IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group III: Mitigation of Climate Change Road transport: mode shifts

Personal motor vehicles consume much more energy and emit far more GHGs per passenger-km than other surface passenger modes. And the number of cars (and light trucks) continues to increase virtually everywhere in the world. Growth in GHG emissions can be reduced by restraining the growth in personal vehicle ownership. Such a strategy can, however, only be successful if high levels of mobility and accessibility can be provided by alternative means.

In general, collective modes of transport use less energy and generate less GHGs than private cars. Walking and biking emit even less. There is important worldwide mitigation potential if public and non-motorised transport trip share loss is reversed. The challenge is to improve public transport systems in order to preserve or augment the market share of low-emitting modes. If public transport gets more passengers, it is possible to increase the frequency of departures, which in turn may attract new passengers (Akerman and Hojer, 2006).

The USA is somewhat of an anomaly, though. In the USA, passenger travel by cars generates about the same GHG emissions as bus and air travel on a passenger-km basis (ORNL Transportation Energy Databook; ORNL, 2006). That is mostly because buses have low load factors in the USA. Thus, in the USA, a bus-based strategy or policy will not necessarily lower GHG emissions. Shifting passengers to bus is not simply a matter of filling empty seats. To attract more passengers, it is necessary to enhance transit service. That means more buses operating more frequently – which means more GHG emissions. It is even worse than that, because transit service is already offered where ridership[26] demand is greatest. Adding more service means targeting less dense corridors or adding more service on an existing route. There are good reasons to promote transit use in the USA, but energy use and GHGs are not among them.

Virtually everywhere else in the world, though, transit is used more intensively and therefore has a GHG advantage relative to cars. Table 5.4 shows the broad average GHG emissions from different vehicles and transport modes in a developing country context. GHG emissions per passenger-km are lowest for transit vehicles and two-wheelers. It also highlights the fact that combining alternative fuels with public transport modes can reduce emissions even further.

It is difficult to generalize, though, because of substantial differences across nations and regions. The types of buses, occupancy factors, and even topography and weather can affect emissions. For example, buses in India and China tend to be more fuel-efficient than those in the industrialized world, primarily because they have considerably smaller engines and lack air conditioning (Sperling and Salon, 2002).

Table 5.4: GHG Emissions from vehicles and transport modes in developing countries

 Load factor (average occupancy) CO2-eq emissions per passenger-km (full energy cycle) 
Car (gasoline) 2.5 130-170 
Car (diesel) 2.5 85-120 
Car (natural gas) 2.5 100-135 
Car (electric)a) 2.0 30-100 
Scooter (two-stroke) 1.5 60-90 
Scooter (four-stroke) 1.5 40-60 
Minibus (gasoline) 12.0 50-70 
Minibus (diesel) 12.0 40-60 
Bus (diesel) 40.0 20-30 
Bus (natural gas) 40.0 25-35 
Bus (hydrogen fuel cell)b) 40.0 15-25 
Rail Transitc) 75% full 20-50 

Note: All numbers in this table are estimates and approximations and are best treated as illustrative.

a) Ranges are due largely to varying mixes of carbon and non-carbon energy sources (ranging from about 20–80% coal), and also the assumption that the battery electric vehicle will tend to be somewhat smaller than conventional cars.

b) Hydrogen is assumed to be made from natural gas.

c) Assumes heavy urban rail technology (‘Metro’) powered by electricity generated from a mix of coal, natural gas and hydropower, with high passenger use (75% of seats filled on average).

Source: Sperling and Salon, 2002.


Public transport

In addition to reducing transport emissions, public transport is considered favourably from a socially sustainable point of view because it gives higher mobility to people who do not have access to car. It is also attractive from an economically sustainable perspective since public transport provides more capacity at less marginal cost. It is less expensive to provide additional capacity by expanding bus service than building new roads or bridges. The expansion of public transport in the form of large capacity buses, light rail transit and metro or suburban rail can be feasible mitigation options for the transport sector.

The development of new rail services can be an effective measure for diverting car users to carbon-efficient mode while providing existing public transport users with upgraded service. However, a major hurdle is higher capital and possibly operating cost of the project. Rail is attractive and effective at generating high ridership in very dense cities. During the 1990s, less capital-intensive public transport projects such as light rail transit (LRT) were planned and constructed in Europe, North America and Japan. The LRT systems were successful in some regions, including a number of French cities where land use and transport planning is often well integrated (Hylen and Pharoah, 2002), but less so in other cities especially in the USA (Richmond, 2001; Mackett and Edwards, 1998), where more attention has been paid to this recently.

Around the world, the concept of bus rapid transit (BRT) is gaining much attention as a substitute for LRT and as an enhancement of conventional bus service. BRT is not new. Plans and studies for various BRT type alternatives have been prepared since the 1930s and a major BRT system was installed in Curitiba, Brazil in the 1970s (Levinson et al., 2002). But only since about 2000 has the successful Brazilian experience gained serious attention from cities elsewhere.

BRT is ‘a mass transit system using exclusive right of way lanes that mimic the rapidity and performance of metro systems, but utilizes bus technology rather than rail vehicle technology’ (Wright, 2004). BRT systems can be seen as enhanced bus service and an intermediate mode between conventional bus service and heavy rail systems. BRT includes features such as exclusive right of way lanes, rapid boarding and alighting, free transfers between routes and preboard fare collection and fare verification, as well as enclosed stations that are safe and comfortable, clear route maps, signage and real-time information displays, modal integration at stations and terminals, clean vehicle technologies and excellence in marketing and customer service. To be most effective, BRT systems (like other transport initiatives) should be part of a comprehensive strategy that includes increasing vehicle and fuel taxes, strict land-use controls, limits and higher fees on parking, and integrating transit systems into a broader package of mobility for all types of travellers (IEA, 2002b).

Most BRT systems today are being delivered in the range of 1–15 million US$/km, depending upon the capacity requirements and complexity of the project. By contrast, elevated rail systems and underground metro systems can cost from 50 million US$ to over 200 million US$/km (Wright, 2004). BRT systems now operate in several cities throughout North America, Europe, Latin America, Australia, New Zealand and Asia. The largest and most successful systems to date are in Latin America in Bogotá, Curitiba and Mexico City (Karekezi et al., 2003).

Analysing the Bogotá Clean Development Mechanism project gives an insight into the cost and potential of implementing BRT in large cities. The CDM project shows the potential of moving about 20% of the city population per day on the BRT that mainly constitutes putting up dedicated bus lanes (130 km), articulated buses (1200) and 500 other large buses operating on feeder routes. The project is supported by an integrated fare system, centralized coordinated fleet control and improved bus management[27]. Using the investment costs, an assumed operation and maintenance of 20–50%[28] of investment costs per year, fuel costs of 40 to 60 US$ per barrel in 2030 and a discount rate of 4%, a BRT lifespan of 30 years, the cost of implementing BRT in the city of Bogotá was estimated to range from 7.6 US$/tCO2 to 15.84 US$/tCO2 depending on the price of fuel and operation and maintenance (Table 5.5). Comparing with results of Winkelman (2006), BRT cost estimates ranged from 14-66 US$/tCO2 depending on the BRT package involved (Table 5.6). The potential for CO2 reduction for the city of Bogotá was determined to average 247,000 tCO2 per annum or 7.4 million tCO2 over a 30 year lifespan of the project.

Table 5.5: Cost and potential estimated for BRT in Bogota

O & Ma) (%) Fuel price per barrel (US$) Cost (US$/tCO2
20 40 11.22 
20 60 7.60 
50 40 12.20 
50 60 15.84 

Note: Assuming 20% of the urban population uses the BRT each day.

a) Operation and maintenance (O & M) costs are expected to be high as the measure involves high demand for management and implementation beyond putting up the infrastructure.

Source: estimate based on Bogata CDM Project (footnote 27)


Table 5.6: CO2 reduction potential and cost per tCO2 reduced using public transit policies in typical Latin American cities

Transport measure GHG reduction potential (%) Cost per tCO2 (US$) 
BRT mode share increases from 0-5% 3.9 66 
BRT mode share increases from 0-10% 8.6 59 
Walking share increases from 20-25% 6.9 17 
Bike share increases from 0-5% 3.9 15 
Bike mode share increases from 1-10% 8.4 14 
Package (BRT, pedestrian upgrades, cycleways) 25.1 30 

Source: Wright and Fulton, 2005.


Non-motorized transport (NMT)

The prospect for the reduction in CO2 emissions by switching from cars to non-motorized transport (NMT) such as walking and cycling is dependent on local conditions. In the Netherlands, where 47% of trips are made by NMT, the NMT plays a substantial role up to distances of 7.5 km and walking up to 2.5 km (Rietveld, 2001). As more than 30% of trips made in cars in Europe cover distances of less than 3 km and 50% are less than 5 km (EC, 1999), NMT can possibly reduce car use in terms of trips and, to a lesser extent, in terms of kilometres. While the trend has been away from NMT, there is considerable potential to revive interest in NMT. In the Netherlands, with strong policies and cultural commitment, the modal share of bicycle and walking for accessing trains from home is about 35 to 40% and 25% respectively (Rietveld, 2001).

Walking and cycling are highly sensitive to the local built environment (ECMT, 2004a; Lee and Mouden, 2006). In Denmark, where the modal share of cycling is 18%, urban planners seek to enhance walking and cycling by shortening journey distances and providing better cycling infrastructure (Dill and Carr 2003, Page, 2005). In the UK where over 60% of people live within a 15 minute bicycle ride of a station, NMT could be increased by offering convenient, secure bicycle parking at stations and improved bicycle carriage on trains (ECMT, 2004a).

Safety is an important concern. NMT users have a much higher risk per trip of being involved in an accident than those using cars, especially in developing countries where most NMT users cannot afford to own a car (Mohan and Tiwari, 1999). Safety can be improved through traffic engineering and campaigns to educate drivers. An important co-benefit of NMT, gaining increasing attention in many countries, is public health (National Academies studies in the USA; Pucher, 2004).

In Bogotá, in 1998, 70% of the private car trips were under 3 km. This percentage is lower today thanks to the bike and pedestrian facilities. The design of streets was so hostile to bicycle travel that by 1998 bicycle trips accounted for less than 1% of total trips. After some 250 km of new bicycle facilities were constructed by 2001 ridership had increased to 4% of total trips. In most of Africa and in much of southern Asia, bicyclists and other non-motorised and animal traction vehicles are generally tolerated on the roadways by authorities. Non-motorised goods transport is often important for intermodal goods transport. A special form of rickshaw is used in Bangladesh, the bicycle van, which has basically the same design as a rickshaw (Hook, 2003).

Mitigation potential of modal shifts for passenger transport

Rapid motorization in the developing world is beginning to have a large effect on global GHG emissions. But motorization can evolve in quite different ways at very different rates. The amount of GHG emissions can be considerably reduced by offering strong public transport, integrating transit with efficient land use, enhancing walking and cycling, encouraging minicars and electric two-wheelers and providing incentives for efficient vehicles and low-GHG fuels. Few studies have analyzed the potential effect of multiple strategies in developing nations, partly because of a severe lack of reliable data and the very large differences in vehicle mix and travel patterns among varying areas.

Wright and Fulton (2005) estimated that a 5% increase in BRT mode share against a 1% mode share decrease of private automobiles, taxis and walking, plus a 2% share decrease of mini-buses can reduce CO2 emissions by 4% at an estimated cost of 66 US$/tCO2 in typical Latin American cities. A 5% or 4% increase in walking or cycling mode share in the same scenario analysis can also reduce CO2 emissions by 7% or 4% at an estimated cost of 17 or 15 US$/tCO2, respectively (Table5.6). Although the assumptions of a single infrastructure unit cost and its constant impact on modal share in the analysis might be too simple, even shifting relatively small percentages of mode share to public transport or NMT can be worthwhile, because of a 1% reduction in mode share of private automobiles represents over 1 MtCO2 through the 20-year project period.

Figure 5.13 shows the GHG transport emission results, normalized to year 2000 emissions, of four scenario analyses of developing nations and cities (Sperling and Salon, 2002). For three of the four cases, the ‘high’ scenarios are ‘business-as-usual’ scenarios assuming extrapolation of observable and emerging trends with an essentially passive government presence in transport policy. The exception is Shanghai, which is growing and changing so rapidly that ‘business-as-usual’ has little meaning. In this case the high scenario assumes both rapid motorization and rapid population increases, with the execution of planned investments in highway infrastructure while at the same time efforts to shift to public transport falter (Zhou and Sperling, 2001).

Figure 5.13

Figure 5.13: Projections for transport GHG emissions in 2020 for some cities of developing countries

Notes: Components of the Low 2020 scenario:

Delhi (Bose and Sperling, 2001): Completion of planned busways and rail transit, land-use planning for high density development around railway stations, network of dedicated bus lanes, promotion of bicycle use, including purchase subsidies and special lanes, promotion of car sharing, major push for more natural gas use in vehicles, economic re-straints on personal vehicles.

Shanghai (Zhou and Sperling, 2001): Emphasis on rapid rail system growth, high density development at railway sta-tions, bicycle promotion with new bike lanes and parking at transit stations, auto industry focus on minicars and farm cars rather than larger vehicles, incentives for use of high tech in minicars – electric, hybrid, fuel cell drive trains, promotion of car sharing.

Chile (O’Ryan et al., 2002): Overall focus on stronger use of market-based policy to insure that vehicle users pay the full costs of driving, internalizing costs of pollution and congestion, parking surcharges and restrictions, vehicle fees, and road usage fees, improvements in bus and rail systems, encouragement of minicars, with lenient usage and parking rules and strong commitment to alternative fuels, especially natural gas. By 2020, all taxis and 10% of other light and medium vehicles will use natural gas; all new buses will use hydrogen, improvements in bus and rail sys-tems.

South Africa (Prozzi et al., 2002): Land-use policies towards more efficient growth patterns, strong push to improve public transport, including use of busways in dense corridors, provision of new and better buses, strong government oversight of the minibus jitney industry, incentives to moderate private car use, coal-based synfuels shifts to imported natural gas as a feedstock

Source: Sperling and Salon, 2002.

  1. ^  The number of passengers using a specific form of public transport.
  2. ^  http://cdm.unfccc.int/Projects/DB/DNV-CUK1159192623.07/view.html.
  3. ^  O & M costs are expected to be high as the measure involves high demand for management and implementation beyond putting up the infrastructure.