【正文】 al social costs of each trip. Marginal social costs are indicated by the MSC curve in figure 1. Figure 1 Effect of congestion pricing If there are n vehicles in the transport system, and mean user cost is represented by MPC, one marginal user added will increase mean user cost to MPC+ △ the marginal social costs can be formulated as MSC = (n + 1)( MPC + ΔMPC ) ? nMPC= MPC + ΔMPC + nΔMPC Congestion pricing is an important means of transportation demand management, and initially only affects transportation decisions. Practices prove that congestion pricing can effectively regulate traffic travel time and space distribution, promote effective utilization of road resources, and enhance the efficiency of transportation operations. Congestion pricing implementation in Singapore has showed that traffic volumes decreased by 17% in peak time periods, and London’s experience also indicated that pricing schemes was successful. As we convert all the savings in travel time resulting from decreased congestion to moary units, we can conclude that the reduction of congestion will promote sustainable economic development. Toll will affect travelers’ budget constraints and will result not only in mode switching but also in broader changes in the economy that will be acpanied by the geographic redistribution of trips. Some concerns that congestion pricing may have negative effects on the economy of the central area, particularly on retail. A counterargument, however, states that the reduced congestion is supposed to lower the costs of the downtown businesses, making them more petitive. The location of retail activity, on the other hand, is also driven by individuals’ preferred shopping locations. As well known, individuals tend to shop near their place of residence, so retail firms cannot easily move out of the core area, because so many people live in center area. This dependence on customer convenience explains why retail production decreases less than the output of other primary industries in the core area. Congestion pricing reduces congestion during the peak time and increases congestion during other periods. By shifting toward public transit and high occupancy vehicles, congestion pricing reduced the number of trips to the pricing areas across all time periods. Except reduced congestion, if improvement of air quality and decrease of fuel consumption are taken into account, the economic efficiency resulting from congestion pricing would be considerable. 3. Environmental protection In China, environmental problem bee increasingly serious. It was reported that china is the second emitter of CO2, and 7 cities are in the list of seriouslypolluted cities in the world. The environmental effects of transportation cover a wide range of different impacts, including for example air pollution, noise, and climate change. Motor vehicles are the dominant producers of urban air and noise pollution, including carbon monoxide, oxides of nitrogen, and airborne particulates. These pollutants are key factors in many respiratory ailments such as asthma as well as “a range of other human health effects, from headaches and eye irritation to cancer”. The World Bank estimates that million people in developing countries die each year from transportrelated air emissions, with a similar death toll from traffic accidents. Transportation cannot be replaced because it is the part of the production chain. For this reason, transportation systems must be developed and standardized, the effectiveness of transportation service must be increased, while the environmental pollution must be decreased or prevented. Emission from road traffic is a plex system with an output that cannot be pletely measured. It is natural to analyze the emissions from a sample of vehicles under different driving conditions. The California Air Resources Board pointes out that congestionstop and go traffic significantly increases emissions. As an example, one report estimates that a 10mile trip, using an average 1987 automobile, results in running exhaust HC emission of 1grams at a speed of 55 mph but that HC emissions would be 7 grams at an average speed of 20 mph, typical of stopandgo conditions Wit