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TIME DISTANCE ANNEX – DRAFT
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TABLE OF CONTENTS
1.
Scope ..................................................................................................................................................... 3
2.
Components of the equipment........................................................................................................... 4
3.
Road section of known length ............................................................................................................ 4
4.
Accurate and stable time reference for traverse duration ............................................................. 5
5.
Sensing the instant of entry and exit from the road section ......................................................... 6
5.1.
Loops.............................................................................................................................................. 8
5.2.
Piezoelectric ................................................................................................................................ 10
5.3.
Light Gate .................................................................................................................................... 10
5.4.
Laser............................................................................................................................................. 11
5.5.
Radar............................................................................................................................................ 11
6.
Trigger event matching ..................................................................................................................... 12
7.
Certainty of vehicle identification .................................................................................................... 12
8.
Photographic evidence ...................................................................................................................... 13
9.
8.1.
Confirmation of the identification of vehicle and driver ....................................................... 13
8.2.
Confirmation of the speed limit within the road section ...................................................... 14
Approval............................................................................................................................................... 14
9.1.
ANPR and registration number matching ............................................................................... 14
9.2.
Simulated measurement cycles................................................................................................ 15
9.3.
Measurement cycles using test vehicles ................................................................................. 15
9.4.
Verification of section distance ................................................................................................ 15
9.5.
Trigger position stability ............................................................................................................ 16
9.6.
Time Synchronisation ................................................................................................................ 16
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FORWARD
The purpose of this document is to assist the interpretation of the R91 in the context of speed
monitoring using time / distance measurement.
Regulations and limitations are to be found in the main part of the recommendation.
SCOPE
This Annex relates to speed measuring equipment in which the time taken for the vehicle to traverse a
known distance is recorded and used to calculate speed.
TERMS AND DEFINITIONS
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1.
COMPONENTS OF THE EQUIPMENT
The components that make up such a system are:
1)
2)
3)
4)
5)
Road section of known and constant distance
Accurate and stable time reference for traverse duration
Accurate sensing of the instants of entry and exit from the road section
Classification of the vehicle and its corresponding speed limit
Method to associate the calculated speed unequivocally with the correct vehicle
Speed monitoring using the time / distance technique, due to the requirements of the installation as
described in this document, are typically fully automatic. The body of evidence required to prosecute an
alleged violation must include photographic evidence related to the event.
To achieve the required speed accuracy – typically ±3% compared to a reference method - the
contribution of each component to the total speed deviation must be considered individually and
minimised.
2.
ROAD SECTION OF KNOWN LENGTH
The length of road section ranges from a few metres to several kilometres, possibly including several entry
and exit gates. In each case the length of the individual road sections must be verified independently
following installation and the value or values permanently stored in the equipment.
For section distance 𝐿 and traverse time 𝑇 the average vehicle speed is:
𝜈̅ =
𝐿
𝑇
If our knowledge of the section distance is slightly incorrect then the associated deviation in the calculated
speed is approximately:
𝛿𝜈 𝛿𝐿
=
𝜈
𝐿
To keep this contribution to total speed error below 0.5% the section distance must be at least 200 times
greater than its associated uncertainty. For example:
A 5-metre road section must be measured to an accuracy of better than 2.5 cm
A 500-metre road section must be measured to an accuracy of better than 2.5 m
A 5 kilometre road section must be measured to an accuracy of better than 25 m
Given that the accuracy to which a short road section can be measured is typically independent of its
actual length, the size of this contribution decreases as the section is made longer. For example, if the
section length can easily be measured to an accuracy of 1cm, then this contribution to speed uncertainty
is:
1% at a section length of 1-metre
0.2% at a section length of 5-metres.
The section distance used in the speed calculation must be the shortest possible between the nominal
trigger event positions, which should be the direction along the lane centre. If the vehicle is travelling at
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an angle to the lane centre then the actual section length for that vehicle is longer than that stored in the
equipment and the calculated speed then lower.
The length of longer road sections is more difficult to ascertain since they can be traversed by any number
of slightly differing routes, each of slightly differing length. The appropriate value of section length in this
case is always the shortest drivable route, which should ensure that the actual route driven by an
individual vehicle is almost always longer than that used in the attendant speed calculation.
Measuring the length of the shortest drivable route through a longer road section to an accuracy of 0.5%
is not trivial, particularly if the section includes a number of bends or even turns. Such road sections must
be carefully surveyed and the section length confirmed using a suitably equipped test vehicle.
Multiple entry and exit points result in several individual sections, some of which may share some
stretches of road. Each route between pairs of gates must then be measured individually when
determining the road section length for that route.
3.
ACCURATE AND STABLE TIME REFERENCE FOR TRAVERSE DURATION
To calculate the average vehicle speed the time at the instant the vehicle enters the road section and the
time at the instant it leaves are needed. For section distance 𝐿 and traverse time 𝑇 the average vehicle
speed is:
𝜈̅ =
𝐿
𝑇
For small inaccuracies in the time taken to traverse the road section the corresponding error in the
calculated average speed is:
𝛿𝜈 −𝛿𝑇
=
𝜈̅
𝑇
The shorter the time to traverse the section, or equivalently the higher the vehicle speed, the greater the
impact of the inaccuracy in the time of arrival at the gate. As a result of this, inaccuracy need only be
considered at the highest average speed for that road section. For example, to reduce the uncertainty
resulting from timing inaccuracy alone to better than 0.5% at 360 km/h, the time taken for the vehicle to
traverse the road section must be known to within:


250µs for a 5-metre road section
25ms for a 500-metre road section
Transit times through short road sections are typically measured using a single electronic counter, for
which a minimum count frequency of 100 kHz (10µs resolution) is recommended. An OXCO time base
reduces the contribution of temperature drift and aging to an insignificant level.
For longer road sections, however, a clock is typically required at each gate. To accurately measure transit
time these multiple clocks be synchronized with each other and/or to a single reference clock of high
accuracy.
Possible reference clocks include:
Technology
Accuracy
Comment
Quartz oven
Achievable relative accuracy better than ±1
Self-contained, but requires
(OXCO)
millisecond per day.
external synchronisation to
absolute time
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Terrestrial time
transmissions
MSF (UK) (for example) transmitted on 60kHz ±1 millisecond absolute time synchronisation
every minute.
regional absolute time
reference
GPS / GLONASS
Better than 100-n.sec possible (one-way)
global absolute time
reference
A time synchronisation to within 1 ms, equivalent to 0.1m travel at 360 km/h, is typically adequate for
multiple clocks installed across long road sections.
Time synchronisation can be easily achieved by referencing all local gate clocks to a terrestrial time
transmission or to GPS / GLONASS. Should the signal reception fail at any gate, due to inclement weather
or interference, for example, then operation of the equipment must be suspended until time
synchronisation has been again re-established.
Alternatively, the time from a central reference clock can be distributed to the gates, using NTP / PTP
protocols, for example. The time required for the synchronization to propagate through the network must
be at least repeatable, which typically requires the network to be exclusive to the installation.
In an installation with multiple clocks synchronised to a central reference, sudden changes in the
correction required for synchronisation are indicative of problems in the clock distribution system.
Operation must be suspended until such time as synchronisation can be re-established.
4.
SENSING THE INSTANT OF ENTRY AND EXIT FROM THE ROAD SECTION
A wide range of sensor technologies are available:
Technology
Description
Characteristics
Repeatability
Loops
Coupling of magnetic field from a coil
embedded in the road surface to the
metal structure of an adjacent vehicle
Invasive; capable of
detecting vehicle presence;
good reliability; good
classification.
Depends on loop size and
vehicle details.
Usable with short and long
road sections.
Piezoelectric
deformation of piezoelectric tape or cable
embedded in the road surface under the
weight of the vehicle
Invasive; capable of very
good identification of entry
and exit instants; good
reliability, basic
classification.
Usable with short and long
Trigger position repeatability for
an individual vehicle in lane
centre: better than 10% of loop
length – i.e. 10cm for 1m loops
Actual trigger position depends
on the profile of the road
deformation.
Trigger position repeatability for
an individual vehicle in lane
centre: better than ±5cm
road sections.
Light gate
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Interruption of the signal directed across
the road to a detector located opposite.
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Non-invasive; very good
detection
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Laser
rate of change of distance derived from
the time of flight of short laser pulses
reflected from vehicle structure
Non-invasive; capable of
good identification of entry
and exit instants,
compromised by dirt from
the environment, good
classification.
Distance to reflector sampled at
discrete intervals due to laser
pulse / scan operation. Trigger
position is the point of
recognition of the vehicle profile
in the distance values
Usable as trigger with long
road sections.
Radar
Distance and speed derived from the
frequency / phase changes induced in a
microwave transmission directed at the
vehicle.
Non-invasive; acceptable
detection; basic
classification
Usable as trigger with long
road sections.
Detection always using signal
amplitude thresholds. Operation
as trigger can be improved by
modulation / multiple antenna
structures.
Trigger position depends on
reflection centres on vehicle.
Video
recognition and analysis of vehicles in
subsequent frames of the video signal
Non-invasive
For any given technology the instant at which a vehicle is recognised may depend on its type and size.
The influence of this trigger point variability on calculated vehicle speed is illustrated in the following
diagram:
Vehicle at
trigger point
D
section distance “D”
measured between
trigger points
X
X
extent of trigger
point variability
Entry gate at t0
Exit gate at t1
The vehicle on the left enters the road section with velocity V and is recognised by the entry trigger sensor
at time t0. Later, after passing through the section it exits to the right and is recognised by the exit trigger
sensor at time t1. Because of uncertainty in the trigger position provided by a given technology there is
also an uncertainty in the actual arrival times of the vehicle at each gate equal to:
±
𝑋
2𝑉
The speed calculated by the instrument for the vehicle is then:
𝑆=
𝐷
(𝑡1 − 𝑡0 ) ± 𝑋⁄𝑉
=
𝐷
𝐷⁄ ± 𝑋⁄
𝑉
𝑉
𝐷
=(
)𝑉
𝐷±𝑋
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If the section distance D is at least 200 times larger than the uncertainty in the trigger point position X
then the contribution to speed error from this source alone will always be <0.5%. Deviations that result in
a longer section length decrease the calculated speed; an increased tolerance may be acceptable in that
case.
The difficulty is to accurately estimate the uncertainty X for a given sensor technology installed at the
gates. When investigated for a representative population of vehicles, it may be found to be significantly
greater than that observed within sub-populations consisting of only a single vehicle type – lorry, car or
motor cycle, for example.
Provided the sensors that generate the trigger events are well matched and similarly orientated relative to
the traffic flow, the smaller uncertainty observed within sub-populations implies that the uncertainty in
section transit time will be potentially much less than the total uncertainty in trigger position measured at
either gate for a representative population if vehicles.
In view of this there are two ways to accurately estimate the uncertainty in transit time between gates

Measure the uncertainty in trigger event time at each gate for individual vehicles in a
representative population, probably by matching images captured synchronously with the trigger
events at both gates.

At both gates individually measure the relative position of the trigger events in a representative
population of vehicles, then divided into sub-populations according to vehicle size and type. The
residual uncertainty in the section transit time is then estimated by the difference in relative
trigger event position within each of the sub-populations. If the sensors that provide the trigger
events are indeed well matched then the difference between sub-populations will be small and the
estimator valid.
4.1.
LOOPS
Each loop is a small number of windings embedded in the road surface, arranged either as a rectangle of
width 2m and length (along the lane) of between 0.5m and 1m, or as a 1m square. The inductance is
circa 100µH.
An alternating current, typically between 20 kHz and 200 kHz in frequency, is excited in the loop, which
results in a magnetic field in its vicinity.
The metal underside of an approaching vehicle removes energy from the magnetic field, which changes
the inductance of the loop. If ΔA𝑣 represents the changing overlap between vehicle and loop then the
change in inductance has the following form:
Δ𝐿1 = 𝑘
(ΔA𝑣 )𝑗
ℎ𝑚
where h is the height of the vehicle underside above the loop and k, j and m are parameters related to the
specific installation and vehicle.
To be of use this change in inductance must be transformed into an electrical signal. A simplified model of
the interaction between loop and vehicle illustrates some approaches for doing this. Consider initially the
loop L1 excited by a current generator of constant frequency close to the resonant frequency.
Representing the approaching vehicle in this simplified model as the shorted inductivity L2 the increasing
overlap changes the coupling coefficient k, and so the mutual inductance 𝑀 = 𝑘√𝐿1 𝐿2
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This moves the resonant frequency of the L1-C
network (or equivalently the L1/Cp network) away
from resonance, which changes the voltage and
relative phase at the generator Gfreq – or equivalently,
imparts both an amplitude and phase modulation to
the generator signal.
Alternatively, Gfreq can be implemented as a freerunning oscillator in which the L1-C network (or equivalently the L1/Cp network) regulates the frequency.
As the vehicle passes over the loop the frequency will change, equivalent to a frequency modulation of the
generator output.
At the output of each loop detector
a signal profile dependent on the
overlap between vehicle underside
and the loop is available, from
which the trigger instant must be
identified. One approach is
illustrated here.
detector o/p
To measure the transit time at a location at least two loops (Loop 1,2) of equal size are installed in short
succession ( a few metres apart) A third loop (Loop 3) can be used to check the speed value and to help
resolve multi-axle vehicles. To avoid interference between adjacent loops their frequencies must be widely
separated (several kHz).
time
As the detector output begins to
increase the developing overlap
t1-1
t1-2
t2-1
t2-2
between vehicle and loop is
recognised at t1-1. If the signal
continues to increase, then a short time later the trigger for that loop is generated at t 1-2. The remaining
signal progression is typically used for vehicle classification.
At the next loop is the signal output is, ideally, similar and results in t 2-1 and t2-2, from which the transit
time for the section S12 is (t2-2 - t1-2).
Uncertainties can result from any of several departures from this ideal, for example:

rapidly changing underbody structure and separation between loop and vehicle, as with some
lorries

breaking or acceleration within the road sections influences the agreement between the two
sections
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
traversing the road sections at an angle relative to the intended direction of travel

inadequacies in the installation of the loops – position, size, depth
The vehicle speed is available only after the vehicle has crossed the final loop. If an intension to prosecute
is established then evidence must be secured, typically using a camera some distance after the exit gate.
To determine the instant at which the vehicle has reached the correct position for the photograph, a
distance Sphoto after the exit gate, the newly calculated vehicle speed is used.
A second photograph, generated shortly after the evidence photograph, can be used to estimate the
vehicle speed, and so confirm the previously calculated speed by an independent method.
When used to generate trigger events at widely separated gates only a single loop is strictly necessary. A
second loop at the exit gate can provide local speed with which to calculate the time to evidence
photograph when an intension to prosecute is recognised.
4.2.
PIEZOELECTRIC
The piezoelectric cable or tape converts the road surface deformation produced by the weight of the
vehicle into an electric charge separation that can be recovered using a suitable amplifier. It is embedded
into the road surface, sometimes in an open metal conduit, and encased in a flexible potting compound.
In contrast to the loop based sensors considered earlier, which respond to the bulk presence of the
vehicle, these piezoelectric sensors generate an output signal at the crossing of every axle. The charge
separation occurs immediately as the weight deforms the potting compound and the piezoelectric cable
within it, which results in a very good trigger localisation and repeatability.
The degree of deformation, and so the amplitude of the resulting signal, depends on the weight of the
vehicle axel and the contact area between tyre and road surface. To detect even very light vehicles, such
as a motorcycle, sufficient amplification must be made available, though excessive amplification can
produce its own problems.
Very heavy vehicles can bring the road surface itself into oscillation, which could be detected by an overly
sensitive cable installation and result in a premature trigger event. A similar effect can be caused by an
object on the road surface immediately preceding the submerged cable, which concentrates the vehicle
weight onto a small area of the road surface.
Similar problems can be caused by cable installations with widely differing sensitivities. The least sensitive
channel may detect only the heaviest axles of the passing vehicle and so present erroneous information to
the analysis method.
To calculate local speed the sensor arrangement is similar to that installed when using loop sensors, with
two short road sections enclosed within three piezoelectric sensors. A camera mounted after the last gate
can be used to collect evidence in the same way.
4.3.
LIGHT GATE
Here the gate is defined by a light sender and a light receiver on opposite sides of the road. A passing
vehicle interrupts the light, which is recognised at the receiver. Very good gate localisation is realised by
using highly collimating optical system, which also allows both the entry and exit gates to be assembled
on a single rigid frame in close proximity (typically less than 2-metres).
Care must be taken to ensure that the light source at both gates impinge on exactly the same part of the
vehicle. This requires that the equipment be assembles exactly parallel to the road surface, which should
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be flat and level prior to the assembly in order to avoid exciting a vertical motion in the approaching
vehicle.
4.4.
LASER
Laser based speed measurement is discussed in a separate annexe and the reader is referred to that.
Here the use of scanning laser sensors to generate trigger events at widely separated gates is
emphasised.
The scanning laser system is mounted either directly above the gate or at the side of the road, such as to
produce a “curtain” of laser beam perpendicular to the direction of travel. From either location the
resulting stream of distance values maps the profile of the vehicle passing through the gate. The trigger
event is coincident with the recognition of the developing vehicle profile.
Using the vehicle profile it is possible to accurately classify the vehicle type, particularly when the scanner
is mounted above the road section. For this arrangement the vehicle has no component of forward
velocity in the direction of the scanner. If the local vehicle speed is need, to calculate the trigger for a for
an additional camera system after the gate, then an additional sensor is required.
4.5.
RADAR
Radar based speed measurement is discussed in a separate annexe and the reader is referred to that.
Here the use of radar sensors to generate trigger events at widely separated gates is emphasised.
The radar sensor establishes an electromagnetic field throughout the road section to be monitored.
Vehicles passing through the field cause part of it to be reflected in the direction on the receive antenna,
where it is analysed to discover information regarding the incursion. In monostatic radar transmit and
receive antenna are coincident.
The required information in the context of time / distance speed measurement is the instant that the
vehicle was present at a specified point, which is impressed on the received signal in a number of different
ways:



The signal amplitude exceeds a given threshold.
The distance between radar and vehicle reaches a specified value.
The vehicle crosses a defined report line.
The instant at which the received signal amplitude exceeds a predefined threshold depends on the
structure and composition of the vehicle. Reflection centres are frequently distributed throughout the
vehicle and their individual contribution to the total signal amplitude cannot be determined in a simple CW
system.
If the transmitted field is arranged to allow the distance between sensor and vehicle to be calculated then
the trigger can be defined at a specific location in the gate, and becomes less dependent on the received
signal amplitude.
The efficacy of radar as a source of trigger events is further improved if multiple antenna channels are
used – either real or virtual – to recover at least 2-dimentional position information. With such a sensor a
report line can be defined at the gate and the approaching vehicle tracked during its approach.
The efficacy of radar as a source of trigger events depends ultimately on a range of characteristics, for
example:

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

The modulation bandwidth
The stability of the specific reflection points that produced the reflected signal.
As a consequence the sensors at the two gates must be closely match in terms of sensitivity, antenna
pattern and location relative to the flow of traffic. Using a sensor that provides 2-dimensional position
information an uncertainty of relative trigger position across gates of better than ±2.5m is possible.
5.
TRIGGER EVENT MATCHING
To arrive at the transit time of a vehicle through the road section it is necessary to associate the trigger
events both with each other and with an individual vehicle.
For short road sections of only a few metres the association is typically given by the proximity of signals
from the two gates and the restricted occupancy of the road section. Possible exceptions are:


two vehicles in close succession are interpreted as a single vehicle
a long vehicle, typically a truck, is interpreted as two vehicles.
Provisions to recognise and correct such false interpretations must be implemented in the speed
measurement equipment. Any uncorrected false interpretations should be automatically annulled or at
least be readily recognisable as such in photographic evidence. A possible way to realise this is a second
independent speed verification at the exit gate.
When the gates are widely spaced, signal proximity to associate trigger events is clearly no longer viable.
Trigger events must therefore be associated with individual vehicles at the gates, and then the vehicle
identities matched to identify the appropriate trigger times.
Vehicle registration number is the feature typically used for automated vehicle identification. Coincident
with the trigger event an image containing the vehicle registration number is generated then, using an
appropriate algorithm, the number is extracted together with a suitable metric of the extraction quality
(probability of accuracy). These are then forwarded, together with the time information from the trigger
at the gate, to be matched.
Obtaining a match for a given vehicle at both gates depends on the accuracy of the vehicle registration
number extraction, as indication by the associated quality metric. If a match is found and the
corresponding quality metrics are adequate then the trigger times can be associated and the transit time
calculated.
For a given road section the probability of finding a match falls rapidly beyond a given potential transit
time. For example, if the automatic number plate recognition (ANPR) at one of the gates failed to extract
the vehicle registration with sufficient accuracy. Consequently, unmatched vehicle registrations should be
cleared after search duration dependent on the road section length and speed limit.
Incorrect vehicle registration number extraction could conceivable result in an incorrect match and an
associated incorrect transit time for that road section. For this reason the calculated transit times should
always be automatically checked for plausibility, with much too short times automatically annulled.
6.
CERTAINTY OF VEHICLE IDENTIFICATION
For short road sections of only a few metres the set of evidence required to prosecute the alleged
violation is perhaps only the calculated vehicle speed and irrevocable evidence of the presence of the
vehicle leaving the exit gate, typically contained in a photograph.
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For longer road sections it is necessary to independently collect and store evidence of the vehicles
presence at each gate at the claimed instants in time. This is typically photographic evidence, with the
vehicle registration number the primary feature used for identification. Enough of the vehicle structure for
it to be recognisable as the same vehicle should be visible in both images, together with the immediate
vicinity of the trigger location.
The image at the gates must be generated coincidently with the trigger event and then passed to the
ANPR algorithm for extraction of the vehicle registration number. If the extracted number actually
matches the true vehicle registration number depends on a number of factures, for example:




position and alignment of the camera relative to the vehicle / registration plate
illumination
weather conditions
clarity of the characters on the registration plate
Position and alignment of the imaging system should be adjusted to minimise the influence of the first two
positions in this list. A quality metric should be defined for the recognition in each image individually,
which indicates the probability of correctness given the above challenges. A minimum level for this metric
must be established, below which the recognition should be abandoned.
If the extraction was successful, then the vehicle registration number is forwarded to be matched with a
registration number from the second gate. Should the resulting speed calculation result in an intension to
prosecute, then a set of evidence of all material corroborating the alleged offence and the irrefutable
identity of the vehicle involved must be assembled. This is to include at least the following:





the images from entry and exit gates from which the vehicle registration number was extracted
in each image the extracted registration number
the time and date of the corresponding trigger event
the location of the gate
enough of the vehicle structure to allow it to be recognised in both images
Additional evidence may also be required to prosecute the alleged violation, including additional images
that include:


Confirmation of the identity of the vehicle and perhaps also the driver identity
Confirmation of the current speed limit in the road section.
The provision of this evidence is addressed in the next section.
7.
PHOTOGRAPHIC EVIDENCE
Evidence can be procured using either front or rear facing imaging equipment, though some vehicles have
only a rear mounted vehicle registration number. Which arrangement is used is to be decided in
consultation with the relevant certifying authority.
Using a camera facing the front of the vehicle it may be possible to identify the driver of the vehicle, which
is a requirement for prosecution of an offence in some jurisdictions.
For widely separated gates, however, which require a photograph of every vehicle at each gate, driver
identification may generate additional civil liberty concerns, preventing the use of front-facing cameras as
part of an ANPR installation.
Ways to address the provision of additional photographic evidence is addressed in the following.
7.1.
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CONFIRMATION OF THE IDENTIFICATION OF VEHICLE AND DRIVER
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Driver identification requires photographic evidence from the front of the vehicle. If the certifying authority
does not allow front photography prior to the recognition of a speed violation then a further camera has to
be positioned after the exit gate and used only after an intension to prosecute has been established.
The instant at which the vehicle is at the correct position to be photographed by the additional camera
depends on the time required to recognise the speed violation and the speed of the vehicle at the exit
gate, which may differ from the average speed of the vehicle through the road section.
To establish the correct delay between the passage of the vehicle through the exit gate and the
generation of the required evidence photograph it is necessary to ascertain the instantaneous speed of the
vehicle at the exit gate. This may be available from the trigger sensor at the exit gates or may require an
additional sensor.
7.2.
CONFIRMATION OF THE SPEED LIMIT WITHIN THE ROAD SECTION
If the speed limit within the road section can be changed by the road operator then it is necessary to
demonstrate that the driver of the vehicle could have been aware of the current speed limit while in the
road section. A possible way to demonstrate this is as follows:

After changing the speed limit within the road section the previous speed limit continues to be
valid until all vehicles within the section at the time of the limit change have evacuated the road
section.

Include evidence of the advertised speed limit at the time the vehicle passed through the road
section in the set of evidence for prosecution of the alleged speed violation. This could be
photographic evidence of the state of the speed limit signs or verifiable entries in a log of speed
limit changes.
An adequate estimate of the time required for vehicles to evacuate the road section following a change in
the speed limit is:
2𝐷
𝑡𝑒𝑣𝑎𝑐𝑢𝑎𝑡𝑖𝑜𝑛 =
𝑆𝑙𝑖𝑚𝑖𝑡
where D is the road section length and Slimit the previous speed limit for that section.
Other evacuation times may be specified by the relevant certification authority.
8.
APPROVAL
Speed measurement based on the time distance-technique includes a number of sub-systems, each of
which must be verified during the approval process. These will be discussed in turn.
8.1.
ANPR AND REGISTRATION NUMBER MATCHING
The vehicle registration number is an unequivocally unique feature that can be used to establish vehicle
identity. If the vehicle is to be recognised automatically then the quality of the ANPR must be established.
This is in two parts:
1. The algorithm used for recognition should indicate the quality of the result using metrics described
in the handbook.
2. The efficacy of the ANPR and the efficacy of the accompanying quality metric should be
established using vehicle images and/or test vehicle registration plates.
The performance and accuracy of the ANPR can be verified using a set of vehicle registration numbers
that meet agreed quality criterion.
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All vehicle registration number extractions are to be accompanied by a quality metric. The minimum value
of this metric, below which the recognition is no longer reliable, is to be stated.
8.2.
SIMULATED MEASUREMENT CYCLES
Synthetic trigger events can be generated using a simulator appropriate to the sensor technology used.
Using such events it is possible to exercise multiple aspects of the system in the laboratory. The accuracy
of such simulated trigger events should be sufficient to allow speed inaccuracies in excess of 1%
originating in the equipment under test to be clearly recognised.
Tests using simulated trigger events are typically not suitable for investigating the following issues:
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Variability of trigger event resulting from differing vehicle structure and form.
Section distance variability due to different route coverage, particularly with long sections.
The ruggedness of the system against these sources of variability should therefore be established by
design and verified at the installation before going live.
8.3.
MEASUREMENT CYCLES USING TEST VEHICLES
If possible the efficacy of the system should be confirmed by comparison against an independent
calibrated method using a number of vehicle types and speeds. This is readily achieved if the section
distance is short, for which a number of certified independent methods are available. Actual traffic may be
adequate here, but if possible the tests should be conducted using dedicated test vehicles.
The accuracy of the available reference methods are typically 1% at 3σ. Using such a reference method
and careful preparation of the speed measurement equipment its accuracy should be verified for a
representative population of vehicle types and speeds. This population should include at least 500
vehicles.
If the section distance is long with widely separated gates then the population of vehicles should be
composed exclusively of suitably equipped test vehicles. Suitable equipment may include GPS or a
calibrated Odometer.
Such tests are easier to realise if some variables are removed from the installation, which could be
achieved if the equipment is assembled on a test track that allows closely defined routes to be repeatedly
traversed.
8.4.
VERIFICATION OF SECTION DISTANCE
It is typically possible to traverse a given section by an infinite number of slightly different routes, each of
which differs in length by a small amount. This is particularly so if the section includes a number of bends
or corners.
It is important that the shortest drivable route is used as section length in the speed calculation, for which
a careful survey of the section, accounting for all bends (inner radius), is necessary. The conclusions
drawn from this survey should be confirmed by a suitably equipped (calibrated Odometer or GPS /
GLONASS tracking) test vehicles that traverse the actual section.
If the Odometer is attached to the side of the test vehicle, then the shorter of two traverses with the
Odometer attached to different sides should be taken as the section length.
Since the end of SA the accuracy of GPS-trackers is in principle adequate to confirm the length of longer
road sections, but there are potential limitations:
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Accuracy depends on dependable reception of a number of satellites.
GPS / GLONASS trackers reports its position at discrete intervals, which shortens the distance
through curves.
All distances are typically calculated on a common surface, which ignores the extra section
distance resulting from elevations or depressions.
The section length must be sufficiently long to render residual uncertainty due to an inadequate
knowledge of the road section length smaller than 0.5%, equivalent to at least 200 times longer than the
estimated distance uncertainty.
8.5.
TRIGGER POSITION STABILITY
Variability in the position of a trigger event at a single gate can be expected to be small within subpopulations of vehicles of similar type and size. Provided the sensors that generate the trigger events at
both gates are matched and similarly orientated then the uncertainty in section transit time of individual
vehicles can therefore also be expected to be small. This has to be verified as part of the approval
process.
For shorter road sections the trigger position variability will contribute to the speed difference observed
during comparison with a suitable reference method, which may be an adequate investigation of this
issue.
For longer road sections the verification process is more complicated, as described in section 4.
8.6.
TIME SYNCHRONISATION
For short section lengths a single clock can be used at both gates. The time base should be sufficiently
high and of adequate stability, as discussed in section 2).
For widely spaced gates the local clock at each gate must be synchronised to each other or both to a
central clock, as also discussed in section 2).
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