chronic pro-
gressive change in the cartilage and bone tissue (arthrosis deformans). Exposure to whole-
body vibration can also exacerbate certain endogenous pathological disorders of the spine.
It is not considered unlikely that the digestive system, the urinary and sexual organs and the
female reproductive organs are affected. Health impairment caused by whole-body vibra-
tion normally only occurs after several years of exposure [10]. Spontaneous abortion is an
exception.

4.6.3 Examples of the effect of extremely low frequency whole-body vibrations
Longwave road roughness produces low frequency vibrations. Vibrations with greater am-
plitudes within the 0.1 – 0.63 Hz frequency band have a particularly strong effect on peo-
ple.

According to the ISO 2631-3 standard, these vibrations cause various degrees of motion
sickness, ”travel sickness”, even after only short exposure. Motion sickness can affect peo-
ple for hours, and even up to days after an arduous trip. It has been observed that motion
sickness lowers performance ability and reduces alertness.

A survey conducted amongst 300 students revealed that about 58% had felt nauseous dur-
ing car rides. Some 33% could remember actually having vomited during car trips before
the age of 12 [11].

A nationwide questionnaire revealed that motion sickness is a frequent working environ-
ment problem amongst ambulance orderlies. 23% replied that they easily felt nauseous
during the ride. [27]. Orderlies in Sollefteå Municipality reported having observed palpable
travel sickness symptoms in patients (in the worst case vomiting, uncontrollable bowel
movements, etc) in 20-25% of the most acute (high speed) transport situations. In the care
unit of the vehicle, it is impossible to watch the horizon.

Vehicle manufacturers are aware that the suspension properties affect the risk of passen-
gers developing motion sickness. A sports car type of suspension is recommended for
people who easily get car sick. It cannot be ruled out that ”comfort suspension” -- by
American standards -- can mean that the high frequency vibrations caused by road surface
damage are converted to an exceptionally high degree into that very type of low frequency
vertical vibration that is known to cause motion sickness. It is also known that rotation
vibrations are a factor in motion sickness. Perhaps even the differences in roll-stability be-

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tween different vehicles has a major impact? Added to the ”motion sickness vibrations”
created by the ride are the low frequency vibrations that ensue from the billowing align-
ment that roads originally built for horse drawn carriages often have.

4.6.4 Origin of whole-body vibration
According to the Academy of Engineering Sciences (IVA) road roughness is a much
greater cause of vibration in road vehicles than in-vehicle factors (wheel imbalance, drive-
line, etc.). IVA has also ascertained that vibrations have a major impact on the steering and
braking properties of the vehicle, and on the working environment, ride quality, health and
possibly even performance ability of vehicle occupants [19].

Whole-body vibrations originate from two different types of force. A random and sudden
force designated as shock. When the wheel hits a bump or sinks into a pothole, shock oc-
curs. If this shock is strong enough, passengers without a safety belt can be thrown from
their seat. They could also be hit by a loose-flying object. Shock can also cause severe spi-
nal injury [32], such as in several Scandinavian cases due to riding in buses over traffic
calming road humps. Less sudden displacements and forces occur during a normal ride on
more or less rough roads. See Figure 13 - Figure 15. These are the most common motion
induced forces that we experience during a normal day [42]. The second law of Newton
can be used to calculate the dynamic forces that vibrations transfer to human organs.

Certain types of vibration are known to cause car sickness. These include extremely low
frequency vertical vibrations (0.1 – 0.63 Hz) and roll (often in combination with lateral
displacement). The low frequency vertical vibrations are caused by exceedingly longwave
roughness (up to 350 m), but can also occur when a vehicle with worn or poorly designed
wheel suspension transforms high frequency vibrations to low frequencies. Roll occurs
when there is an unfavourable variation in the gradient between the wheel tracks (crossfall);
this often is caused by roadway deformations and all too sharp curves in the alignment.
The limits for whole-body vibration in the ISO 2631-3 Standard can be converted into
standard specifications for the road roughness profile. See Figure 16. On roads where there
is substantial roll (caused for example by sharp curves or deformed edges) the acceptable
longwave road roughness must be reduced by 25%.

Major vertical
motion

Figure 13 Origin of vertical vibrations on roads where the roughness wavelength
coincides with the distance between the vehicle axles. Adapted from [66]

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Major pitch and thus
major longitudinal
vibration

Little vertical
movement

Figure 14 Vibrations in the direction of travel occur when the wavelength of
the road roughness does not coincide with the distance between the vehicle axles.
Adapted from [66].

Major roll and thus
substantial lateral vibration

Figure 15 Origin of lateral vibrations on a road with deformed edges

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2.00
1.80
1.60
1.40
1.20
1.00
0.80
0.60
0.40
0.20
0.00
153 122 95 76 61 49 38 31 24
Length of the hollow or ridge
(i.e., half the roughness wavelength), [m]

Figure 16 Limits for longwave roughness at a speed of 110 km/h, set with respect
to the criterion for decreased performance ability. The values are derived
from the vibration limits in the ISO 2631-3 standard. The corresponding limits with
respect to the discomfort criterion are considerably stricter. The limits assume no
surface defects at all (aggregate stripping, potholes, etc) or damage that cause rota-
tion vibration (unevenness at culverts, edge deformation, etc).

4.6.5 Measurement of whole-body vibration
The measurement of whole-body vibration must comply with ISO 2631 “Evaluation of
human exposure to whole-body vibration” (1997). The equipment consists of acceleration
sensors, arranged as shown in Figure 17 and Figure 18.

The reaction time for the sense of motion has been found to be 0.24 – 0.80 s, with a mean
value of 0.72 seconds [57]. This is one of the reasons why comfort-related measurements
are normally done through integration over 1-second intervals. A vehicle travels 20 m in a
second, at the speed of 72 km/h. This means that vibration data measured in compliance
with ISO 2631 at rural highway speeds on sub-stretches are fairly comparable in length to
road roughness data in the SNRA road surface condition database, which after sampling at
the mm-level was ultimately averaged over 20 m intervals.

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Figure 17 Vibration measurement gauge on the seat

Figure 18 Vibration measurement gauge on the floor

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5 Method
Field surveys were conducted between the 27´th and 29´th of October 1999. This late date
in the season meant risking wintry road conditions, which also proved to be the case on the
morning of the 28´th. The light snowfall during the night meant that the highest frequency
vibrations caused by the roadway texture were somewhat lower. As these are not particu-
larly high energy, this situation was not considered to have affected the study in a way that
would result in any greater underestimation of the vibration problem.

5.1 Test stretches

1

2

Figure 19 Location of the roads surveyed. Sollefteå Municipality, Väste rnorrland
County. The stretch on National Highway 90 is indicated as 1, and that on County
Road 950 as 2.

5.1.1 National Highway No. 90
The stretch of highway surveyed is located north-west of Sollefteå, between Näsåker and
Remsle, see Figure 19. The survey was conducted in an easterly direction. The roughness
measurements heading towards Sollefteå began (not counting an approach of a little over
300 m) at the intersection by Flintabaren in Näsåker. The IRI20 values on 32 kilometres of

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Whole-Body Vibrations When Riding on Rough Roads

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