Transport for London, Congestion Charging
Technology Trials, Stage 1 Results
Dan Firth
Transport for London, United Kingdom
1. CENTRAL LONDON CONGESTION CHARGING
The central London congestion charge, an £8 daily fee for entering a 22 sq km
area of central London, was introduced in February 2003 with the aim of reducing
congestion, as well as further encouraging the use of public transport in central
London, benefiting business efficiency by speeding up the movement of goods
and people, and creating a better environment for walking and cycling. Two years
after its introduction, congestion charging had seen a 30% reduction of
congestion within the charging zone 1. The revenues raised by congestion
charging are being used to fund transport improvements in London.
The present scheme is enforced by means of cameras situated at entry points to,
and at points within, the charging zone as well as on a number of mobile units
within the zone. The cameras send live video images over secure
communications lines to a central location where Automatic Number Plate
Recognition (ANPR) equipment identifies the Vehicle Registration Mark (VRM) of
vehicles which have passed through the field of view of the camera and compiles
an evidential record of the passage of the vehicle. This evidential record is used
as the basis of enforcement action against drivers who have not paid the charge
Transport for London (TfL) believes that there could be benefits to be gained
from using alternative technologies within the existing zone as well as for any
future developments. A series of technology trials commenced in August 2003 to
investigate the different technologies available.
2. THE TRIALS
The Stage 1 trials were intended to provide a proof of concept of the technologies
trialled, rather than a detailed assessment of their viability, whether from an
operational or cost perspective. They have demonstrated which technologies are
worth further investigation.
The trials addressed four different groups of technologies:
•
Cameras and automatic number plate recognition (ANPR) technology;
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•
“Tag & beacon” detection systems, of which Dedicated Short Range
Communications (DSRC) is the leading solution for road tolling;
•
Satellite navigation (Global Positioning System - GPS) technology;
•
Digital mobile phone technologies (GSM – Global System for Mobile).
The first two groups of technologies are associated with identifying a vehicle as it
passes a fixed point, whilst the second two groups are concerned with identifying
the location of a vehicle, without any fixed detection points.
In principle, the assessment of a technology’s applicability to congestion charging
should be measured against the functional and performance requirements of a
given charging scheme. This was not possible in the case of these technology
trials, for two reasons:
•
the shape or scope of any likely changes is not determined;
•
at the start of the trials there was insufficient knowledge of the
performance of the technologies for performance requirements to be
sensibly set; for example, setting an accuracy requirement of 1 metre is
not realistic if the technology is working at accuracies of 100s of metres.
The results of the trials should therefore be judged as an initial assessment of the
technologies.
In order to ensure the robustness of the trials and the results, TfL invited
acknowledged experts from the public, academic and private sectors in the
relevant fields to review, at a high level, the methodology used in conducting the
trials, the methodology of the results analysis and the results of the trials. TfL
would like to take this opportunity to acknowledge their valuable input to these
trials.
3.
CAMERAS,
NUMBER
COMMUNICATIONS
PLATE
READERS
AND
DIGITAL
3.1 Background
The technology presently employed for congestion charging in London allows for
a good audit trail, with a dedicated communications network, as well as having
the lowest evidential and technical risk at the time the congestion charging
scheme was procured. However, the cost associated with installing and operating
this telecommunications system is high and there are also limits to which it could
be used to support more flexible charging structures.
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The key issues associated with the camera and ANPR technologies, which the
trials have considered are:
•
The evidential integrity of digital photography: Digital images are
easier to manipulate than analogue ones, and there is therefore a need to
ensure that any digital technologies used are able to satisfy the evidential
requirements of the courts, to prove that they have not been tampered
with, whether deliberately or not;
•
High data volumes: Digital image capture can produce very large
volumes of data. The trials therefore needed to ascertain whether the
volumes produced are greater or lesser than for the existing configuration,
whether the existing infrastructure is capable of handling these large
volumes and whether different transmission technologies can be used to
transmit digital photographs in place of the existing analogue fibre-optic
transmission;
•
The costs of telecommunications versus on-street processing: If
ANPR processing is moved from a central site to the roadside, then invalid
images (e.g. of incomplete number plates), duplicates or images of
vehicles which are exempt or have already paid could be discarded at the
roadside. This would reduce the volume of data which needs to be
transmitted to the hub site, and hence reduce the associated
telecommunications costs and could also lead to significant back-office
savings. However, the costs of roadside equipment would be increased.
The trials therefore needed to provide sufficient data for the trade-off
between these costs to be evaluated;
3.2 Methodology
There are a number of variables around the present camera and ANPR
configuration which could be refined to enhance performance or reduce costs.
These variables have been tested in a series of trials, using existing congestion
charging enforcement cameras as well as cameras employed as part of the
Journey Time Monitoring System (JTMS) and specific trials cameras at other
locations outside the congestion charging zone. These were performed using
data collected from the general vehicle population, in accordance with the
provisions of the UK Data Protection Act 1998. Dedicated trials vehicles were
also used in some instances to perform specific manoeuvres or to test driver
behaviour to be tested.
Focusing on relatively minor adjustments to the current technology, attempts
were made to optimise the existing congestion charging cameras, through
adjusting lens, illumination and filter combinations. At the hubsite, multiple inputs
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to a single ANPR processor were tested and different generations of ANPR
processors were compared.
3.3 Results
It is feasible to use number plate readers with multiple video inputs, thus reducing
the cost of the system. This result applies primarily to JTMS, which does not
require encryption and compression, rather than to the main congestion charging
system, which does. Further work is required to ensure that this result is
applicable to the main congestion charging system.
New, improved ANPR systems generate number plate reads in excess of 90%
for a single pass, compared to 70%-80% for a single pass for the current
systems. Further work is required to ensure that these results are not
compromised when encryption and compression algorithms are added.
It is feasible to deploy roadside ANPR systems; however, there are few systems
available that have been ruggedised for roadside use.
Digital Subscriber Line (DSL) broadband technology is suitable for carrying
images from roadside systems to a central point without loss of quality and brings
significant cost reductions. The difference in costs of different broadband
strategies is small in relation to the overall costs and therefore should not be the
key driver in selecting the broadband strategy. The decision is more likely to be
driven by issues relating to integration with the back office.
4. TAG & BEACON – MICROWAVE & INFRA-RED DSRC
4.1 Background
Tag and beacon technology is already in widespread use for road user charging
– for example on the Dartford crossing and M6 Toll. In an urban context it has
been deployed in Singapore, Oslo, Rome and elsewhere, however, the
infrastructure required tends to be bulky and intrusive, generally consisting of
gantries crossing the carriageway. Such infrastructure would not be acceptable in
London and the trials have thus sought to test how it could be applied in a
London-specific road charging context exploring the various design issues which
London’s road network and traffic behaviour present.
The technology trials concentrated on two solutions, generically known as
Dedicated Short Range Communications (DSRC), one using microwave and one
using infra-red (IR) communications.
The key issues associated with tag & beacon technologies, which the trials have
considered, are:
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•
Obscuration and roadside beacons: Normal configurations of tag &
beacon solutions (e.g. motorway tolls) mount beacons on overhead
gantries above the lane being monitored. In London, beacons would have
to be mounted at the roadside, meaning that a tall vehicle in the lane
nearest the beacon could potentially obscure the tag in a vehicle in the
next lane from the beacon, resulting in that vehicle not being detected.
The trials sought to assess how important this obscuration problem was
and what geometries of pole and road would minimise it, taking into
account the aesthetics of tall poles and the range of the beacon;
•
“Communication zones” and road geometry: In many existing toll
situations, vehicles can be compelled to stay in lane, and a beacon can
easily be dedicated to each lane. In an urban environment, however,
vehicles may straddle lanes and while a beacon can still be dedicated to
each lane, tailoring the “communications zone” to cover only that lane is
more difficult when the beacon is on a roadside pole rather than on an
overhead gantry. A vehicle’s tag may therefore be picked up by more than
one beacon and a beacon may pick up tags in any one of two or three
lanes. The trials sought to address this issue and have focused on
tailoring “the communications zone” to match London’s carriageways and
roadside geometry;
•
Matching camera images and tag data: Tag & beacon technology will
still require cameras for enforcement where vehicles do not have a
functioning tag and to support any challenges to the charge (at least until
the record of the tag/beacon transaction on its own is accepted as
sufficient evidence of the vehicle passage). It is therefore important to
accurately match the photograph taken by the camera with the information
read from the tag by the beacon. One of the aims of the trials was to
address this matching problem.
4.2 Methodology
A set of practical on-street trials of microwave DSRC was carried out, with
various roadside beacon mounting configurations, and also at the manufacturer’s
test track. These tests were supplemented by computer modelling. The trials also
included camera/ANPR systems both to act as a benchmark and to test how well
the overall system could match DSRC transactions and readings from the
camera/ANPR system.
A fleet of trials vehicles was used for all the DSRC trials, due to the requirement
for on board equipment, although during the trials, tags related to other charging
schemes were detected, including tags from Dartford crossing users and tags
from the Austrian lorry road user charging scheme. Test vehicles deliberately
sought to evade detection by the roadside infrastructure by, for example,
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shadowing large vehicles and passing near the edges of the charge-point
detection zone. These tests provided a worst-case evasion scenario.
Due to commercial difficulties, it was not possible to commence a trial of infra-red
DSRC until late in the trials process. Due to the short time available, the trials
were only conducted at a test track and not at a roadside location in London. The
results are not statistically significant but are indicative of whether further
assessment is warranted. The tests focused on those key elements of IR DSRC
which could offer benefits over and above microwave DSRC, as well as some of
the key issues identified as potential difficulties for infra-red technology.
Figure 1: DSRC trials site at Commercial Road
4.3 Results
Microwave DSRC
Overall the trial DSRC system correctly detected the tag in the vehicle passing
the beacon in 99.55% of cases. The results showed no significant difference in
performance when vehicles passed charge points singly or in groups. However, a
series of tests where parking, U-turns and other manoeuvres which might
confuse the DSRC system were carried out with standard vehicles showed that
under these circumstances detection rates fell by 1.8%.
The computer modelling of the potential for obscuration showed that there is no
obscuration in the near-side lane in any configuration and no obscuration in any
lane if a 3.5m outrigger is used. Despite consistent and determined attempts
during long periods of testing, the opportunities for evasion, even on a major bus
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route, were not abundant and evasion had a greater effect on the camera/ANPR
recognition rate than on the DSRC system.
The DSRC system’s ability to determine direction of travel was tested. This could
be significant for a policy which sought to charge for entering a defined area, but
not for leaving, or for charging traffic flowing in a particular direction, for example
during rush-hours. It was possible to detect the direction of travel for 100% of
passages.
The key overall design issue for DSRC is ensuring that the communications
footprint of the microwave transceiver is coincident with the capture zone of the
accompanying enforcement cameras. In order for enforcement cameras to obtain
a sufficiently good image for the ANPR system to process, they must be set at a
shallow angle. In the current congestion charging scheme configuration, this
means that the zone observed by the camera is some 20m away from the pole
on which it is mounted, whilst the range of DSRC transceivers is typically 8m
when mounted on a 6m pole. So for the two zones to be coincident, the cameras
and the DSRC transceiver would need to be mounted on two separate poles. In
the London environment this is unlikely to be desirable. The trials explored how
DSRC and camera configurations could be adjusted to allow them to be mounted
on a single pole. Although a single pole solution could not be demonstrated in
Stage 1, sufficient progress was made to believe that there are good prospects
for demonstrating such a solution in Stage 2.
The trials successfully demonstrated that spatial matching between the record of
the detection of the tag and the corresponding read of the VRM is possible. The
prototype system achieved correct matches in 95.5% of cases and of the
remaining 4.5%, the correct VRM was identified as the most probable match.
Limiting the DSRC footprint to a specific area of the road was key to achieving
this result.
Infra-red DSRC
The results confirmed that the IR equipment operates successfully at distances of
up to 18.9 metres from the transceivers, dependent upon the angle of vehicle
windscreens. This suggests that co-ordination with ANPR cameras on a single
pole might be more easily achieved.
Given the short duration of the trials, together with a period of fine weather, no
specific testing was carried out in varied adverse weather conditions. Testing was
carried out in bright and direct sunlight, at dawn, dusk and at night and in light
showers, with no adverse effects on performance. Of the IR DSRC systems
deployed around the world, for example in Korea, Germany, USA, Malaysia and
Japan, no detrimental effects on performance have been reported due to adverse
conditions, including heavy rain, large rain droplets, fog, snow and ice and mud
thrown up on windscreens.
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The IR DSRC equipment has been certified in Austria as a “Class 1 Laser”
allowing it to be deployed safely on public roads. This certification confirms that
there is no harmful effect on human sight from the operations of the DSRC
system, even when directed in line of sight to drivers. Obtaining appropriate
certification for use in the UK remains to be investigated.
Despite the short time scale of these trials and the fact that they were only
carried out on the test track, the results show that IR DSRC technology is
adaptable, may solve some of the issues relating to microwave DSRC and is
suitable for further trials on road and with some form of integration with an ANPR
camera system.
5. MOBILE POSITIONING SYSTEMS
5.1 Background
Mobile positioning systems are used to identify the position of a vehicle. This
information can then be used either as the basis of an automated charging
system or in order to provide information or services to users. Two broad
techniques for mobile positioning have been tested in these trials – satellite
navigation systems and digital mobile telephony.
Satellite positioning
The Global Positioning System (GPS) became operational in 1994. It is a system
of 24 satellites orbiting the Earth in 6 different orbital planes, each transmitting a
unique identification signal, orbit information and a precise time signal. At any
one time, at any point on Earth, there are 5-8 satellites above the horizon, from
which a GPS receiver on Earth would in principle be able to receive a signal –
although in reality not all satellites would be visible, due to obstruction by
buildings, hills etc. By calculating the difference between the times of arrival of
signals from the visible satellites, a GPS receiver is able to calculate its position.
The more satellites that it can see, the more accurately it can calculate its
position.
GPS systems will often deploy supplementary systems to increase accuracy, for
example by using additional location survey data, compasses, gyroscopes,
electronic mapping or location predictions.
The key issues associated with GPS technologies, which the trials have
considered, are:
•
“Canyoning”: As discussed, the accuracy of a GPS position is heavily
influenced by how many satellites are visible. This is particularly an issue
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in an urban environment where GPS receivers can be subject to a
“canyoning” effect which decreases accuracy. Canyoning occurs when the
receiver is surrounded by tall buildings (as if it were in a deep canyon),
restricting its view of the sky and hence of the number of satellites. The
received GPS signal can also be reflected and distorted by the
surrounding buildings. The trials examined the impact of this effect by
identifying those areas of London where the accuracy was best and worst
and characterising them according to how built-up the areas are, taking
into account height of buildings, density of buildings, narrowness of streets
and therefore number of satellites visible.
•
Other key issues addressed include location accuracy, integrity of the
GPS signal, availability of the satellites, continuity of service and the
potential for jamming and interference. One issue which the trials have not
addressed (but which will be addressed in Stage 2 trials) is that of
communicating location data from a GPS on-board unit to a charging
system.
Figure 2: Canyoning and multi-path reflection errors in GPS systems
Digital mobile telephony
Digital mobile telephony has been implemented in the UK using GSM (Global
System for Mobile) technology and, more recently, with third generation (3G)
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technology. This trial has mainly tested GSM technology, although limited trials
have been carried out using 3G.
For the purposes of these trials, there are two ways in which the location of a
handset within a given network can be identified:
• Identify the cell with which the handset is communicating – the smaller the
cell, the more accurate the estimated location;
• Query the network for location-based information. Based on the
knowledge of the network design, the LBS (Location Based Services)
system on the network is able to calculate a circle within which the
handset is most likely to be, based on the last base station with which it
communicated.
There are other ways of identifying the location of a handset, but these are more
complex and unsuitable for use in a large scale system.
The key issues associated with digital mobile telephony, which the trials have
considered, are:
•
Accuracy of location information: The locations provided by a GSM
network are approximations to the actual locations of the handset/vehicle.
The trials aimed to identify how accurately these locations approximate to
the true location;
•
The influence of network characteristics: As described above, the cell
with which the handset is communicating is not necessarily the nearest
cell. As base stations have more capacity for signalling between handsets
and base stations than for handling conversations, the use of a more
distant base station is more likely when a user is talking than when the
handset is in standby mode. Frequently there will therefore be an apparent
shift in the location of the vehicle as the user switches the handset from
standby mode to talk mode. This is likely to make location identification
less accurate and was an issue tested during these trials;
•
Accuracy of time-based information: Whilst cell ID information is up to
date, this is not necessarily the case with LBS information. This delay is
not long but it can be sufficient for the vehicle to have moved significantly.
Networks do not necessarily use a consistent reference clock for time (in
the way that the central London congestion charging scheme is
synchronised to the Atomic Clock at the National Physical Laboratory in
Rugby). There is even more variation in time on individual handsets.
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5.2 Methodology
A set of on-street trials was carried out between January and May 2004. Due to
the varied nature of London’s environment, routes were designed to encompass
as many types of conditions as possible. Routes included clockwise and anticlockwise drives around the boundary roads of the central London congestion
charging zone, as well as cross-zone routes, routes on approaches into London
and other routes within London. A number of methods were employed to
determine the true exact position of the various location systems and devices
under test in the trials at the time that trial data were collected, allowing a
comparison of the real location and the location given by the mobile positioning
systems.
Figure 3: Example route driven during mobile positioning trials
5.3 Results
Satellite positioning
Of more than 800,000 GPS positions analysed during the trial, using seven
different GPS devices, the average location error was 9.7 metres, with a
maximum error of more than a kilometre. In order to account for such errors a
charging zone, or location, would need a ‘buffer zone’ around it to ensure that
only vehicles which had entered the charging zone were in fact charged. Vehicles
in the buffer zone, although inside the charging zone, would not be charged to
ensure that there were no “false positives”. Using the results obtained in the trial,
the following calculations have been made:
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•
for a confidence level of 75% that a vehicle was inside the zone a buffer
zone of 14 metres on average would be sufficient;
•
to accommodate 90% of all possible errors the buffer zone would need to
be approximately 28 metres on average;
•
to have a 99% certainty the buffer zone would need to be extended to
approximately 57 metres on average.
Figure 4: Required buffer zone for the central London congestion charging
zone for
99% confidence level
As a result of the different geographic and physical conditions in central London,
errors vary across the central London congestion charging zone. On average, a
buffer zone of 60m around the boundary of the zone would be required to be
99% confident that a position reported as being within the zone was actually so.
Around some parts of the boundary this increases to 250m or more, illustrated in
Figure 4. While the performance of different GPS on-board units varied, none
gave a significant improvement on this average result. This included GPS with
additional support, such as dead reckoning or differential signals. Some units
gave a significantly worse result.
Digital mobile telephones
The Location Based Services of GSM mobile telephone networks used to
establish the position of a vehicle gave results indicating that the position of the
vehicle lay within a circle with a radius, on average, of 800m. The observed
distance between the quoted centre of the circle and the actual position was, on
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average, 2400m. This level of accuracy is inadequate to support congestion
charging.
The timing information related to LBS was also highly inaccurate. In part this is a
result of the way the system operates; in part it appears to be a result of incorrect
synchronisation of clocks which would be relatively easy to solve in any actual
deployment.
A desk study has examined the potential for the deployment of ‘pico cells’, with a
radius of only 25m. These cells could cover a junction or charging point and
behave like a tag and beacon system using GSM as the communications
medium and a mobile telephone as an onboard unit. It is proposed to further test
such a solution within Stage 2.
The trend is for new mobile phones, particularly 3G phones, to have GPS
capability. The accuracy of such phones proved to be no greater than standalone GPS or GSM units, but as the technology evolves
this may change and in any case eases the problem of communicating the
location back to the centre.
6. STAGE 2 TRIALS
Stage 2 of the trials is currently taking place and will continue during 2006. A
number of practical scenarios will be constructed and the output of Stage 2 will
be a number of tested designs which address those scenarios. Stage 3 of the
trials, which would take place during 2006 and 2007, would take one or more of
these tested designs and subject them to large scale usability trials.
The primary trials activity is intended to be the implementation of a tag and
beacon solution in two areas:
•
a ‘mini-zone’ within the current charging zone with approximately 20 trial
charging points;
•
a strategic route, again in the charging zone, where the practical issues of
charging on a route which suffers from congestion could be explored.
The location for the Stage 2 trials is south of the river within the existing
congestion charging zone. It is intended that at least 100 vehicles should be
involved, in addition to 10 dedicated trials vehicles. These would typically be
recruited from vehicle fleets, such as licensed mini-cabs or delivery vehicles and
volunteers such as ordinary TfL employees not involved in the trials. The manner
of recruitment of these trials volunteers, the incentives (if any) offered to
participate and the resultant costs are being planned.
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Stage 2 would also continue with the resolution of design issues for roadside
ANPR solutions as well as research into GPS-based onboard units and the
potential for GSM solutions for improved customer experience and the use of
pico cells.
Figure 5: Indicative location of the stage 2 ‘mini-zone’ and montage of a
charge point
It is envisaged that Stage 3 trials, which should take place in 2006, will trial a
system designed to support a specific policy or policies and evaluate how it
works from a user and operational perspective.
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NOTES
Further details of the results of the comprehensive congestion charging
monitoring programme will be covered in a separate paper to be presented at
this conference: The social impacts of the central London congestion charge on
Londoners, by Alison Cairns, Transport for London.
1
BIBLIOGRAPHY
Department for Transport (2004), Feasibility study of road pricing in the UK,
London, DfT.
Directive 2004/52/EC of the European Parliament and of the Council on the
Interoperability of Electronic Road Toll Systems in the Community, Brussels.
Transport for London (2005). London Congestion Charging Technology Trials,
Stage 1 Report, London, TfL – www.tfl.gov.uk/congestioncharging
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