None of the strategy categories alone are likely to have greater than a moderate impact on GHG emissions. However, each of the strategies can have significant reinforcing effects with other strategies. For example, vehicle electrification complements renewable power (by reducing emissions per kilometer), travel demand management and land use changes (by reducing the amount of travel and thereby avoiding range constraints with battery electric vehicle or gasoline driving with plug-in hybrid electric vehicle). Similarly, travel demand management can be more effective with land use changes to achieve a more compact and walkable metropolitan area. In our judgment, the GHG reduction potential taking advantage of these different complementary effects is high. GHG emission reduction goals could be attained by aggressively pursuing both existing policies and the four strategies discussed here: fuel/ vehicle strategies, travel demand management, land use change, and renewable power.

—Mashayekh et al.

Summary findings for each of the major strategies examined include:

• Fuel/vehicle strategies. They team re-emphasized that while fuel and vehicle changes—such as vehicle electrification—do offer the possibility of reduced life cycle GHG emissions, those reductions alone are unlikely to achieve the national 50−80% reduction goal. In addition, they noted, the largest reductions would be expensive and dependent upon development of new technology and supporting infra- structure.

  to achieve the required GHG reductions in transportation, dramatically more efficient vehicles will need to travel fewer kilometers using an energy source with substantially reduced GHG emissions.

  —Mashayekh et al.

• Low carbon and renewable power strategies. Assuming low-carbon power can always be used to charge vehicles is an unlikely scenario for the next few decades, the team concludes. To be able to balance instantaneously demand and supply, a fleet of power plants using different fuels is operating at any given time. Wind and solar resources are intermittent and require changes in the operations of the grid at appreciable generation fractions, they note.

  Though there is consensus that PHEVs can reduce the GHG emissions associated with transportation, the actual reductions are difficult to estimate and greatly depend on assumptions about time of charging, grid operations, driving patterns, and electricity generation mix. The role that renewable resources will play in these emission reductions is not clearly understood. Certainly, exclusively charging PHEVs with renewable resources would provide the greatest emission reductions. Given the requirements in current grid operations, considerable changes in grid operations are required to realize this scenario.

  —Mashayekh et al.

• Travel demand management benefits. Travel demand management (TDM) measures include pricing (e.g., mileage fees, tools, congestion pricing); broder use of transit, non-motorized and intermodal travel; land use and parking management; commute travel reduction (e.g., telework, ridesharing, carpooling); and public information campaigns.

  While some of the TDM measures have a significant impact on GHG emissions, they too are insufficient to achieve the overall targets, the team concludes.

• Land use strategies. Land use strategies that can reduce travel activity by decreasing trip lengths and the number of trips include compact developments; mixed-use developments; pedestrian and bicycle friendly communities; and transit oriented developments.

  While each of the above developments has a potential of lessening carbon intensive travel activity, combining them would result in synergistic impact on VKT reduction. Hence in achieving the maximum reduction in travel activity and its associated GHG emissions, the key is to combine as many possible elements of each of the above-mentioned developments.

  Land use strategies can be an effective tool to achieve sustainable transportation...One disadvantage for land use changes is that they only take effect over long period of time as only small changes in land uses typically occur annually.

  —Mashayekh et al.

The authors propose two concepts for achieving more sustainable transportation systems:

1. Strategy Combination Impact. The combination of various strategies or development types (in the case of land use) results in maximum impact.

2. Agencies Collaboration. Cooperation and collaboration between public agencies on local, state and regional levels as well as between public and private entities is crucial. The goal, the authors say, should be to create platforms, tools and processes that enhance this collaboration and to make decisions more effectively.

Resources

• Yeganeh Mashayekh, Paulina Jaramillo, Constantine Samaras, Chris T. Hendrickson, Michael Blackhurst, Heather L. MacLean, and H. Scott Matthews (2012) Potentials for Sustainable Transportation in Cities to Alleviate Climate Change Impacts. Environmental Science & Technology doi: 10.1021/es203353q