Energy efficiency in transportation

Walmart's truck fleet logs millions of miles each year, and the company planned to double the fleet's efficiency between 2005 and 2015.[1] This truck is one of 15 based at Walmart's Buckeye, Arizona distribution center that was converted to run on a biofuel made from reclaimed cooking grease produced during food preparation at Walmart stores.[2]
The Tesla Roadster, first delivered in 2008, uses lithium ion batteries to achieve 220 mi (350 km) per charge, while also capable of going 0-60 in under 4 seconds.

The energy efficiency of different types of transportation ranges from some hundred kilojoules (kJ) per kilometre for a bicycle to tens of megajoules (MJ) for a helicopter.

Units of measurement

Efficiency can be expressed in terms of:

Transportation types

For freight transport, rail and ship transport are generally much more efficient than trucking, and air freight is much less efficient.[4][5]

Walking

A 68 kg (150 lb) person walking at 4 km/h (2.5 mph) requires approximately 210 kilocalories (880 kJ) of food energy per hour, which is equivalent to 4.55 km/MJ.[6] 1 US gallon (3.8 L) of gasoline contains about 114,000 British thermal units (120 MJ)[7] of energy, so this is approximately equivalent to 360 miles per US gallon (0.65 L/100 km).

Velomobile

Velomobile
Main article: Velomobile

Velomobiles seem to have the highest energy efficiency in personal transportation. At a speed of 50 km/h (31 mph) the WAW manufacturer claims they need only 0.5 kW·h of food energy per 100 km to transport the passenger, which is around 1/5 (20%) of a normal bicycle, and 1/50 (2%) of an average fossil fuel or electric car. This corresponds to 4700 miles per US gallon (2000 km/L or 0.05 L/100 km).[8] Other sources give a figure of 1/3.4 (29.5%) of the energy efficiency of a normal bicycle.[9]

Bicycling

A relatively light and slow vehicle with low-friction tires and an efficient chain-driven drivetrain, the bicycle is one of the most energy-efficient forms of transport. Compared with walking, a 64 kg (140 lb) cyclist riding at 16 km/h (10 mph) requires about half the food energy per unit distance: 43 kcal/mi, 27 kcal/km or 3.1 kW·h (11 MJ) per 100 km.[6] This converts to about 732 mpg-US (0.321 L/100 km; 879 mpg-imp).[10] This figure depends on the speed and mass of the rider: greater speeds give higher air drag and heavier riders consume more energy per unit distance.

A motorized bicycle such as the Velosolex allows human power and the assistance of a 49 cm3 (3.0 cu in) engine, giving a range of 160 to 200 mpg-US (1.5–1.2 L/100 km; 190–240 mpg-imp). Electric pedal-assisted bikes run on as little as 1.0 kW·h (3.6 MJ) per 100 km, while maintaining speeds in excess of 30 km/h (19 mph). These best-case figures rely on a human doing 70% of the work, with around 3.6 MJ (1.0 kW·h) per 100 km coming from the motor.

Human power

To be thorough, a comparison must also consider the energy costs of producing, transporting and packaging of fuel (food or fossil fuel), the energy incurred in disposing of exhaust waste, and the energy costs of manufacturing the vehicle. This last can be significant given that walking requires little or no special equipment, while automobiles, for example, require a great deal of energy to produce and have relatively short lifespans. In addition, any comparison of electric vehicles and liquid-fuelled vehicles must include the fuel consumed in the power station to generate the electricity. In the UK for instance the efficiency of the electricity generation and distribution system is around 0.40.

Automobiles

Automobile fuel efficiency is most commonly expressed in terms of the volume of fuel consumed per one hundred kilometres (L/100 km), but in some countries (including the USA, UK and India) it is more commonly expressed in terms of the distance per volume fuel consumed (km/L or miles per US or imperial gallon). This is complicated by the different energy content of fuels such as petrol and diesel. The Oak Ridge National Laboratory (ORNL) states that the energy content of unleaded gasoline is 115,000 British thermal unit (BTU) per US gallon (32 MJ/L) compared to 130,500 BTU per US gallon (36.4 MJ/L) for diesel.[11]

A second important consideration is the energy costs of producing energy. Bio-fuels, electricity and hydrogen, for instance, have significant energy inputs in their production. Because of this, the 50-70% efficiency of hydrogen production has to be combined with the vehicle efficiency to yield net efficiency.[12]

A third consideration to take into account is the occupancy rate of the vehicle. As the number of passengers per vehicle increases the consumption per unit distance per vehicle increases. However this increase is slight compared to the reduction in consumption per unit distance per passenger. We can compare, for instance, the estimated average occupancy rate of about 1.3 passengers per car in the San Francisco Bay Area[13] to the 2006 UK estimated average of 1.58.[14]

Finally, vehicle energy efficiency calculations would be misleading without factoring the energy cost of producing the vehicle itself. This initial energy cost can of course be depreciated over the life of the vehicle to calculate an average energy efficiency over its effective life span. In other words, vehicles that take a lot of energy to produce and are used for relatively short periods will require a great deal more energy over their effective lifespan than those that do not, and are therefore much less energy efficient than they may otherwise seem. Compare, for example, walking, which requires no special equipment at all, and an automobile, produced in and shipped from another country, and made from parts manufactured around the world from raw materials and minerals mined and processed elsewhere again, and used for a limited number of years.

It has been researched that driving practices and vehicles can be modified to improve their energy efficiency by about 15%.[15][16]

Example consumption figures

Two American Solar Cars in Canada

Aircraft

Solar Impulse 2, a solar aircraft

A principal determinant of energy consumption in aircraft is drag, which must be opposed by thrust for the aircraft to progress.

Concorde fuel efficiency comparison (assuming jets are filled to capacity)
Aircraft  Concorde[29] Boeing 747-400[30]
passenger miles/imperial gallon 17 109
passenger miles/US gallon 14 91
litres/100 passenger km 16.6 3.1

Passenger airplanes averaged 4.8 l/100 km per passenger (1.4 MJ/passenger-km) (49 passenger-miles per gallon) in 1998. Note that on average 20% of seats are left unoccupied. Jet aircraft efficiencies are improving: Between 1960 and 2000 there was a 55% overall fuel efficiency gain (if one were to exclude the inefficient and limited fleet of the DH Comet 4 and to consider the Boeing 707 as the base case).[31] Most of the improvements in efficiency were gained in the first decade when jet craft first came into widespread commercial use. Compared to advanced piston engine airliners of the 1950s, current jet airliners are only marginally more efficient per passenger-mile.[32] Between 1971 and 1998 the fleet-average annual improvement per available seat-kilometre was estimated at 2.4%. Concorde the supersonic transport managed about 17 passenger-miles to the Imperial gallon; similar to a business jet, but much worse than a subsonic turbofan aircraft. Airbus states a fuel rate consumption of their A380 at less than 3 l/100 km per passenger (78 passenger-miles per US gallon).[33]

As over 80% of the fully laden take-off weight of a modern aircraft such as the Airbus A380 is craft and fuel, there remains considerable room for future improvements in fuel efficiency. The mass of an aircraft can be reduced by using light-weight materials such as titanium, carbon fiber and other composite plastics. Expensive materials may be used, if the reduction of mass justifies the price of materials through improved fuel efficiency. The improvements achieved in fuel efficiency by mass reduction, reduces the amount of fuel that needs to be carried. This further reduces the mass of the aircraft and therefore enables further gains in fuel efficiency. For example, the Airbus A380 design includes multiple light-weight materials.

Airbus has showcased wingtip devices (sharklets or winglets) that can achieve 3.5 percent reduction in fuel consumption.[34][35] There are wingtip devices on the Airbus A380. Further developed Minix winglets have been said to offer 6 percent reduction in fuel consumption.[36] Winglets at the tip of an aircraft wing, can be retrofitted to any airplane, and smooths out the wing-tip vortex, reducing the aircraft's wing drag.[36]

NASA and Boeing are conducting tests on a 500 lb (230 kg) "blended wing" aircraft. This design allows for greater fuel efficiency since the whole craft produces lift, not just the wings.[37] The blended wing body (BWB) concept offers advantages in structural, aerodynamic and operating efficiencies over today's more conventional fuselage-and-wing designs. These features translate into greater range, fuel economy, reliability and life cycle savings, as well as lower manufacturing costs.[38][39] NASA has created a cruise efficient STOL (CESTOL) concept.

Fraunhofer Institute for Manufacturing Engineering and Applied Materials Research (IFAM) have researched a shark skin imitating paint that would reduce drag through a riblet effect.[40] Aircraft are a major potential application for new technologies such as aluminium metal foam and nanotechnology such as the shark skin imitating paint.

Propfan propulsors are a more fuel efficient technology than jets or turboprops, but turboprops have an optimum speed below about 450 mph (700 km/h).[41] This speed is less than used with jets by major airlines today. However, the decrease in speed reduces drag. With the current high price for jet fuel and the emphasis on engine/airframe efficiency to reduce emissions, there is renewed interest in the propfan concept for jetliners that might come into service beyond the Boeing 787 and Airbus A350XWB. For instance, Airbus has patented aircraft designs with twin rear-mounted counter-rotating propfans.[42] NASA has conducted an Advanced Turboprop Project (ATP), where they researched a variable pitch propfan that produced less noise and achieved high speeds.

Related to fuel efficiency is the impact of aviation emissions on climate.

Small aircraft

Ships

Queen Elizabeth

Cunard states that the RMS Queen Elizabeth 2 travels 49.5 feet per imperial gallon of diesel oil (3.32 m/l or 41.2 ft/US gal), and that it has a passenger capacity of 1777.[46] Thus carrying 1777 passengers we can calculate an efficiency of 16.7 passenger miles per imperial gallon (16.9 l/100 p·km or 13.9 p·mpg–US).

Emma Maersk

Emma Maersk uses a Wärtsilä-Sulzer RTA96-C, which consumes 163 g/kW·h and 13,000 kg/h. If it carries 13,000 containers then 1 kg fuel transports one container for one hour over a distance of 45 km. The ship takes 18 days from Tanjung (Singapore) to Rotterdam (Netherlands), 11 from Tanjung to Suez, and 7 from Suez to Rotterdam,[47] which is roughly 430 hours, and has 80 MW, +30 MW. 18 days at a mean speed of 25 knots (46 km/h) gives a total distance of 10,800 nautical miles (20,000 km).

Sailboats

A sailboat, much like a solar car, can locomote without consuming any fuel. A sail boat such as a Dinghy using just wind power requires no input energy in terms of fuel. However some manual energy is required by the crew to steer the boat and adjust the sails using ropes. In addition energy will be needed for demands other than propulsion, such as cooking, heating or lighting. The fuel efficiency of a single-occupancy boat is highly dependent on the size of its engine, the speed at which it travels, and its displacement. Due to the high viscosity of water, with a single passenger, the equivalent energy efficiency will be lower than in a car, train, or plane.

Trains

Trains can be an efficient means of transport for freight and passengers. Efficiency varies significantly with passenger loads, and losses incurred in electricity generation and supply (for electrified systems),[48][49] and, importantly, end-to-end delivery, where stations are not the originating final destinations of a journey.

Actual consumption depends on gradients, maximum speeds, loading and stopping patterns. Data produced for the European MEET project (Methodologies for Estimating Air Pollutant Emissions) illustrate the different consumption patterns over several track sections. The results show the consumption for a German ICE high-speed train varied from around 19 to 33 kW·h/km (68–119 MJ/km; 31–53 kW·h/mi). The data also reflects the weight of the train per passenger. For example, TGV double-deck Duplex trains use lightweight materials, which keep axle loads down and reduce damage to track and also save energy.[50]

Freight

Energy consumption estimates for rail freight vary widely, and many are provided by interested parties. Some are tabulated below.

Source CountryYear Fuel Economy (weight of goods)Energy Intensityref
Association of American RailroadsUSA 2007185.363 km/L (1 short ton)energy/mass-distance[51]
Network Rail UK —87 t·km/L 0.41 MJ/t·km (LHV)[52]

Passenger

Source countryyearTrain EfficiencyPer passenger-km (in kJ)reference(s)
East Japan Railway Company Japan200420.6 MJ (5.7 kWh)/car-km350 kJ/passenger-km[53]
EC EC 199718 kW·h/km (65 MJ/km)TGV Duplex assuming 3 intermediate stops between Paris and Lyon.[54]
Colorado Rail USA year1.125 mpg-US (209.1 L/100 km; 1.351 mpg-imp)468 passenger-miles/US gallon (0.503 L/100 passenger-km)Colorado Railcar double-deck DMU hauling two Bombardier Bi-level coaches[55]
SBB-CFF-FFS Switzerland20112300 GWhr/yr470 kJ/passenger-km[56]
SiemensBasel, Switzerlandyear1.53 kWh/vehicle-km (5.51 MJ/vehicle-km)85 kJ/passenger-km (150 kJ/passenger-km at 80% average load)[57][58]
Amtrak USA2009 2,435 BTU/mi (1.60 MJ/km)[59]
Comboios de Portugal Portugal 2011 8.5 kW·h/km (31 MJ/km; 13.7 kW·h/mi) 77 kJ/passenger-km [60]

Considering only the energy spent to move the train, and taking as example the urban area of Lisbon, train seems to be on average 20 times more efficient than automobile for transportation of passengers, if we consider energy spent per passenger-km.[60] Considering an automobile which has a consumptions of around 6 l/100 km (47 mpg-imp; 39 mpg-US) of gasoline, the fact the on average cars in Europe have an occupation ratio of around 1.2 passengers per automobile and that one litre of gasoline amounts for about 8826 Wh, one gets on average 441 Wh (1,590 kJ) per passenger-km. On the other hand, a modern urban train with an average occupation of 20% of total capacity, which has a consumption of about 8.5 kW·h/km (31 MJ/km; 13.7 kW·h/mi), one gets 21.5 Wh per passenger-km, 20 times less than the automobile.

Braking losses

Stopping is a considerable source of inefficiency. Modern electric trains like the Shinkansen (the Bullet Train) use regenerative braking to return current into the catenary while they brake. A Siemens study indicated that regenerative braking might recover 41.6% of the total energy consumed. The Passenger Rail (Urban and Intercity) and Scheduled Intercity and All Charter Bus Industries Technological and Operational Improvements – FINAL REPORT states that "Commuter operations can dissipate more than half of their total traction energy in braking for stops." and that "We estimate head-end power to be 35 percent (but it could possibly be as high as 45 percent) of total energy consumed by commuter railways."[61] Having to accelerate and decelerate a heavy train load of people at every stop is inefficient despite regenerative braking which can recover typically around 20% of the energy wasted in braking. Weight is a determinant of braking losses.

Other references

AEA study of road and rail for the United Kingdom Department for Transport: Final report

Buses

The Bus Rapid Transit of Metz uses a diesel-electric hybrid driving system, developed by Belgian Van Hool manufacturer.[62]

Other

International transport comparisons

UK Public transport

Rail and bus are generally required to serve 'off peak' and rural services, which by their nature have lower loads than city bus routes and inter city train lines. Moreover, due to their 'walk on' ticketing it is much harder to match daily demand and passenger numbers. As a consequence, the overall load factor on UK railways is 35% or 90 people per train:[67]

Conversely, Air services work on point-to-point networks between large population centres and are 'pre-book' in nature. Using yield management overall loads can be raised to around 70-90%. However, recently intercity train operators have been using similar techniques, with loads reaching typically 71% overall for TGV services in France and a similar figure for the UK's Virgin trains services.[68]

For emissions, the electricity generating source needs to be taken into account.[69][70] [71]

US Passenger transportation

The US Transportation Energy Data Book states the following figures for passenger transportation in 2009:[59]

Transport mode Average passengers
per vehicle		 BTU per passenger-mile MJ per passenger-kilometre
Rail (Intercity Amtrak) 20.9 2,435 1.596
Motorcycles 1.16 2,460 1.61
Rail (Transit Light & Heavy) 24.5 2,516 1.649
Rail (Commuter) 32.7 2,812 1.843
Air 99.3 2,826 1.853
Cars 1.55 3,538 2.319
Personal Trucks 1.84 3,663 2.401
Buses (Transit) 9.2 4,242 2.781
Taxi 1.55 15,645 10.257

US Freight transportation

The US Transportation Energy book states the following figures for freight transportation in 2010:[59][72][73][74]

Transportation mode Fuel consumption
BTU per short ton mile kJ per tonne kilometre
Domestic Waterborne 217 160
Class 1 Railroads 289 209
Heavy Trucks 3,357 2,426
Air freight (approx) 9,600 6,900

From 1960 to 2010 the efficiency of air freight has increased 75%, mostly due to more efficient jet engines.[75]

1 US gal (3.785 l, 0.833 imp gal) of fuel can move a ton of cargo 857 km or 462 NM by barge, or 337 km (209 mi) by rail, or 98 km (61 mi) by truck.[76]

Compare:

Canadian transportation

Natural Resources Canada's Office of Energy Efficiency publishes annual statistics regarding the efficiency of the entire Canadian fleet. For researchers, these fuel consumption estimates are more realistic than the fuel consumption ratings of new vehicles, as they represent the real world driving conditions, including extreme weather and traffic. The annual report is called Energy Efficiency Trends Analysis. There are dozens of tables illustrating trends in energy consumption expressed in energy per passenger km (passengers) or energy per tonne km (freight).[77]

Caveats

Comparing fuel efficiency in transportation has several challenges:

Footnotes

  1. ↑ Nishimoto, Alex (March 10, 2014). "Walmart Debuts Turbine-Powered WAVE Semi Truck Prototype". Motor Trend.
  2. ↑ "Wal-Mart To Test Hybrid Trucks". Sustainable Business. February 3, 2009.
  3. ↑ Aeroplane Efficiency, Fédération Aéronautique Internationale, "FAI – The World Air Sports Federation"
  4. ↑ technology.newscientist.com, graph
  5. ↑ David Strahan, "Green fuel for the airline industry", New Scientist, 13 August 2008, pp. 34-7.
  6. 1 2 Energy expenditure for walking and running
  7. ↑ EPA (2007). "Appendix B, Transportation Energy Data Book". Retrieved 16 November 2010.
  8. ↑ sites.google.com
  9. ↑ The velomobile: high-tech bike or low-tech car?
  10. ↑ "Calculation of conversion from dietary calories per mile to miles per gallon gasoline, using the energy density of gasoline listed by Wolfram Alpha". 2011. Retrieved 19 Jul 2011.
  11. ↑ Oak Ridge National Laboratory (ORNL)
  12. ↑ Hydrogen Internal Combustion Engine (ICE) Vehicle Testing Activities
  13. ↑ Maps and Data – Metropolitan Transportation Commission for the nine-county San Francisco Bay Area, California
  14. ↑ "Transport trends: current edition". UK Department for Transport. 8 January 2008. Retrieved 23 March 2008.
  15. ↑ Beusen; et al. (2009). "Using on-board logging devices to study the long-term impact of an eco-driving course". Transportation Research D 14: 514–520.
  16. ↑ "Do lower speed limits on motorways reduce fuel consumption and pollutant emissions?". Retrieved 2013-08-12.
  17. ↑ MIT Unveils 90 MPH Solar Race Car
  18. ↑ NEV America US Department of Energy Field Operations Program – 2005 Global Electronic Motorcars e4 4-Passenger
  19. ↑ eere.energy.gov
  20. ↑ "Best on CO2 rankings". UK Department for Transport. Retrieved 2008-03-22.
  21. ↑ "Vehicle details for Polo 3 / 5 Door (from Nov 06 Wk 45>) 1.4 TDI (80PS) (without A/C) with DPF BLUEMOTION M5". UK Vehicle Certification Agency. Retrieved 22 March 2008.
  22. ↑ "Vehicle details for Ibiza ( from NOV 06 Wk 45 > ) 1.4 TDI 80PS Ecomotion M5". UK Vehicle Certification Agency. Retrieved 22 March 2008.
  23. ↑ Jerry Garrett (2006-08-27). "The Once and Future Mileage King". The New York Times.
  24. ↑ "2008 Toyota Prius". EPA. Retrieved 2007-12-25.
  25. ↑ "2008 Most and Least Fuel Efficient cars (ranked by city mpg)". United States Environmental Protection Agency and United States Department of Energy. Retrieved 2007-12-25.
  26. ↑ "Vehicle details for Prius 1.5 VVT-i Hybrid E-CVT". UK Vehicle Certification Agency. Retrieved 2008-03-22.
  27. 1 2 2012 Most and Least Efficient Vehicles
  28. 1 2 3 Barney L. Capehart (2007). Encyclopedia of Energy Engineering and Technology, Volume 1. CRC Press. ISBN 0-8493-3653-8, ISBN 978-0-8493-3653-9.
  29. ↑ "Powerplant." concordesst.com. Retrieved: 2 December 2009.
  30. ↑ "Technical Specifications: Boeing 747-400". Boeing. Retrieved 11 January 2010.
  31. ↑ National Aerospace Laboratory
  32. ↑ Peeters P.M., Middel J., Hoolhorst A. (2005). Fuel efficiency of commercial aircraft An overview of historical and future trends. National Aerospace Laboratory, The Netherlands.
  33. ↑ "The A380: The future of flying". Airbus. Archived from the original on 2007-12-14. Retrieved 2008-03-22.
  34. ↑ nzherald.co.nz
  35. ↑ ylcrafts.com
  36. 1 2 gizmag.com
  37. ↑ Ecogeek Article
  38. ↑ "Boeing to Begin Ground Testing of X-48B Blended Wing Body Concept." Boeing, October 27, 2006. Retrieved: April 10, 2012.
  39. ↑ Lorenz III, Phillip. "AEDC testing brings unique blended wing aircraft closer to flight." AEDC, U.S. Air Force, July 3, 2007. Retrieved: April 10, 2012.
  40. ↑ smartplanet.com
  41. ↑ Spakovszky, Zoltan (2009). "Unified Propulsion Lecture 1". Unified Engineering Lecture Notes. MIT. Retrieved 2009-04-03.
  42. ↑ US application 2009020643, Airbus & Christophe Cros, "Aircraft having reduced environmental impact", published 2009-01-22
  43. ↑ Contact, Experimental Aircraft and Powerplant Newsforum for Designers and Builders, Issue 55, March–April 2000
  44. ↑ "Tecnam P92 Echo Classic". Tecnam costruzioni aeronautiche s.r.l. Retrieved 22 May 2012.
  45. ↑ "Tecnam P2002 Sierra De Luxe". Tecnam costruzioni aeronautiche s.r.l. Retrieved 22 May 2012.
  46. ↑ "Queen Elizabeth 2: Technical Information" (PDF). Cunard Line. Retrieved 2008-03-31.
  47. ↑ Emma Mærsk schedules Mærsk, 5 December 2011.
  48. ↑ Fuel Efficiency of Travel in the 20th Century, Appendix
  49. ↑ Fuel Efficiency of Travel in the 20th Century)
  50. ↑ Commission for integrated transport, Short haul air v High speed rail
  51. ↑ progressiverailroading.com
  52. ↑ freightonrail.co.uk
  53. ↑ Environmental Goals and Results, JR-East Sustainability Report 2005
  54. ↑ Estimating Emissions from Railway Traffic, page 74
  55. ↑ Colorado Railcar: "DMU Performs Flawlessly on Tri-Rail Service Test"
  56. ↑ SBB Facts and Figures Traffic
  57. ↑ European Environment Agency Occupancy Rates, page 3
  58. ↑ Combino – Low Floor Light Rail Vehicles Tests, Trials and Tangible Results
  59. 1 2 3 Davis, Stacy C.; Susan W. Diegel; Robert G. Boundy (2011). Transportation Energy Data Book: Edition 30. US Department of Energy. pp. Table 2.12. ORNL-6986 (Edition 30 of ORNL-5198). Retrieved 2012-02-22.
  60. 1 2 veraveritas.eu
  61. ↑ Bus and Rail Final Report
  62. ↑ "Van Hool presents the ExquiCity Design Mettis.". Retrieved 5 June 2012.
  63. ↑ "Passenger Transport (Fuel Consumption)". Hansard. UK House of Commons. 2005-07-20. Retrieved 2008-03-25.
  64. ↑ Demonstration of Caterpillar C-10 Duel-Fuel Engines in MCI 102DL3 Commuter Buses
  65. ↑ carris.pt
  66. ↑ NASA Crawler-Transporter statistics
  67. ↑ ATOC
  68. ↑ Delivering a sustainable railway White paper, p43
  69. ↑ Energy & Emissions Statement
  70. ↑ Defra 2008 Guidelines to Defra’s GHG Conversion Factors
  71. ↑ Kilograms of CO2 per passenger kilometre for different modes of transport within the UK
  72. ↑ US Environmental protection, 2006
  73. ↑ Energy Efficiency – Transportation sector (from the United States Department of Energy's Energy Information Administration)
  74. ↑ Energy Table 2.15
  75. ↑ Trend in Aircraft Fuel Efficiency
  76. ↑ Transportation and Energy
  77. ↑ 2010 data
  78. ↑ Newman, Peter; Jeffrey R. Kenworthy (1999). Sustainability and Cities: Overcoming Automobile Dependence. Island Press. ISBN 1-55963-660-2.

See also

External links

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