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HIGHLIGHTSDrivers contributing to GHG emissions of road transport are identified and analyzed. Decomposition analysis based on Modified Laspeyres Index (MLI) is applied to the Spanish case. Economic crisis and changes in mobility patterns and GHG emissions are analyzed. Policies for the decarbonization of road transport are recommended. ABSTRACTRoad traffic is the greatest contributor to the carbon footprint of the transport sector and reducing it has become one of the main targets of sustainable transport policies. An analysis of the main factors influencing greenhouse gas (GHG) emissions is essential for designing new energy-and environmentally efficient strategies for the road transport. This paper addresses this need by (i) identifying factors which influence the carbon footprint, including traffic activity, fuel economy and socioeconomic development; and (ii) proposing a methodological framework which uses Modified Laspeyres Index decomposition to analyze the effect of important drivers on the changes in emissions of road transport in Spain during the period from 1990 to 2010. The results demonstrate that the country's economic growth has been closely linked to the rise in GHG emissions. The innovative contribution of this paper is the special analysis of the changes in mobility patterns and GHG emissions during the economic crisis, when, for the first time, Spanish road traffic emissions decreased. The reduction of road transport and improved energy efficiency has been powerful contributors to this decrease, demonstrating the effectiveness of energysaving measures. On the basis of this analysis, several tailored policy recommendations have been suggested for future implementation.

Global demand for mobility is increasing and the environmental impact of transport has become an important issue in transportation network planning and decisionmaking, as well as in the operational management phase. Suitable methods are required to assess emissions and fuel consumption reduction strategies that seek to improve energy efficiency and furthering decarbonization. This study describes the development and application of an improved modeling framework -the HERA (Highway EneRgy Assessment) methodology -that enables to assess the energy and carbon footprint of different highways and traffic flow scenarios and their comparison. HERA incorporates an average speed consumption model adjusted with a correction factor which takes into account the road gradient. It provides a more comprehensive method for estimating the footprint of particular highway segments under specific traffic conditions. It includes the application of the methodology to the Spanish highway network to validate it. Finally, a case study shows the benefits from using this methodology and how to integrate the objective of carbon footprint reductions into highway design, operation and scenario comparison.

motorized modes are favored over other forms of transport, road transportation is the main energy consumption mode and, consequently, the main transportation pollutant source at 80% of the total transport energy demand (4). Passenger emissions are rising more rapidly than freight transport emissions, caused by an overall increase in daily mobility. These numbers put into perspective the need to set specific emissions targets for passenger mobility and to develop policies aimed at cohesive and concrete emissions reductions in passenger transport (5).Local mobility is important, because 40% of all transport-related CO 2 is emitted in cities. The need to decarbonize urban mobility is a priority if countries are to achieve reduction targets for GHG emissions. Moreover, the car is the main mode: 75% of all kilometers traveled (passenger kilometers) in European urban areas are produced by car journeys (6). The public transport mode share is decreasing almost everywhere and now accounts for only 16% of journeys (6). To achieve reduction targets for GHG emissions, more emphasis must be placed on modal split policies that highlight public transport and nonmotorized transport as viable options. For instance, Lapillonne et al. report that public transport is four times more energy-efficient than cars (7). Moreover, where rail infrastructures and bus lanes are available, public transport can compete with cars because of its efficiency and the fact that travel times during peak hours tend to favor public transport users. Overall, public transport offers a better level of service, mainly because of its regularity and reduced travel times.A shift is required in both travel behavior and the perception of public transport as an unsafe, time-consuming, inconvenient option among populations accustomed to traveling by car (8). A European Union white paper sets challenging targets for a shift to more sustainable modes in urban transportation in European countries, encouraging cities to increase the modal share of nonmotorized modes (9). Modal shift policies are consistently among the best practices in urban areas for reducing the environmental effects of urban transport. Rail modes are seen as an ecological form of transportation (10). Buses offer flexibility, can be used quickly in response to changing demand, and do not need specialized infrastructure, unlike trains (11). Walking and cycling are carbonless, environmentally friendly solutions for individual urban transport (12). In Europe, cycling and walking account for approximately 13% of urban passenger kilometers (13).In Spain, motorized modes are favored over other forms of transport, and much investment is made in new road infrastructure in dense urban areas. Moreover, Spanish daily commute patterns indicate that the population is slowly reverting from public transport to carbon-intensive automobile transport (5). However, the difference in the use of public transport in large and small urban areas is significant. In dense cities, travelers are more likely to use public tran...

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