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Workshop on Policies to improve the contribution of Urban Buses and other captive fleets to Urban Air Quality

Brussels, 14 January 2005, in co-operation with

The workshop brought together the most recent research findings on the effectiveness of emission performance enhancement options for urban buses and other captive fleets, and helped identify the scope for action at EU level, in the context of C.A.F.E. Thematic Strategy.


  • Give a clearer picture of the potential contribution of urban buses and other captive fleets (such as waste collection, taxis, urban freight delivery, etc.) to air quality improvement in urban areas.
  • Analyse the advantages and disadvantages of different emissions performance enhancement options.
  • Identify the best methodologies and the parameters to be taken into account for the assessment.
  • Assess the scope for action at E.U. and Member State level, making the link with other Policy objectives such as climate change and public transport promotion.


The present air pollution levels of particulate matter, ozone, nitrogen dioxide are higher than limit and target values of EU legislation. The impact of particulate matter has been estimated to cause about 100.000 people dying prematurely every year. The CAFE Baseline shows that present policies of emission reductions will lead to strong improvements, but still problems will remain by 2020 and additional measures are called for. This is particularly true for urban areas – where the majority of the EU population live. Although a significant proportion of the air pollution problem in many urban areas is caused by non-transport sources and/or sources external to the area (particularly in the case of PM), there is a large scope for improvement for urban transport.

The objectives of the workshop were to:

·        Give a clearer picture of the potential impact on air quality from urban buses and other captive fleets, such as waste collection trucks, taxis, etc.

·        Analyse the advantages and disadvantages of different emissions performance enhancement options. - Identify the best methodologies and the parameters to be taken into account for the assessment.

·        Identify the scope for action at EU level, making the link with other policy objectives such as climate change and public transport promotion.


The workshop was introduced by Olivier Colla, Director of Strategic Studies at the Brussels Public Transport Company (STIB), and chaired by Jacques Delsalle, European Commission - Environment DG.

Leonidas Ntziachristos, LAT/AUTh presented the background document drafted for preparing the workshop and which was the basis for the discussion (slides presentation).

This document has now been updated reflecting the conclusions of the workshop and the contributions from the participants. FINAL REPORT

Contribution of urban captive fleets to air pollution

Captive fleets have a measurable contribution of PM and NOx in urban inventories, and their significance increases with spatial resolution (bus routes, depots, etc.). While aggregate estimates of transport pollutant emission – such as TRENDS or TREMOVE - may not give a reliable picture of the problem, a proper assessment of Air Quality should include a finer resolution (regional, urban, hot-spots) which will eventually highlight these issues (an example is given by Udo Lambrecht, IFEU Heidelberg). It is therefore useful to improve the fleet estimates, the activity data and in particular the actual driving conditions and the emissions factors as a function of these actual driving conditions, used to assess the contribution of captive fleets in inventories.  On this point, see contributions from the projects SORT (Jean-Paul Etienne, TEC) and ARTEMIS (Michel André, INRETS).

Local societies have been taking measures to reduce emissions from captive fleets, but these are not widely known and might not be representatively taken into account in inventories. It is even more difficult to estimate the actual Air Quality benefit of such measures due to the lack of detailed emission information. There is a need to develop a central mechanism for monitoring pilot and voluntary experiences of improvement of public transport and environmental performance of vehicles to quantify the potential environmental and economic impacts.

Evaluation of technological options for improving environmental performance

The workshop has been looking at the wide set of technological options which may help to go quicker and further in the reduction of pollutant emissions:

·        Maintenance: There is no general evidence that taxis, busses or refuse trucks are badly maintained with respect to their emission control, although many companies focus maintenance mainly on fuel consumption and vehicle safety. Measures in that field have a limited potential, and may be focused on a modification of the inspection and maintenance procedure, including a better PM emission characterization, and taking into account the forthcoming HDV OBD regulation.

·        Biodiesel and biogas may be used with none or minor engine modifications to existing fleets but offer limited air quality benefits over low sulphur diesel and NG respectively and are produced at a higher cost, although major changes are expected in the coming years.

·         The use of emulsified fuels is limited, despite of air quality benefits demonstrated on relatively older vehicles, due to compatibility restrictions on new vehicles, an increased fuel consumption, noise and costs, and the lack of standardization. Use of emulsions usually also voids the warranty offered by manufacturers.

·        Emissions factors of new NG vehicles are already within Euro 5 emission limits, as highlighted by the presentations of Alfredo Martín, IVECO, on stoichiometric engined vehicles, and from Nils-Olof Nylund, VTT on the comparison with diesel engines. However, NG is not offered as a retrofit option and is rather relevant for fleet replacement activities.

·        LPG is a good alternative for taxis and light duty trucks, offering substantial reductions to PM and NOx and comparable operation costs, but its use is limited due to its limited availability compared to diesel (production and fuel stations).

·        Retrofit: The combination of CR-DPF (FB-DBF for LGV) and EGR or SCR systems show the largest potential in terms of emission performance and operation. (see also presentation of Guido Lenaers, VITO). There is a need to develop a technical specifications / applicability list of retrofit devices for use in different fleets (cf USEPA, Switzerland).

The situation is evolving quickly, and there is a need for integrating this information in a systematic way, so that it can form the basis for policy making, at local, national and European level. Gabriel Plassat, from ADEME, presented the programme of evaluation of technological options launched in 1999 to provide elements of decision-making aid. A multi-criteria analysis shows well that each pathway has advantages and disadvantages. The integration of environmental and economic variables can be performed using a valuation of pollution and GHG external costs, and a detailed assessment of operational conditions for each bus operator, thanks to the SIMULIBUS decision tool. This experience could be extended to the European level.

This could also be the basis for a further improvement of the model TREMOVE, presented by Bart Van Herbruggen, TMLeuven. TREMOVE is a policy assessment model, designed to study the effects of different transport and environment policies on the emissions of the transport sector.

Implications for policy making

Ulrich Weber, UITP, presented the vision of public transport operators. According to the recent review performed by UITP, diesel technology is still the most economic solution and when equipped with EGR/SCR and particle filters offers compliance with highest emission standards (EEV). Local authorities and transport undertakings should be free to choose their best strategy on how resources are allocated to obtain a clean fleet, and extra costs for changing to clean vehicles and alternative fuels have to be taken into account and should not lead to a counterproductive cut in the PT service offer.

David Marshall, US Clean Air Task Force, presented the US diesel retrofit experience in terms of regulations (California and NYC) and voluntary programs. It should be further assessed how far this can be transposed to the European Context.

Peter Danielsson, VOLVO, presented the Swedish Public Transport Association (SLTF) environment program, applied on an optional basis for regional/local public transport authorities in the traffic procurement.

Finally Jacques Delsalle, Environment DG, presented two concrete actions that could be undertaken as a follow-up of this workshop:

·        In the context of Green Public Procurement, as a follow-up of Handbook on Environmental Public Procurement, it may be useful to make available more detailed information for urban  captive fleets, aiming at presenting a detailed an updated environmental know-how to set the right environmental specifications or criteria. These guidelines could be used also in the context of State Aids, Structural Funds, Environmental Zones, Charging, etc.

·        The development of Low Emission Zones across the EU may trigger the need for common EU system approval of retrofit systems, need to base on special local or national regulations.

A conclusion – already highlighted in the UITP presentation – was that comprehensive strategies to solve transport related air quality problems in urban areas are necessary and the scope of analysis must not be reduced to technological measures. Other measures aiming at modal split and at increasing the mean speed of public transportation may already have a strong impact on major pollutants and GHG emissions.

The Environment DG will assess the opportunity to organise a follow-up of this workshop in order to define cost-effective approaches (comparing technological and non technological measures) for public transport and other urban captive fleets, in the context of the Urban Strategy and the Air Quality directives.