It is clear that reduced travel demand in peak periods reflects fewer and shorter trips being made and results in higher travel speeds during peak periods, with consequent reductions in vehicles emissions during peak periods. However, the bulk of daily travel occurring in off-peak periods may be negatively affected, as some peak travelers shift their time of travel to off-peak periods to avoid tolls. Overall beneficial impacts on air quality may thus be smaller than that suggested simply by peak period travel demand reductions. This reduction cannot be measured directly, but will have to be calculated from the information on traffic diversion, congestion reduction, and ride-sharing increase.

In the long term, congestion pricing could lead to increases in use of alternative travel modes for all daily trips, thus reducing total daily emissions.

4. Social justice

Sustainable development has three widely agreed meta-goals: sustainable economic development, environmental protection and social justice. All three goals must be addressed together if development is to be sustainable. Much attention has been paid to economic development and environmental protection, but less attention has been paid to social justice. Social justice (equity) contains economic equity and environmental equity.

Economic equity issues resulting from congestion pricing are difficult to address completely. Some people have argued that road pricing is regressive, in that it will bear more heavily on poorer car users, short distance journeys, and on those living adjacent to the cordons; more generally concern has been expressed at the impact on those who, for whatever reason, have no choice but to travel by car. These arguments can be countered to some extent by devising more flexible charging regimes. It has to be accepted that any form of road pricing will introduce some inequities. The key is to keep these to minimum,and to find ways of compensating those who do not benefit from congestion pricing. In practice, the lowest income travelers, who typically travel by public transport or on foot, are most likely to benefit.

For the conventional transportation system, the tax rates on gasoline, which are the same regardless of whether transport users are traveling during congested or un-congested periods Congestion pricing is expected to reduce this unfair by implementing surcharge for the use selected congested facilities during peak traffic periods.

Equity issues addressed in transport have been largely concerned with economic equity, including the relationship between public and private transport, the impacts of

congestion pricing on peripheral areas and underprivileged population groups.However, environmental-equity issues have been little within a transport context. Transport produces direct effects such as atmospheric emissions and noise, and also indirect effects, through its influence on the location of polluting facilities and affected people.

Environmental equity refers to the social distribution of environmental quality (and specifically the distribution of NO2 by deprivation status). That is equal access to a clean environment and equal protection from possible environmental harm irrespective of income, class or other differentiating feature of socio-economics status. Transportation planners should point out that the reduction of congestion levels and increased trip speed on an entire facility will benefit users of public transit.

From an environmental equity perspective, the effectiveness of congestion pricing is sensitive to the spatial distribution of socio-economic characteristics; hence the best scheme design may be different for each application city.

As can be seen, social justice is most difficult to address, and practices indicate that the revenue from road pricing plays a crucial role. If the revenue is recycle to permit investment in public transport, or maintenance of higher service levels, it helps to provide a clear alternative, which both reinforces the impact on mode choice and reduces the concern of those on whom the charge is imposed. If it is used to improve the road infrastructure, it can be seen that traffic diverted from the controlled area is being catered for. If it is used to pay for environmental improvements, it will help to reinforce the wider benefits of charging. How to use the revenue and improve the efficiency of urban transport system is play a key role in sustainable development.

5. Conclusions

With the continuing growth in traffic demand and decreasing scope for expansion of road infrastructure, traffic congestion is not a problem that will go away soon. Congestion pricing is proposed as an important demand- management strategy for tackling traffic, but still receives strong social and political opposition because of the uncertain impacts on economic development, environmental protection and social justice.

This paper reviews the relationship between congestion pricing and three goals of sustainable transportation system, and the results show that congestion pricing is a promising traffic management strategy, which can promote urban economic development, improve environmental quality and reduce inequity. Recent advances in electronic toll collection (ETC) technologies have made congestion pricing technologically feasible. It is safe to predict that coming decades will witness an increasing number of implement or

attempts to implement congestion pricing. The design and evaluation of congestion pricing will require a deeper understanding of the direct and indirect impacts on sustainable development.

Acknowledgement

This work was supported by the Young Talents Project of Fujian province

(2007F3078). Appreciation is also extended to reviewers for their helpful comments and suggestions on improving this paper.