fully effective. in some cases attainment of health-based ... · all motor vehicle emissions. air...

The range and effectiveness of short-term

measures to reduce traffic emissions during high

air pollution episodes

Derek M. ElsomAir Quality Management Research Group, Department of Geography,Ck/%W #roo&&? (7mver.?zYy, Cb/brd, QA3 O&P, C/mW ATrngdomE-mail: [email protected]

Abstract

Concern for continuing poor urban air quality, caused primarily by motor vehiclesemissions, and the slow progress being made towards reducing total vehicleemissions by long-term measures, such as improving fuel and vehicle technologies,has prompted some authorities to try to reduce the severity and duration of highair pollution episodes by implementing short-term traffic restraint measures. Thispaper reviews the range of episodic air quality management schemes applied incities around the world and comments on the effectiveness of such schemes. Thedifficulty of targeting vehicles according to the contribution they make to the airquality problem is highlighted. The problem of some schemes simply causing adisplacement of the area of excessive vehicle emissions rather than reducing totalemissions is reviewed. Rapid developments in telematics and improved urban airquality and traffic monitoring networks (e.g. Urban Traffic Management andControl systems) may offer significant improvements in the effectiveness ofepisodic management schemes in the future.

1 Short-term measures: a response to the limited effectiveness

of permanent measures in reducing vehicle emissions

Cities faced with severe air pollution problems in central and inner areas causedby motor vehicle emissions are implementing a range of permanent measures toreduce pollutant emissions. These include vehicle-targeted measures (improvedvehicle and fuel technologies), transport and traffic management measures, andplanning policies. Unfortunately, such actions may take many years to become

Transactions on Ecology and the Environment vol 29 © 1999 WIT Press, www.witpress.com, ISSN 1743-3541

240 Air Pollution

fully effective. In some cases attainment of health-based air quality standards maynot occur because attempts to reduce emissions through improved vehicle and fueltechnologies are being undermined by growth in the number of motor vehicles aswell as their frequency of use and increasing urban distances travelled. In otherwords, the reduction of emissions per vehicle achieved by technologicalimprovements may be offset by an increase in total vehicle emissions. Indeveloping countries, the potential for air quality improvement by vehicle and fueltechnologies may be limited further by economic factors. Introducing majortechnological changes requires large initial financial investment which may be toocostly to implement in some countries. Additionally, the low fleet turnovercharacteristic of many cities in developing countries will minimise the overall airquality impact that is offered by the introduction of a small number of newvehicles with relatively low emissions. Even the air quality benefits offered bysome permanent traffic management measures such as park-and-ride facilities maybe overwhelmed by emissions generated by the rapid growth in traffic flows.

In response to concern for poor air quality and the slow progress being madeby permanent measures, some city authorities and national governments haveintroduced short-term or emergency measures to reduce emissions during highpollution episodes. Episodes pose a serious risk to public health. Most episodicmanagement schemes implement traffic restraint measures but some incorporatecurbs on industrial and residential emission sources too.

2 Episodic air quality management

The objective of most episodic air quality management is to reduce the severityand duration of high pollution episodes (Elsom*). The ultimate objective would beto implement measures in advance of, and during, a period of stagnantmeteorological conditions in order to prevent pollution concentrations evenreaching unacceptable levels. This would require an accurate forecast of a highpollution episode together with a detailed understanding of what each type ofemission source contributes to the air quality problem.

It is common to employ two or three pollutant threshold concentrations (smogalert levels) at which to alert the public, especially sensitive groups within thecommunity, to periods when air quality may pose an increasing risk to theirhealth. These pollutant thresholds trigger the introduction of specific measures toreduce emissions, often voluntary initially but then mandatory. The pollutant levelswhich trigger the implementation of episodic management vary from country tocountry as does the stringency of the measures implemented.

Not all countries, even where urban air quality exceeds national air qualitystandards, implement short-term actions to curb high air pollution episodes. Someauthorities may consider it more appropriate to direct all their attention andresources to introducing and improving the effectiveness of permanent measures.In some countries the cost of taking short-term action is considered prohibitiveand no episodic management is practised. Other authorities argue that episodicmanagement is a necessary step towards adopting radical permanent measures as


Air Pollution 241

the public's experience of stringent short-term measures helps them to recognisethe seriousness of air pollution problems and to generate support for permanentmeasures to tackle these problems. In some climates the occurrence of severalmonths of stagnant meteorological conditions (e.g. light winds and a low-level,intense temperature inversion) may result in a lengthy period of high pollutionconcentrations. In such situations a seasonal air quality management scheme ratherthan a short-term scheme may operate during which traffic restraint measures areapplied for the entire "pollution season". In some cities, such as Mexico City,seasonal measures may be strengthened on the worst pollution days.

3 Transport and traffic management measures

Transport and traffic management measures play an important part in urban airquality management. They may be applied permanently, seasonally and/or in theshort-term. They aim to reduce the number of vehicles, their frequency of use, andthe distances they are driven. In particular they attempt to encourage a modalswitch from private cars to public transport, walking and cycling. In addition,traffic management measures aim to create situations where vehicle enginesoperate efficiently so consuming less fuel and keeping pollutant emissions as lowas possible. Eliminating congestion, reducing the frequency of stop-start situationsand smoothing of traffic flows help to reduce vehicle emissions. Transport andtraffic management measures include vehicle restraint (access restrictions), parkingcontrols, traffic calming and smoothing, urban traffic control, park-and-ridefacilities, and priority for public transport.

This paper examines the range and effectiveness of traffic managementmeasures which attempt to restrict the access of vehicles to the central part of acity during high pollution episodes. Short-term access restrictions may be appliedto all vehicles or to specific types of vehicles. The latter may include heavy goodsvehicles, private vehicles, taxis, vehicles not meeting specified emission standards,vehicles older than a specified year, vehicles with an odd (even) numbered licenseplate, and cars with less than two (or three) people in them. Restrictions mayapply for several days, a day or simply for a few hours.

4 Limit of emissions reduction achievable by traffic restraint

The degree to which pollutant concentrations can be reduced during an episode bytraffic restraint measures depends upon the relative contribution that motorvehicles make to total emissions. Detailed annual pollutant emissions inventoriescompiled for ten major UK urban areas indicate that road transport often accountsfor a very high percentage of total emissions of benzene (QHJ, 1,3 butadiene(QjHg) and carbon monoxide (CO) and a significant percentage of nitrogen oxides(NOJ, non-methane volatile organic compounds (VOC) and fine particulates(PMjo) (table 1). For some pollutants and for some urban areas, notablyMiddlesbrough and Swansea/Port Talbot with their large numbers of major


242 Air Pollution

industrial sources, the table highlights that road transport is not the primarycontributor. However, emission inventories compiled for an entire city oragglomeration are likely to underestimate the contribution that road transportmakes to total emissions in a city centre during a high pollution episode. Even so,industrial, commercial and residential emissions will contribute to a high airpollution episode unless restrictions are applied to these sources too. Even if allprivate vehicles were denied access to the city centre some road vehicle emissionswould still occur there, such as from public transport vehicles. In Bristol, Glasgowand London, buses contribute 8%, 9% and 4% respectively of annual totalemissions of NO, and 7%, 10% and 6% respectively of PM,Q (Buckingham etal.'4). Essential road vehicles such as emergency service vehicles (police, fire,ambulance) would be exempt and continue to add emissions. Some episodicmanagement schemes permit the continued operation of public service vehicles(electricity, gas, water), special use vehicles (for the disabled), motorcycles, all orsometimes half of the taxis, and perhaps even all or some delivery vehicles. Thecontinued circulation of these vehicles further reduces the maximum reduction intotal vehicle emissions likely to be achieved from short-term traffic restraintmeasures. Train emissions would be unaffected too.

Table 1: Annual pollutant emissions attributable to road traffic (%) in ten UKurban areas (BR=Bristol, GL=Glasgow, LOLondon, MA-Manchester,ME=Merseyside, MI=Middlesbrough, SO=Southampton & Portsmouth,SW=Swansea& Port Talbot, WM=West Midlands, WY=West Yorkshire). Source:London Research Centre/RSK Environment Ltd on behalf of DETR.

Poll.

CaH*

CA

CO

CO,

NO,

VOC

PM,o

SO,

BR

89

97

97

26

61

60

36

5

GL

92

98

95

29

76

56

73

28

LO

86

97

97

30

76

62

79

22

MA

93

96

95

21

63

20

31

3

ME

92

78

91

18

42

28

8

1

MI

17

42

22

3

17

10

19

1

SO

94

92

93

24

47

46

24

2

SW

80

92

83

17

28

38

13

1

WM

99

96

98

43

85

46

56

16

WY

65

98

95

17

73

42

64

25

Other factors affecting the potential improvement that can be achieved by,say, restricting traffic within a designated area is the size of that area becausepollutants will drift in from surrounding areas, some of which may experiencehigher emissions due to displaced traffic. The problem of pollutants being


Air Pollution 243

imported from outside the traffic restraint area is greater for small restricted areas(e.g. a single street, a few streets) compared with large areas. However, even fora large restraint area pollutants may be transported in to the city centre fromoutside by urban heat-island induced light winds, perhaps enhanced bytopographical effects if a city lies within a basin or valley.

For some pollutants, such as O, and PM,o, high concentrations are associatedwith regional transport mechanisms so this will limit the impact on local pollutantlevels produced by a reduction in local traffic emissions. The nature of thephotochemical reactions involving O^ means that there is no simple linearrelationship between a reduction in precursor emissions and changes in O^concentrations. A reduction in ozone-scavenging NO emissions in a city centrecould result in O, levels increasing there [Jol and Kielland*].

Pollutant measurements in three cities in Taiwan provide an insight into themaximum reduction in pollutant concentrations that may be achieved by stoppingall motor vehicle emissions. Air quality monitoring conducted during routine airraid drills in April 1998 highlighted that CO concentrations fell by 67 to 83%within 20-30 minutes of the traffic coming to a halt in Taipei, Taichung andKaohsiung (table 2). NOx concentrations fell by 75 to 83%. Pollutant levelsquickly returned to normal levels once vehicles began to move again (Walsh*).

Table 2: Changes in pollutant concentrations following all traffic being stoppedduring routine air raid drills in three cities in Taiwan in 1998.

Pollutant

CO

NO;

Average cone, (ppm),with moving traffic

Average cone, (ppb),stationary traffic

Reduction (%)

Average cone, (ppm)with moving traffic

Average cone, (ppb),stationary traffic

Reduction (%)

Taipei

3.00

0.50

83

300

50

83

Taichung

1.50

0.50

67

60

10

83

Kaohsiung

3.00

0.50

83

200

50

75

Information may be available for situations analogous to that in Taiwan inarchives of air pollution data in other cities around the world. There are probablymany occasions when traffic has been denied access to certain streets or even theentire city centre for some reason other than for episodic management. Occasions


244 Air Pollution

may include closure for road repairs, accidents, public demonstrations, terroristbomb alerts, and festivities as well as special occasions when the city centre isclosed in order to show residents and visitors what the centre would be likewithout cars should a proposed permanent traffic restraint or pedestrianisationscheme be adopted. All of these occasions have occurred in the author's home cityof Oxford. Closing some of city streets to traffic on selected Sundays helpedpersuade the Oxford public to support the implementation of permanent trafficaccess restrictions (7.30 am to 6.30 pm) in the city centre from mid-1999.

Some studies and experiments have attempted to quantify the effects of short-term traffic restrictions. A theoretical study of the effects of a total ban on all carswithout catalytic converters entering Graz, Austria, during a smog alert level 2estimated that CO emissions would fail by 85%, NO, by 86%, HC by 69% andparticulates by 63%.(Sturm et al/). An experiment at Heilbronn/Neckarsulm,Germany, was conducted in June 1994 .when it was closed for four days to allvehicles except those fitted with three-way catalytic converters and the lowestemission diesel engines in order to measure the effect on air quality. Road trafficfell by around 40%. The reduction in light and heavy duty vehicles within the cityas well as the effects of the lowered speed limit (60 kmph) on the nearbymotorway resulted in NO% emissions being almost halved and N0% concentrationsfalling by 30-40%. VOC emissions and benzene concentrations were almost halvedand this is attributed to the reduced emissions from passenger cars in the trafficrestraint area. Peak ozone concentrations within the traffic restraint area remainedunchanged during the experiment (Jol and Kielland*).

5 Traffic restraint during episodes: the Paris experience

Traffic restrictions were introduced in Paris on 1 October 1997 after the previousday's pollution levels reached smog alert level 3 (NO% 1-h threshold of 400pg/nf) for the first time since legislation empowering the authorities to restrictvehicle movement came into force. Around 3 million vehicles enter or leave thecity each day and 1.1 million use the ring road around Paris. On this date onlyvehicles with an odd numbered license plate were allowed access into the city.This follows the practice of many other episodic management schemes such as inAthens, Geneva, Rome and Tehran. Public transport was free (being subsidised bythe government), speed limits were reduced (from 80 to 60 kmph) on the Parisring road, residential parking was free to try to persuade commuters to leave theircars behind, only priority vehicles were used by Parisian authorities, sports werestopped in schools, tourist buses banned, industrial activities decreased (the largestof three power stations near Paris stopped operating) and some polluting activitieswere suspended. Vehicles with three or more passengers, delivery vans andemergency vehicles were allowed to enter the city as well as electric and LPG-powered vehicles. Around 1000 police officers were deployed to enforce therestrictions but, as this was the first time the alert procedure had beenimplemented, drivers breaking the ban were not fined FFr 900 (S150) but wereasked to park their cars and walk or use public transport. The number of


Air Pollution 245

passengers on buses increased by 15% and on the Metro and trains by 10%.Traffic levels in central Paris fell by 17%, on the ring road by 6%, and aroundParis by 14%. Emissions of NO and CO were reduced and NO% levels fell andremained below alert level 1 (1-h of 200 pg/nf). In the future, exemptions (greenstickers) will be given to less polluting vehicles, such as those fitted with a three-way catalytic converter (Mercier ).

6 Seasonal plus episodic traffic restraint

Several major cities (Athens, Manila, Mexico City, Sao Paulo, Santiago) haveintroduced a seasonal scheme banning the use of cars on one day of the week, theday being determined by the license-plate number. In Sao Paulo, the one-day-a-week vehicle ban from June through August in 1997 produced a mean reductionin CO emissions of around 20%. This helped ensure that 8-hour average levelsremained below the 9 ppm air quality standard for "most of the time" (Szwarc andAlonso ). Santiago applies a seasonal one-day-a-week ban on private cars whichbecomes a two-days-a-week ban in the central area when the air pollution indexexceeds 300. If the index rises above 500 then the two-days-a-week ban extendsto the whole city. Mexico City has operated a seasonal (November through March)one-day-a-week ban ('Hoy No Circula') since 1989. This scheme has beenextended to 'Doble Hoy No Circula' during high pollution episodes since 1997.Pre-1993 cars, which are not fitted with catalytic converters, are banned on twodays of the week rather than simply the seasonal one-day-a-week ban. Weekendsare included in this phase with odd and even license plate number restrictionsswitching on alternate weekends. Car models dated 1993 and newer, displayingthe appropriate windscreen hologram sticker, are granted exemptions on theassumption that they are less polluting. Half of the vehicles operated bygovernment offices are banned and one-in-five of petrol stations are closedrandomly, with stations being exempted only if they are fitted with vapourrecovery systems. Emergency vehicles, buses, taxis, and passenger trucks, someof which are highly polluting, are exempted from the restrictions.

7 Urban Traffic Management and Control Systems

Many cities around the world have implemented or plan to implement extensiveUrban Traffic Control (UTC) systems. If linked with an air quality managementsystem to create an Urban Traffic Management and Control (UTMC) system thena range of alternative strategies can be selected in response to worsening pollutionlevels. Bangkok is implementing the SCOOT-based adaptive traffic control systemin the inner part of the city, encompassing approximately 350 junction(Chantavaliet al.'°). It is proposed to develop this into the Bangkok Transport ManagementSystem which will offer the capability of warning the public if adverse air qualityproblems are expected in a particular area unless traffic to and from that area ismodified on that day. The system will strengthen priority measures for publictransport and apply gating strategies to protect an area from worsening congestion


246 Air Pollution

by redirecting vehicles along alternative routes. Additional tactical traffic controlcould then be applied to reduce the congestion further to lower vehicle emissionsand improve air quality in the area of concern. Detailed information about airquality, road traffic, public transport capacity (mass rapid transport, railway, busservices, park-and-ride) will be needed for the system to be effective. Effectiveways will be required to communicate to car drivers what actions they should takeor are required to take. Communication may be via roadside variable-messagesigns, radio and television, telephone, internet, interactive public terminals andeven in-vehicle units. Kanagawa, Japan, plan a similar system, the EnvironmentalProtection Management System, which will provide alternative routes to vehiclesto avoid them entering a heavily polluted area (Giannopoulos and McDonald").

8 Telematics developments in traffic management in the EU

An increasing number of European Union (EU) schemes are exploring telematicsapplications in linking urban traffic control systems and air quality managementsystems (Miles et al. ). A demonstration scheme in Leicester, UK (LeicesterEnvironmental Road Tolling Scheme, LERTS) links SCOOT traffic control withair quality forecasts. It is part of the EFFECT (Environmental Forecasting for theEffective Control of Traffic) project which aims to forecast poor local air qualityand then instigate effective traffic demand management measures to reducepollution levels at predicted 'hotspots'. Athens is developing the APOLLONconcept (part of QUARTET) which is based on early traffic control to reduce theprobability that pollution concentrations will reach alert thresholds.

There is growing interest in two-stage episodic traffic management schemes.The first stage uses variable-message signs, radio and television information toadvise motorists travelling along specific routes served by park-and-ride schemesof poor city centre air quality and to urge them to use these facilities. Publictransport services can be given increased priority on these occasions so amotorist's journey would likely take longer than usual. Information comparing carand park-and-ride trip times can offer motorists an incentive to opt for park-and-ride. Charges for parking at the park-and-ride site and for use of the bus (tram,train) can be reduced or made free during pollution episodes to add a furtherincentive for its use. Each vehicle taking part in the Helsinki demonstrationscheme has an in-vehicle unit fitted which not only receives detailed informationbut allows the motorist to book a parking space, provide access to this reservedspace and pay for a public transport service at one of the park-and-ride stationson the outskirts of the city. The second stage, for those who continue to drive, isthat the driver is charged electronically (via an on-board smartcard) for enteringthe city centre and that the charge reflects the level of pollutant concentrations.This type of scheme is being demonstrated in Leicester, mentioned earlier, Bristol,UK (Environmental Led Guidance and Restraint, ELGAR) and Helsinki, Finland(Demand Management Toolbox, part of ADEPT II). The Gothenburg (ADEPT II)demonstration is exploring the use of telematics and multi-application informationand payment systems in relation to parking and tolling and could readily be


Air Pollution 247

extended to take into account air quality in determining the level of payments.

9 Pollution-related charges for electronic road pricing schemes

Existing electronic road pricing systems offer the potential to charge motoristsduring periods of high air pollution concentrations and to set the charge accordingto the pollution level. The aim would be to select a charge which would deter asufficient number and/or type of vehicles from entering the city centre in order toprevent air quality from deteriorating further and even to improve it. Existingsystems applying variable charges according to congestion already exist but couldbe extended to vary charges according to air quality too. Singapore's electronicroad pricing (ERP) scheme began full operation in September 1998 at the entrypoints on the Area Licensing Scheme in the Central Business District (weekdays,0730-1900h; Saturdays, 0730-NOOh) as well as the manual Road Pricing Scheme(introduced in June 1995) on the East Coast Parkway, Central Expressway andPan Island Expressway (0730-0930h). Cars are charged $2 per pass during thecongested 0800-0900h period but this drops to $1 during the half an hour beforeand after this period. This is intended to smooth the peak morning traffic flow butpollution-related charges could be employed to discourage use of cars, promotecar-sharing and encourage transfer to public transport.

10 Effectiveness of short-term traffic restraint measures

Detailed monitoring and modelling assessments of the spatial and temporalchanges in emissions and pollution concentrations throughout the urban area areneeded to fully assess the air quality effectiveness of short-term traffic restraintschemes implemented during high pollution episodes. Many such assessments areneeded to provide guidance to other urban areas considering adopting similarschemes. This is because the effectiveness of schemes is likely to vary in relationto factors such as the size of the traffic restraint area, type of vehicles selected forrestraint, general age and composition of vehicle fleets, number and type ofexempted vehicles, level of enforcement, and availability and quality ofalternatives to using the car. Episodic traffic management schemes will need toovercome several problems experienced by current schemes if they are to becomefully effective.

First, the inadvertent consequence of some traffic restraint schemes is toreduce city centre pollution levels by simply shifting the worst air quality toanother area, often the area immediately outside the area of restraint. This pointsto the need for greater effectiveness in reducing total traffic emissions and forimproved tactical application of restraint measures. Current developmentscombining traffic control and air quality management systems hold much promisefor being able to make effective traffic redirection and restraint decisions. Theneed to reduce traffic flows throughout the city during high pollution episodeshighlights the importance of communicating with travellers well before they set


248 Air Pollution

off on their journey, to give them sufficient time to change their travel plans andto opt for park-and-ride, public transport, sharing cars, etc.

Second, schemes to control traffic during high pollution episodes need totarget those vehicles which contribute most to the air quality problem.Developments in telematics can offer help in this respect. For example, dependingupon the severity of the air pollution episode, episodic management schemes mayexempt electric- and natural gas-powered vehicles and petrol-powered vehiclesfitted with catalytic converters. Telematics offer the potential to monitor allvehicles passing a tolling or restraint point and to check they are displaying theappropriate license plate or carrying a valid exemption smartcard. Futuredevelopments could even link such a system with remote sensing roadsideemissions testing in order to identify those exempt vehicles which, because of poormaintenance or failed pollution-control equipment, have become 'gross polluters'and should not be granted access to the city centre.

Third, buses and taxis may be significant polluters in some cities. Indeveloping countries, emissions from exempted motorcycles and autorickshawsmay be high too. If the decision is taken to exempt such vehicles then additionalmeasures may be needed to be taken during an episode, such as frequent vehicleemissions testing, to ensure that such vehicles are not 'gross polluters'. Localemission standards may need to be applied to, say, buses and taxis to require themto conform to low emission standards. If episodic management were to target PM^(or smaller particles) then the contribution to emissions from exempted dieselbuses and taxis would need careful attention. During episodic management stage2 in Helsinki and Turku, Finland, there is enhanced surveillance and enforcement(issuing fines) for vehicles left with their engines idling unnecessarily (Daly ).

Fourth, the emissions reduction achieved by simple schemes which permit aspecified proportion of the existing vehicle fleet, such as cars with an odd- (oreven-) numbered license plates, may be offset by growth nationally in the numberof cars. Even if access to the city centre is restricted to low emission vehicles, anincrease in the number of these vehicles can lead to congestion and risingemissions. Initially, tactical reduction and redirection of traffic flows using acombined traffic control and air quality management system may be able to copewith the annual increase in the number of vehicles but there may come a timewhen a quota system for vehicle centre access may need to be adopted.

Finally, the type of pollutant used to justify the introduction of trafficrestraint needs careful consideration. Existing schemes show that traffic restraintmeasures may have a significant effect on CO levels within a restricted area andNO, to a lesser extent. High concentrations of PM,o are associated with regionaltransfer into the city as well as local sources so local traffic reductions caninfluence only the latter component. Levels of O, are more likely to be reducedif use of vehicles is restricted over a wide area, not simply the city centre. If onepollutant is targeted by episodic management then care must be taken to ensurethe measures implemented do not result in another pollutant increasing inconcentration. For example, applying restraint measures to diesel-engine ratherthan petrol-engine vehicles may lower PM,o concentrations but allow benzene


Air Pollution 249

concentrations to rise.

11 Conclusion

Short-term traffic restraint measures offer the potential to reduce the severity andduration of high pollution episodes. Existing episodic management schemes havehad some success, in addition to generating support amongst the public forpermanent measures to tackle air pollution problems, but there is scope forimprovement. Recent developments in telematics applications which integrate anurban traffic control system and an air quality management system should helpimprove existing episodic management schemes and establish effective new ones.

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2. Buckingham, C. et a!., Atmospheric Emissions Inventories for Four UrbanAreas: Merseyside, Bristol, Southampton/Portsmouth, Swansea/Port Talbot,London Research Centre/RSK Environment, London, 1997.

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Paulo Metropolitan Region, Paper presented at the llth World Clean Air andEnvironment Congress, Durban, South Africa, September, paper 10G-3,1998.

10. Chantavali, K.N., Paksarsawan, S. & Gammons, T., The Bangkok TransportManagement System, Traffic Engineering & Control, February, pp. 82-85,1998.

11. Giannopoulos, G.A. & McDonald, M., Developments in transport telematicsapplications in Japan, Transport Reviews, 17, pp. 37-59, 1997.

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