Transport solutions for cleaner air
Recently, professor Zhu Tong from College of Environmental Sciences and Engineering, Peking University and professor Frank Kelly from King's College London published an article Transport Solutions for Cleaner Air in Science. Frank KellyBy facilitating the movement of people and goods and providing employment, road transport supports economic growth and plays a considerable role in global urbanization. The consequent dependence on fossil fuels (increasingly diesel in Europe) has, however, created specific air pollution challenges. In particular, road transport remains an important source of particulate matter (PM) and nitrogen dioxide (NO2), which can cause or aggravate asthma and impair lung development in children (1). Here, we ask whether the policy interventions already introduced, or under consideration, in two of the world’s largest and most economically powerful cities—London, England, and Beijing, China—are good exemplars for other cities to follow (Figs. 1 and 2). We consider some of the changes required so that urban areas do not grind to a standstill because of continued growth and transport demand.
PHOTO: MILES ERTMAN/ROBERT HARDING
PHOTO: © HUFTON AND CROW/VIEW/CORBIS
A tale of two cities—London
In view of widespread public concern about the health effects of air pollution, the mayor of London launched an air quality strategy in 2002, entitled Cleaning London’s Air (2). The main aim of the strategy was the reduction of pollution from road traffic, which was the largest source of the pollutants of concern—PM2.5 and NO2—in the city. PM2.5 refers to particles that pass through a size-selective inlet with a 50% efficiency cutoff at 2.5 μm aerodynamic diameter, which means that they are small enough to penetrate deep into the lungs. To this end, the goals were to reduce the number of vehicles on the road and to lower emissions from individual vehicles through modernization of the fleet vehicle stock.
To help achieve the first goal, a congestion-charging scheme (CCS) was introduced in central London in February 2003 (3). The CCS initially reduced the number of private vehicles entering central London by 18%, but the scheme’s effectiveness has since been eroded; since 2012, congestion has broadly been back at pre-2002 levels.
The London-wide LEZ, introduced in February 2008, aimed to tackle the second goal by restricting the entry of the oldest and most polluting vehicles (such as diesel engine heavy goods vehicles, buses and coaches, and larger vans and minibuses) across greater London (4). However, although black carbon pollution has gradually decreased on many roads in London since 2008, NO2 concentrations have either not changed or have gone up (5). As a consequence, London does not comply with European Union limit values for NO2 and is unlikely to do so until 2025 at the earliest. The main reason for this is the increased share of new diesel cars on London’s roads, up from 1 in 10 cars in 2000 to 6 in 10 in 2014. Real-world NO2 emissions from most of these vehicles do not comply with European exhaust emission standards, which were introduced in 1993 to improve air quality (6).
A tale of two cities—Beijing
In contrast to the UK’s long industrial heritage, China has undergone rapid industrialization over the past few decades, adding thousands of kilometers of urban road and hundreds of millions of vehicles. PM2.5 emissions from traffic have contributed to increasingly poor air quality in Beijing (7), threatening public health (8). The severity of air pollution in Beijing was first acknowledged when China was bidding for the Olympic Games in the 1990s. To address the problem, in 1998 the Beijing municipal government introduced the first of 16 stages of air pollution control measures. Initial measures mainly focused on shutting down small industrial and domestic coal burning stoves and controlling dust from road and construction sites, as well as phasing out old vehicles that did not meet emission standards. These standards were, however, much less strict than those in place today.
Given fast urbanization and social and economic development in Beijing, later measures were more comprehensive because they needed to deal with the complicated nature of the air pollution sources in Beijing. Long-term and strategic measures included replacing coal with natural gas, creating a public transport system with 554 km of subway, and introducing current European emission standards. During the Beijing Olympics in 2008 (9) and the Asia-Pacific Economic Cooperation meeting in Beijing in 2014, aggressive measures were taken to reduce the number of vehicles on the road by allowing cars to be used on only certain days of the week, determined by the last digit of the car license number. This measure resulted in substantial temporary reductions in congestion and pollution concentrations in Beijing and was considered so successful that it was made a routine rule immediately after the Olympics. Today, every car is allowed to drive 4 days per 5 weekdays.
These efforts to control air pollution in Beijing have shown some success. PM2.5 concentrations have started to fall in recent years. However, these benefits have been compromised to some extent by the rapid growth in vehicle numbers from 1.5 million in 2000 to 5.6 million in 2015. To start dealing with this problem, the Beijing government introduced a range of vehicle emission policies, including a lottery system for new car license plates and further modernization of the fleet.
Given that the vehicle fleet in China, especially in Beijing, now has the most advanced engine technologies (10), further control measures to cut vehicle emissions will necessitate further restrictions on car movements. Importantly, the Chinese government has not provided incentives for increasing the proportion of private diesel vehicles (11). As a result—in line with other countries, such as Japan—the problems of diesel NO2 emissions from private vehicles contributing to Beijing’s poor air quality have been avoided.
Looking to the future
The experiences in London and Beijing clearly show that more fundamental changes in urban transportation systems are required for air quality improvements to be achieved and for city streets to become more pleasant environments. In London, the introduction of an ultralow emission zone (uLEZ) is planned for 2020 (12). However, banning all but the cleanest vehicles and incentivizing the use of zero-emission cars and taxis in a small area of central London will be insufficient to achieve compliance with NO2 standards until at least 2025 (13). Beijing will grind to a standstill through excessive congestion, and air quality will suffer further if car ownership levels are allowed to move toward U.S. levels (840 cars per 1000 people). Car ownership (currently 280 per 1000 people) must therefore continue to be strictly controlled.
In many European cities, attitudes toward car ownership appear to be changing. Many urban youth of today, it seems, would rather use taxis and car share than own their own car. Recent polls in London associated with the mayoral elections in May 2016 put air quality issues high on the political agenda and suggest that there is popular support for a further tightening of vehicle use in the city. Cars promote a sedentary lifestyle, and a shift to cycling and walking has health benefits for urban dwellers (14).
Upon recognizing the negative impacts that congestion, when entering cities, has on the economy and on air quality, several states in the northeastern United States have introduced electronic toll collection technology, which has helped to reduce delays at toll plazas. Improved traffic flows near these toll plazas were linked to more healthy infants born to mothers living nearby (15). Other interventions, such as the Hoy No Circula (HNC) program in Mexico City, have proved less successful. Introduced in 1989 to address the severe air pollution problems in Mexico City, the HNC banned most drivers from using their vehicle 1 weekday per week, determined by the last license plate digit (16). Although the program was largely successful in removing ~20% of vehicles from the roads, no evidence could be found that it led to increased use of public transportation or that it improved air quality (16).
“As cities continue to grow in size and population, even zero emission vehicles are not the solution. Moving increasing numbers of people efficiently around a city can only be achieved by expanded mass transit systems.”
The car industry has recognized the need for change, and companies such as BMW and Ford have launched car-share schemes. Car-sharing has obvious benefits to cities. Zipcar estimates that every shared vehicle replaces up to 20 private cars, thus reducing total vehicle miles and land devoted to parking. The car industry is also pushing forward the development of city-friendly plug-in electric vehicles. Worries over battery life and the lack of coordinated charging networks have, however, held back vehicle sales, even with generous government subsidies. In the UK, the electric vehicle fleet has grown in the past 2 years from 3500 to more than 50,000 vehicles, suggesting that the tide is turning in favor of zero-emission vehicles. In contrast, goals for fuel cell (hydrogen)–powered cars have not been achieved because of limited fueling networks and high initial vehicle costs.
As cities continue to grow in size and population, even zero-emission vehicles are not the solution. Moving increasing numbers of people efficiently around a city can only be achieved by expanded mass transit systems. The increased use of bus networks and traditional subways are providing city dwellers with more flexible, cheaper, and less polluting options. For instance, China is building 87 new mass transit rail lines over the next 5 years. If construction continues at this pace, China’s cities will have half of the world’s metro tracks by 2050 (17).
Rapid transit systems offer only a partial solution to city congestion and poor air quality, however, because many users may not live within easy walking distance to the transit points. To be effective, rapid transit systems must be linked with other transport options at the start and end of the journey. The creation of Autonomous Mobility-on-Demand networks may solve this problem (17). Modeled after bicycle-share programs, users would have access to a network of lightweight electronic vehicles (LEVs) distributed at charging stations throughout the city. Customers could summon a vehicle through a smartphone app and abandon it upon reaching their destination, from where the vehicle would find its way back to a charging station or new user. Trials of this technology are planned in California and in Milton Keynes, UK (18, 19).
In the city of the future, mass public transport systems may lead to many more car-free roads, transforming the landscape and enhancing the urban experience. Moving toward such an environment depends, however, on continued and exacting scientific research, the translation of such research into realistic and effective policies, and successfully encouraging members of the public to use public transport, value exercise, and not drive for short journeys (20). The reward will be improved health and quality of life for the growing urban populations around the world.
Edited by: Zhang Jiang