Road bumps reduce vehicle speeds on residential streets and other densely populated areas, thus improving safety and comfort for pedestrians and bicyclists. Unfortunately, the effects on health and safety for drivers and passengers passing these obstacles are rarely considered by road agencies, consultants and contractors. Many current bumps induces harmful whole-body vibration and shock when passing, even at legal speeds. Injuries can be immediate, e.g. fracture of vertebrae, or long term, e.g. low back pain. Bus drivers in many cities pass up to 40.000 bumps per year. The bumps will also cause additional longitudinal and vertical stress to the vehicle. A literature review shows that previous research on the subject of road bumps has been severely misinterpreted. As a result, bumps are in many countries today designed to cause a high maximum vertical acceleration (shock) level. To cause shocks on purpose, is in obvious conflict with ergonomic knowledge such as in directive 2002/44/EC. In a project described in this paper, a “shock-free” speed hump has been designed using vibration engineering. The new hump cause uncomfortable vibration at high driving speeds, but only a minimum of shock occur when passing. In 2003 the first hump ramps of this design were casted in Portland Cement Concrete, available from www.gunnarprefab.se

1. Johan Granlund, Page number 1(8)
DESIGN OF A SHOCK-FREE SPEED HUMP
Johan Granlund, Fredrik Lindström
Swedish National Road Administration, Consulting Services
Pavement Engineering Division
Röda Vägen 1
S-781 87 Borlänge, Sweden
E-mail: johan.granlund@vv.se
Abstract
Road bumps reduce vehicle speeds on residential streets and other densely populated
areas, thus improving safety and comfort for pedestrians and bicyclists.
Unfortunately, the effects on health and safety for drivers and passengers passing
these obstacles are rarely considered by road agencies, consultants and contractors.
Many current bumps induces harmful whole-body vibration and shock when passing,
even at legal speeds. Injuries can be immediate, e.g. fracture of vertebrae, or long
term, e.g. low back pain. Bus drivers in many cities pass up to 40.000 bumps per
year. The bumps will also cause additional longitudinal and vertical stress to the
vehicle. A literature review shows that previous research on the subject of road
bumps has been severely misinterpreted. As a result, bumps are in many countries
today designed to cause a high maximum vertical acceleration (shock) level. To cause
shocks on purpose, is in obvious conflict with ergonomic knowledge such as in
directive 2002/44/EC. In a project described in this paper, a “shock-free” speed hump
has been designed using vibration engineering. The new hump will cause
uncomfortable vibration at high driving speeds, but only a minimum of shock will
occur when passing. A full-scale test will be done during 2003.

2. Johan Granlund, Page number 2(8)
INTRODUCTION
Since the introduction of “Vision Zero” for road safety in Sweden, the construction of
road humps/bumps has boomed. Complaints about ride related back pain among
urban bus drivers have also boomed. In a handful cases, car and bus passengers have
been severely injured when passing the obstacles; fracture of vertebrae, ruptured liver
et c.
Road hump regulations and design codes in many countries -- Sweden and
Denmark for instance -- specify that these obstacles should cause a 0,7 g (7 m/s2)
vertical acceleration level at the design speed. A quick literature review reveals that
this target is the result of a serious misunderstanding.
The famous Watt hump (1973) was designed on panel rating basis. Today there
are demands for new designs that for instance suit disabled pedestrians better, such as
a flat plateau. The introduction of the 0,7 g target changed the design focus, and
opened up for quite sharp obstacles that induce high shock levels.
The objective of the work described in this paper, is to develop a hump that
fulfils both new demands from disabled pedestrians and Watts original criteria for
speed control without causing harmful shocks. Manufacturer Gunnar Prefab AB in
Rättvik, Sweden, is owner of this “shock-free” hump design, and pattern protection is
pending.
LITERATURE REVIEW
The Watt Speed Hump
Watts (1973) stated sound criteria for speed humps; "The ideal speed control hump
should probably exhibit the following characteristics:
At and below the design speed all drivers should be able to cross the hump
without damage to load or vehicle, or loss of control and they should
suffer no discomfort.
Above the design speed the driver should suffer a degree of discomfort
depending on the amount by which he violates the design speed but there
should still be no damage to load or vehicle or risk of loss of control.
When testing various humps, Watts used panel ride quality ratings of the
designs. This way he experimentally developed a 3,7 m long and 10 cm high arc-shaped
hump, that was a great improvement to the bumps in frequent use at that time.
A Watt hump is compared with a typical bump in Figure 1.
Watts also measured the peak value of positive vertical acceleration in the
vehicles. This may have been in order to learn about the risk for damage and loss of
control. For the best hump, later known as the Watt hump, peak acceleration was
measured to 0.93 g in a small passenger car at 32 km/h.

3. Johan Granlund, Page number 3(8)
Figure 1 A typical bump, compared with a Watt hump. From Stephens (1986)
Confusing the Definition of Peak Acceleration
Alppivuori Laitakari (1981) tested the 3,7 m * 10 cm arc-shaped Watt speed hump
with large Nordic type vehicles.
Panel ride quality ratings was the main evaluation method. Also vertical
acceleration was again measured in the test vehicles. However “peak acceleration”
was defined in an erroneous way, see Figure 2. Thus their data refer to peak-to-peak
acceleration, which for typical ride vibration is roughly twice as high as the positive
peak acceleration measured by Watts.
Figure 2 Confusing peak acceleration with peak-to-peak acceleration. From
Alppivuori Laitakari (1981)
Alppivuori Laitakari came up with 0.5 to 0.9 g “peak acceleration” (rather
peak-to-peak) in a passenger car driving at 30 km/h over the Watt hump. This
corresponds to about 0.25 to 0.45 g true peak acceleration, which is more than 50 %
less than what Watts measured. However the panel ride quality ratings matched well
between the two studies. This is very interesting, since it tells something about the
(lack of) correlation between peak acceleration measurement and panel ride quality
rating.
The Serious 0.7 g Mistake
Stephens (1986) is referring to Watts study as well as the Alppivuori Laitakari
study. He writes Best-fit analyses of discomfort and the peak vertical acceleration of
vehicles passing over humps yield good-fit linear relationships. Assuming that a

4. Johan Granlund, Page number 4(8)
midpoint discomfort value is a threshold for speed control at the road hump, peak
vertical accelerations of about 0.69 g (6.8 m/s2) will induce the typical driver to
reduce speed.
With this work, Stephens caused a change of focus from panel ratings to peak
acceleration measurements. The unfortunate change of focus, opened up for poor
bump designs with large Slope Variance.
Comfort Rating of Transient Vibration
The manner in which vibration affects human is dependent on the vibration frequency
content. Thus the vibration data must be frequency-weighted. This has not been done
by neither Watt (1973), nor Alppivuori Laitakari (1981).
Spång (1997) presents a comprehensive study of comfort ratings of vibration
containing single event shocks. This work shows how different vibration definitions
correlate with human ratings. The results show a large lack of correlation between
transient peak acceleration and comfort (38 % for unweighted peak value). The best
fit with comfort rating was derived for the running rms of weighted acceleration; 92
%. Other studies have verified the Swedish study; peak acceleration, especially
unweighted, is a poor indicator for the degree of discomfort.
In ISO 2631-1 (1997) the running rms of weighted acceleration is
recommended when evaluating human exposure of very transient vibration (a high
crest factor).
Current Knowledge About Shocks
Long-term exposure to vibration containing multiple shocks, such as when travelling
over rough surfaces, bring an increased risk to the lower lumbar spine.
Exposure to shocks can bring very large health effects, while the risk is very
difficult to measure. Directive 2002/44/EC states that in risk assessment, particular
attention shall be given to any exposure of intermittent vibration or repeated shocks.
From acceleration data describing the number and magnitudes of peak
compression in the spine, ISO/DIS 2631-5 predicts the response of the bony vertebral
endplate (hard tissue) for a seated person in an upright position, while also dealing
with effects of multiple shocks and the posture on the intervertebral disc (soft tissue).
ISO/DIS 2631-5 also give guidance on assessment of health effects to the spine.
The AASHO Road Test
In the early 1960’s, the huge AASHO road test was done. The objective was to find
suitable taxation principles for trucking, and to develop know-how in pavement
engineering. An important measure was the road surface’s “Present Serviceability
Index”, PSI, that predicts the “Present Serviceability Rating” expected from a road
user panel. The PSI-value is usually affected to more than 95 % by short-wave Slope
Variance of the surface roughness at the constructed test roads. This strongly indicate
how the properties of the road profile corresponds to ride quality.

5. Johan Granlund, Page number 5(8)
DESIGN OF A SHOCK-FREE SPEED HUMP
Slope Variance, SV, is the second (space domain) derivative of the road profile
elevation, and it excites the second (time domain) derivative of the ride displacement;
namely the vertical acceleration. Thus a shock-free speed hump must have a
minimum of short-wave SV to avoid wheel axle hop shocks, but enough long-wave
SV to cause resonance in chassis bounce at high speeds.
A simple tool for hump design layout is MS Excel®, where one easily can
compute slopes and SV for a given profile. SV is then minimized over a step
corresponding to wheel axle hop (about 10 Hz) at the design speed. For 30 km/h, SV
shall be minimized over 4 dm length. After a couple of hours of work in Excel®, a
layout for a shock-free hump design is prepared, see Figure 3. To avoid resonance in
vehicle pitch, the rear ramp has a very low gradient. This makes the hump
asymmetric, thus it requires an channelising island on streets with opposite traffic.
Another possibility is to extend the plateau to at least 8 m (longer than typical bus
wheel base), and then use a mirror of the front ramp as rear ramp.
The software Ride Quality Meter®, RQM, is an advanced tool for evaluating
road roughness, as well as speed hump profile designs. In Figure 4 a 10 cm * 5 m
flat-top hump is tested with RQM, in this case with a quarter car simulation using
“The Golden Car” chassis specification. The specification is developed by the World
Bank as a standard for road roughness evaluations with quarter car simulations.
The result in the middle window of Figure 4 shows that peak weighted
acceleration reach about 0.6 g (not 0.7 g!) at 30 km/h over the flat-top hump. In the
lower window the running rms of weighted acceleration turn out to reach 2 m/s2,
corresponding to “Extremely uncomfortable” according to the comfort scale in ISO
2631-1 (1997). Clearly the 10 cm flat-top hump is unnecessary tough for 30 km/h.
In Figure 5 results are given for a similar simulation over the shock-free profile
showed in Figure 3. As seen in the middle window, peak weighted positive
acceleration reach about 0.4 g. Thus it causes much less hazard to health, earning the
name “shock-free”. In the lower window the running rms of weighted acceleration
turn out to reach 1.4 m/s2, corresponding to “Very uncomfortable” according to the
comfort scale of ISO 2631-1 (1997).
Real road vehicles have both front and rear axles. This brings a “smoother”
chassis dynamics than shown in the quarter car simulation. Real vehicles can be well
modelled with a “full car” simulation, where all wheels are considered. In Figure 6
the vertical acceleration (here no attention has been given to fore-aft or pitch
vibration) results are given for a full car simulation of a 30 km/h drive over the shock-free
hump. The chassis specification used is for a Chevrolet 3500 Mobile Intensive
Care Unit Ambulance. The middle window shows vertical peak weighted acceleration
of about 0.25 g. The lower window shows that the running rms of weighted
acceleration reaches 0.8 m/s2, corresponding to “Uncomfortable” as per ISO 2631-1.
This brings confidence in that the shock-free hump will be efficient in speed control.
In addition to vertical vibration and shock, speed humps also induce
longitudinal and rotational shocks. The longitudinal loads affect vehicle wear, since
vehicle suspension is designed mainly for vertical loads. Rotational shocks can

6. Johan Granlund, Page number 6(8)
occasionally add to vertical vibration and bring severe injuries to vehicle occupants.
Tests not shown in this paper show a large decrease in such vibration when replacing
traditional Watt humps and flat-top humps with shock-free humps.
The next development phase, running during the spring 2003, is to construct the
shock-free hump in pre-cast concrete, and make full scale tests in reality. The tests
will be done in various vehicles and speeds. After initial testing, large scale testing
(incl. noise, ground born vibration, speeds et c.) will be done at a hand full of places
in the Gothenburg area.
Slope Variance is often significantly affected by construction errors, wear and
tear. If existing humps are accurately measured with a laser/inertial Profilograph, all
SV can be evaluated with Ride Quality Meter®.
SUMMARY
The widely used design target 0.7 g for speed control humps is not relevant at all. It is
merely a product of scientific misunderstandings.
Ergonomic “shock-free” humps can have high profile elevation, high average
ramp slope, high max. ramp slope, but must have low short-wave Slope Variance.
Various speed hump designs can be evaluated with the Ride Quality Meter®
software, designed for road roughness evaluations. If profiles of existing humps are
accurately measured with a laser/inertial Profilograph, also construction errors, wear
and tear can be included in the evaluation.
Figure 3 Geometry of a shock-free hump

7. Johan Granlund, Page number 7(8)
Figure 4 Quarter car result for a 30 km/h ride over a flat-top hump
Figure 5 Quarter car result for a 30 km/h ride over a shock-free hump

8. Johan Granlund, Page number 8(8)
Figure 6 Full car result for a 30 km/h ride over a shock-free hump
REFERENCES
Alppivuori, K. Laitakari, P. The effect of a hump and an elevated pedestrian
crossing on vehicle comfort and control. Technical Research Centre of Finland, Road
and Traffic Laboratory Report 69, Espoo 1981
Cirkulære 188 om udformning af hastighedsdæmpende bump. Trafikministeriet, 2000
Directive 2002/44/EC on the minimum health and safety requirements regarding the
exposure of workers to the risks arising from physical agents (vibration) (sixteenth
individual Directive within the meaning of Article 16(1) of Directive 89/391/EEC).
The European Parliament and the Council, 2002
ISO 2631-1. Mechanical vibration and shock. Evaluation of human exposure to
whole-body vibration. Part 1: General Requirements. The International Organization
for Standardization, 1997
ISO/DIS 2631-5. Mechanical vibration and shock. Evaluation of human exposure to
whole-body vibration. Part 5: Method for evaluation of vibration containing multiple
shocks. The International Organization for Standardization, 2001
Nilsson, U. Ride Quality in Ambulances – Modelling and Model Validation. TRITA-FKT
2003:XX
Spång, K. Assessment of whole-body vibration containing single event shocks. Noise
Control Eng. J., 45(1), 1997, pp 19-25
Stephens, B.W. Road Humps for the Control of Vehicular Speeds and Traffic Flow.
Public Roads, 1986
The AASHO Road Test. Special Reports 61A-61E. HRB, National Research Council.
Washington, D.C., 1961
Vägutformning 94 (VU94). Del 5.5.6.4, Gupp. Vägverket, publ 113-128. 2002
Watts, G.R. Road humps for the control of vehicle speeds. TRRL lab. report 597,
1973