VEHICLE SPEED MEASUREMENT II
LESLIE C L FELIX
SA 5093
This paper
discusses uncertainties and errors in vehicle speed measurement and the legal
implications of these. It provides a
proven method of measuring vehicle speed over its working range, without the
use of extrapolation, which is conducted in a controlled environment rather
than on public roads.
Keywords: speed, speedometers
Introduction
Both federal and
state legislation set standards for the accuracy of speedometers installed in
motor vehicles. Unless these legislative provisions are compatible, and
prosecution policies recognise the accuracy
achievable by speedometers installed in vehicles, there is danger that
motorists could offend unwittingly. This paper will discuss the interaction of
the federal design standard, individual state prosecution policies and the
performance of speedometers and associated testing equipment.
The Australian
Motor Vehicle Standards Act, (known as the Australian
Design Rules, or ADR [1]) sets requirements for speedometers
installed in vehicles to be used on the road throughout
“indicate
the actual speed, for all speeds above 40 km/h, to an accuracy of ± 10
percent.”
State Legislatures
have also set their own minimum requirement.
For example New South Wales Traffic Law [2]
requires that speedometers:
“indicate,
when the vehicle is travelling at a speed in excess of 50 km/h, a speed that is
not more than 10% less than actual speed”.
The individual
State requirements are all worded differently and may impose different
constraints on the performance of speedometers.
However none change the “10% less”
requirement, which is a main contributing factor to the system failure. This
accuracy guide method has severe limitations
and is only used by
persons with a lack of understanding of measurement.
Uncertainties
are an integral part of regulations administered by the National Standards
Commission, such as those concerning the weighing of products in commerce. Since there is a trend to base the level of fines on exactly how much the
speed limit is exceeded, the policy should recognise the effect of uncertainty of measurement
and fall into line with other measurements
with financial implications. The ADR [1]
should take account of the requirements of the ISO Guide to the Expression of Uncertainty in
Measurement [3]. This reference to uncertainty is an integral part of weight
measurement and is found in
There
is a system that would enable drivers to reliably determine if they are travelling
within the posted speeds limits. This paper will endeavour to prove the accuracy and safety
aspects of a test system that once used, will enable the public to travel
within the posted speed and furthermore be expected to do so.
The Monash
University Accident Research Centre published research notes with the heading
“Accuracy of vehicle speedometer
readings with respect to speed enforcement tolerances” [5]. Table 1 gives a compilation
of statistics summarised in the notes.
Actual speed relationship to indicated speed in km/h |
|||||
Actual
|
40 |
60 |
80 |
100 |
120 |
|
Max indicated |
43 |
64 |
83 |
108 |
130 |
|
Min indicated |
27 |
48 |
71 |
84 |
105 |
Table 1:
Summary of results of speedometer tests carried out by
The University used some collated results from other sources and whilst the test methodology was not described these results indicate either a failure by manufacturers to meet the minimum requirement of the relevant ADR [1], or that other mechanical factors are affecting the results.
Speed indication errors and
variations
Speedometers in
vehicles respond to the rotational velocity of the wheels. Errors and variations in vehicle speed indication will then
be due to either the relationship between a rotation of the wheels and the
actual distance travelled, or to the errors in measuring rotational
velocity. The nature of the tyres
contribute the first type, and instrument errors the second.
The speed indication in a vehicle is tested by either
measuring the time to travel a known distance (measured by numerous methods),
or on an apparatus consisting of rollers with known circumference and
measurable rotational velocity (a “rolling road”). Some
instrument repair companies merely “check” the odometer over a distance and
conclude the speedometer accuracy from this data. Some have recently used GPS
units. The latter options require conducting tests on public roads.
Testing of speedometers should ideally be conducted throughout the usable range as this eliminates the need for extrapolation. There are obvious safety implications if speedometers installed in vehicles are tested throughout their range on public roads. However using a rolling road for such measurements reduces the safety issues and the latest computerised rolling road machines provide a printout of the parameters tested.
Another
machine that utilises rollers is the dynamometer and these can be used to test
speedometers. Most rolling road testers are primarily a dynamometer. Its main
function is to introduce resistance to wheel rotation by absorbing test vehicle
energy into a load, and measuring the force developed by the drive wheels. Care
should be taken when using a dynamometer that slippage is not induced by the
machine’s resistance. Some operators use the loading to minimise
hunting (the failure to maintain a constant speed due to engine behaviour). Load generation should be minimised as
should tie-down pressures. It is normal
practice to chain or strap the vehicle under heavy loading conditions for
measuring engine torque to avoid the vehicle climbing up and out of the roller
valley. In these tests, lateral restraining of
the vehicle was used instead of tie down,
since vertical restraining caused tyre distortion, which
can lead to an error in the region of 2 km/h. It would be difficult to balance
normal tyre load distortion, aerodynamic and centrifugal force to a
corresponding offset for the rollers, because the forces are not linear and
combined to create a complex response curve.
At best only a “best fit” correction can be given.
Except where indicated otherwise, the tests described in this paper were carried out on a free-running rolling road, that is, without applying a load to the wheel rotation. This machine held a current NATA accredited certificate of accuracy. The measurements described in this paper are traceable to an Australian National Standard and have adhered to the requirements of ISO 17025 [7].
Errors due to
tyres may be long-term (e.g. tyre type and size), medium-term (e.g. tread
wear), or short-term (e.g.
pressure and loading). The author
undertook measurements of both true and indicated vehicle speed with varying tyre brands, wear and tyre fill pressures.
Inflation pressure:
Increase in
pressure will occur as the tyre increases with heating
due to use. This pressure increase is as much as 28 kPa (4 psi). An increase in tyre temperature will increase
pressure and cause the indicated speed to be lower. The tyre inflation
pressures referred to in the following tests were hot pressures and should not
be confused with cold pressures settings recommended by manufacturers.
To examine how
pressure affects the tyres, they were initially inflated to 160 kPa. The first
run at this pressure was followed by tests in
increments of 30 kPa to a maximum tyre pressure of 280 kPa. One of the tests
was conducted with a standard tyre pressure of
190 kPa and the equivalent weight of four adult males in
the car,
all the other tests in this series were with one adult male only. The
deviations from true speed occurring at
indicated speeds of 30, 60, 80 and 120 km/h were recorded. Three readings were
made at each speed and pressure, and mean of the readings were calculated. Results of these tests are given in Table 2.
|
Speedometer error versus tyre pressure |
||||||
|
speed ß |
280 kPa |
250 |
220 |
190 |
160 |
190+ load |
|
30 |
1.5 |
1.4 |
1.4 |
2.3 |
2.6 |
1.8 |
|
60 |
1.9 |
1.6 |
1.8 |
2.3 |
3.6 |
3.8 |
|
80 |
1.8 |
1.6 |
2.3 |
2.6 |
3.3 |
3.7 |
|
120 |
3.4 |
3.6 |
3.4 |
4.1 |
4.8 |
5.1 |
Table 2:
Speedometer error variation with tyre pressure.
Brand and model:
Examination of
model and brands were undertaken using 17 and 18inch rims with low profile
tyres. Some 20 different tyre models were tested to consider variations between
brands. It was found that a variation of speedometer reading of 1.5% resulted
from the same vehicle and speedometer calibration settings over the twenty
types.
Wear:
The change in the
tread depth of a Dunlop Monza 205/65R15 tyre, from new through to the 1mm above wear indicator bars, was measured to change the diameter by 12 mm (although the
diameter change can be 14 mm if worn completely). This is equivalent to a
change in circumference during its life of 2.0%.
On the face of it,
the circumference of a tyre is constant whatever the tyre pressure. However
tyres compress as the tyre surface changes shape when it meets the road surface
squeezing and then stretching each portion of tread during a cycle so that the
distance travelled per revolution of the wheel changes. It was found that a
worn tyre does not compress to the same amount as a tyre with new tread
although smaller in circumference. During these experiments it was found that
tyre growth under the influence of centrifugal force was only significant when
the tyres were under-inflated and at speeds of
more than 120 km/h. A Dunlop 215/60R16 95V inflated to 240kPa was roller-driven
to 160 km/h and had expanded 3.5mm on radius or approximately 1.1% of indicated
speed. This expansion increases with speed in an approximate logarithmic
fashion.
Experiments showed
that a Dunlop Monza 205/65R15 tyre fitted to a rim had an undistorted radius of
320mm at a pressure of 220 kPa, a compressed radius of 295mm and, a compressed
radius of 290mm
at a pressure of 190kPa. The calculated circumferences for the three radii were
2011mm, 1854mm
and 1822mm respectively. The distance travelled in
one rotation, for the compressed tyres
was measured to be 1966.5mm at 220kPa and
1908.0mm at 190kPa. The difference in the measured distances travelled was 0.7% yet the radii differed by 1.7%. Further clarification of this phenomenon
would require test throughout the pressure range for a number of combinations
of vehicle and tyres. The actual results
from direct comparison to laser and radar measurements at speeds from 30 to
160km/h had indicated only a 0.7% difference at 100km/h dropping to 0.4% at
160km/h. This suppression may be a
result of aerodynamic behaviour of the vehicle. The results are given in Table 3.
Indicated |
30.0 |
60.0 |
80.0 |
100.0 |
120.0 |
160.0 |
|
Rollers |
29.7 |
57.3 |
76.3 |
96.4 |
116.1 |
157.3 |
|
Laser |
30.0 |
57.0 |
77.0 |
96.0 |
116.5 |
156.5 |
|
Radar |
29.5 |
57.0 |
77.0 |
96.0 |
116.0 |
156.5 |
Table 3:
Comparisons of different methods of speed measurement.
When speed is
measured using rollers the compressed diameter of the tyre varies from the compressed diameter of the same tyre on the
road surface. This is due to the rollers creating two curved surfaces rather
than one flat surface on the tyre (load
surface area and shape, or tyre footprint).
The effective
circumference of a tyre on the road can differ with brand, ply rating, belt
type (steel or nylon) and tread depth. This circumference variation can be
minimised when the vehicle is on the rollers
by increasing the tyre pressure. The required increase will depend on tyre
type, but early test results indicate it is about 30 kPa.
Experiments on the
tyre distortion with different diameter rollers was undertaken starting with
203mm (8.0 inch) to 266mm (10.5inch) in 12.5mm intervals
Some experiments
are still being analysed that look at leading edge roller speed sensing verses
trailing edge roller speed sensing. This plays a roll in the effective diameter
seen be the rolling road tester.
Instrument Errors
Systematic
corrections that are not eliminated during
calibration or applied as a correction, will contribute with opposite sign to the results of speed
measurement by a police pursuit vehicle. For example, consider a police car tested at 100 km/h with a reported error with
new tyres of +1.5 km/h (that is, the true speed is 1.5 km/h lower than
the indicated speed) and which eventually has
tyres at half wear equating to 1km/h. A motorist’s
vehicle is then perceived to be travelling 2.5 km/h faster than actual.
If the motorist has a speedometer error of -1.5 km/h and
is travelling at an indicated speed of 100 km/h we can see that it has been measured to
exceed the speed by 4 km/h, enough to be considered a breach of traffic
rules. These errors created by, (a) tyre wear, (b) not applying
calibration corrections, and/or (c) the roller-to-road anomaly, are
critical to the overall picture, since the accumulative affect can be as much
as 4 km/h. These three items were intentionally not calculated in this first
view of the uncertainty assessments (subject discussion to follow) since the
corrections may or may not be deemed as uncertainty components.
To calculate the
uncertainty associated with a driver’s knowledge of the true speed of his or
her vehicle, a review of the components of
uncertainty arising from interpretation of
speedometer indication, vehicle load, engine power management and tyre
behaviour was undertaken by the author.
The driver’s ability to accurately determine the vehicle speed
using an ordinary speedometer is affected by:
*The intrinsic
accuracy of the instrument (the residual systematic
error after calibration).
*Parallax error.
*Size
of minor graduations (normally 5 or 10 km/h).
*Readability
(usually one fifth of one
minor graduation).
Based on these
factors uncertainty (expressed as 95% confidence
intervals) for a speedometer read to 2
km/h was as follows:
60 km/h is ±8 km/h
80 km/h is ±10 km/h
110 km/h is ±13 km/h.
A calibrated
speedometer read to 2 km/h and tested with certified speedometer testing reaches a better accuracy than the ADR18.5.1.2, that is the
accumulated uncertainty described in this paper is less than the ±10% specified
by ADR. The calculated uncertainty is ±4.9 km/h at
110 km/h without any account for tyre wear and
roller to road anomaly. This assumes that the
speedometer was either adjusted to read true or the calibration correction was
applied. Failing this, the uncertainty must be calculated with an uncertainty
components added for the systematic errors.
The needle in an
analogue speedometer will be about 2 mm from the gauge face. This results in a
parallax error, which will depend on the position of the driver’s dominant eye.
The maximum error derived from experimentation was
less than 2 km/h. With the advent of liquid crystal displays with either
synthesized analogue or numerical readout, parallax problems are not an issue.
On the other hand rounding of the displayed speed may create errors but
these would be less than 1 km/h.
Analogue
instruments display information by indicating with a needle or a pointer. The
graduations on the display face limit the precision of the instrument
readability. With a minimum division of 5 km/h and a needle width of the
equivalent of 1 km/h, resolution to a fifth of a division or 1 km/h can be
expected. Examples of the application of this convention can be found in
Australian Standard AS1349 [6]
Since
infringements can occur in just a few metres, we investigated other sources of
speed control and measurement and found a significant problem with smaller
vehicles. Measurements with an air-conditioned four-cylinder vehicle showed a
5km/h variation in speed with the air-conditioning compressor cycling. This
variation is created by the driver compensation for power fluctuations by his
efforts to maintain constant speed. Policy makers may wish to include this in
the big picture.
Calibration of the testing
machine
The measurements of the roller diameters and rotational speed gives
a standard uncertainty component of less than 0.1 to 0.3 km/h between the
speeds of 30 and 180 km/h. The stability of performance of all the roller
machines tested throughout most of Australia over the last six years has been
in the region of ± 0.2 km/h. Plotted roller wear on the Adelaide based
unit was less than 0.01% over six years.
Police tolerances for speed
infringements
The inconsistency
between Australian States in their tolerance of small infringement of speed
limits means that there is no single system in use. The most widely used system is the decade
method. The posted speed limit can be exceeded by 9 km/h eg 69 in a 60km/h zone
(89 in 80 km/h zone etc) and incurs a fine if 70 km/h is detected. This method
was introduced to compensate for the ADR 18.5.1.2 speedometer error of ±10%.
One State has
recently introduced a 3km/h tolerance, since their detecting equipment carried
an uncertainty in the region of ±2 km/h. This system has the implicit
assumption that the drivers must not exceed the speed limit regardless of
measurement errors and the onus is upon the driver to ensure that they comply
with the law irrespective of accuracy of their speedometer.
Discussion
Achievable aims:
The statistics
collated by the Monash University, the police departments, the Royal Automobile
Association and myself, indicate that a high proportion of speedometers are set
to read 3 km/h high to minimise liability and supposedly to compensate for
possible drift. There has been no response from manufacturers confirming this
practice. The application of this offset does
not improve the accuracy of speedometers. The
latest manufactured vehicles have an accuracy of 3% or better, of reading with
one brand offering an adjustable version correct to within 2% of full scale. In
the first instance, the use of “3%” is an archaic method of describing accuracy
and creates a distorted view of the errors expected.
Statistics have shown that ADR [1] should be amended to read:
“an accuracy of
±(0.65% of full scale + 1.75% of reading)”,or “±(1.5
km/h + 1.75% of reading)”.
This
would ensure that the
tolerance does not limit the lower values to impossible accuracies or the upper
value becoming too large.
The tests
conducted were not intended to measure individual effects of tyre behaviour on
speed but was a measure of an overall effect.
The “lumping” of the tyre effects was purely to extract expected overall
variations in speed measurement.
Tyre wear and low
fill pressure just resulted in a higher indicated speed, which may not be of
concern in a motorist’s vehicle, but in a police vehicle will result in a high
reading of the speed of motorists. A
worry for motorists is the fact that tyre pressure increases from cold to hot,
lower indicated speed.
Improved method:
With
the adoption of the suggested changes to the design rules, and with roller
anomaly taken into account, we can then address the policy of dealing with the
error caused by tyre wear, so that the uncertainty can be calculated
considering all significant components.
The author suggests taking measurements for the tyre wear at the half
wear point since a tread depth at time of test can be obtained and results of
the speedometer test mathematically corrected to the half wear point. The tyre
wear can then be included in the uncertainty to reflect results by tyres
wear being other than half worn. The combined uncertainty components mentioned
earlier and these latest additions were calculated to be ±6.7 km/h
for a Dunlop Monza 205/65R15 tyre at 110 km/h.
No mans land:
In some Australian
States road works and children’s crossing
zones are automatically classed as 25 km/h zones. As the wording of the design rules (ADR 18.5.1.2) does not call for any
accuracy for speeds below 40 km/h, the driver has no
assurance of the vehicle’s true speed in these
zones.
Driver’s responsibility:
Other errors that
have been attributed to outside interference (for example incorrect tyre size
fitted, or differential ratio altered), or a
deviation from manufacturers specifications are a separate issue. With vehicles
made to the amended ADR as suggested above in paragraph “Achievable aims”, the uncalibrated speedometer would have a lower
calculated uncertainty of speed measurement and can be expected to perform
within a smaller infringement tolerance.
Breach of natural justice:
The calculation of uncertainty associated with speeds up to 120km/h shows that the decade method used by police forces allows infringement notices to be issued to drivers travelling within the region of uncertainty. The issuing of infringement notices using the 3 km/h tolerance system can be even unfair to drivers who use a speed-measuring instrument conforming to Australian design rules.
A temporary measure:
A suggested
policing policy is to allow 7 km/h at speeds of up to 50 km/h and an additional
1 km/h for every 10 km/h of speed up to 110 km/h speed. This policy will
prevent infringements notices being issued for
the region of uncertainty and therefore should not be legally challengeable.
This policy of expanded tolerances would only be an interim measure to correct
the present situation, prior to public testing facilities being introduced.
The solution:
I believe that
this paper lays the groundwork to give the Federal Government, State Governments, State Police Forces and motorists the tools to
operate motor vehicle speed control measures correctly and fairly. If all recommendations are accepted, a fixed
tolerance of 7 km/h (or a sliding scale of tighter constraint but more
cumbersome to apply) can be used without compromising the motorists and afford
them their right to an accurate form of speed measurement. However this policy assumes the application of
calibration offsets to correct the speed value.
The process of testing and calibration of rolling road testers that is traceable to a national standard must be made publicly
available. A series of approved testing stations should be available so that
motorists can confirm their speedometer accuracy and drive accordingly.
Acknowledgements:
I would like to
thank the following people and companies for their assistance in producing this
paper.
Royal Automobile
Association of South Australia for the use of testing
equipment.
R
Laslett (retired) of S.A. Police Traffic Technical Support for Encouragement and guidance.
J Lipman Traffic
Technical Support NSW Police, for duplicate testing to expose systematic
errors.
Injection
Perfection NSW, for duplicate testing to expose systematic errors.
South Australia Police Force for road to
roller comparisons
J Tapping for his editorial assistance.
Abstec Calibrations
For my employer’s continued support
References:
1. The Australian Motor Vehicle Standards
Act 1989, Design Regulation 18.5.1.2
2.
3. ISO Guide to the Expression of
Uncertainty in Measurement.
International Organisation for Standardisation,
4. Organisation Internationale De
Métrologie Légale” Recommendation R111 titled Weights of classes E1, E2, F1, F2,
M1, M1-2, M2-3 and M3
5. Monash University Accident Research
Centre, “Accuracy of Vehicle Speedometer readings with respect to speed
enforcement tolerances”.
6.
Australian Standard AS1349-1986, Bourdon tube pressure and vacuum gauges,
Section 1.3 (Definitions), 1.3.4 (Scales and scale markings).
7. Australian Standard ISO/IEC 17025-1999,
“General requirements for the competence of testing and calibration
laboratories.”