交通基础设施与城市空气污染——来自中国的经验证据

The Effects of Transportation Infrastructure on Air Quality: Evidence from Empirical Analysis in China

为了探寻现阶段加强交通基础设施能否提升城市空气质量,本文构建理论模型分析了居民效用、交通基础设施与空气污染的相互作用机理,并运用2000—2012年83个城市的面板数据进行实证研究。为控制内生性问题,本文采取反映样本城市地形特征的城市坡度指数作为城市交通基础设施的工具变量,估计了交通基础设施对城市空气质量的影响。研究发现:增加交通基础设施投资能够改善城市空气质量;通过工具变量缓解内生性偏误后,改善作用更加明显,且大于城市机动车保有量增加对空气污染的边际影响;与道路投资相比,道路面积的增长对空气质量的改善效应更好。本文的政策启示是:现阶段城市建设应该注重基础设施建设与生态环境保护并举,通过交通基础设施的供给侧角度,寻找城市空气污染难题的解决思路,实现城市生态文明建设的平衡与充分发展。

The process of urbanization in China continues to accelerate, and the explosion in the urban population has led to “urban illnesses” such as traffic congestion and air pollution. Accelerated urbanization leads to an increased demand for transportation due to logistics and commuting, while the construction of municipal transportation infrastructure cannot keep up with the increased number of motor vehicles. This growing disparity results in increased road traffic, leading to lengthy travel times and traffic jams. As most vehicles are petrol-driven at present, idling cars continue to burn gasoline;thus, the emissions of harmful substances will be 2 to 3 times greater due to idling. Traffic congestion thus aggravates air pollution. Therefore, taking measures to keep the roads clear may be an effective way to improve urban air quality. Traffic congestion can be managed by compulsory policies from the demand side, such as traffic restrictions, odd/even license plate rules, or purchase restriction, which reduce trip frequency or slow down the increase in number of vehicles. However, several studies have indicated that this approach may not be the best choice at this stage. In this case, it is necessary to improve municipal transportation infrastructure from the supply side, to make roads less congested, improve the support capacity of the road network system, and reduce air pollution caused by traffic congestion. In this context, this paper studies the relationship and reaction mechanism between residents utility, traffic infrastructure, and air pollution. We introduce air pollution and traffic smoothness into residents utility function and derive a theoretical model to explain how traffic infrastructure affects urban air quality on the basis of utility maximization. We can thus obtain the empirical equations. Investment in urban roads is selected as a proxy variable for transportation infrastructure. Thus, the impact mechanism of transportation infrastructure on air pollution can be found through empirical research, and thus we can determine whether the enhancement of transportation infrastructure improves urban air quality. The key difficulty for this research lies in the complex relationship between urban traffic infrastructure and air pollution, as the endogeneity problems caused by reverse causality and possible missing variables cannot be easily resolved. To solve this issue, an innovative city slope index constructed based on topographic features is proposed as an instrument variable. The data for this index come from a cloud platform for monitoring geographical conditions. The empirical results show that increased investment in transportation infrastructure contributes to improved urban air quality, and the effect becomes more pronounced after endogeneity bias is removed with using the instrument variable. If the growth rate of urban road investment remains consistent with that of the number of urban vehicles, it will have a marginal effect on the control of air pollution. Using road area instead of investment in roads as an indicator to measure the level of transportation infrastructure can significantly improve the positive effects of on urban air quality. Our research also suggests that as the process of urbanization continues, air pollution problems are more likely to occur in large and medium-sized cities. This paper provides empirical evidence and a new justification for policies supporting urban transportation infrastructure construction and environmental governance. The economy is changing from high-speed growth to high-quality development in today s China. For this reason, it is important from the supply side perspective to increase investment in urban transportation infrastructure to solve the serious problem of air pollution.