The solar resource. Solar energy is the source of all energy on earth, available to us in a number of derivatives. Plant matter for example, which relies on solar energy for nutrition, experiences natural compression and decomposition over millions of years to form the the fossil fuels we use today for electrical generation and transportation. Other examples of this can be seen in use of biomass for fuel or the harvesting of wind energy which is reliant on solar heated air for the formation of currents.
We are also able to utilize the solar resource directly. Solar thermal technologies take advantage of this resource to heat a working fluid that can transfer energy to an air stream or water for domestic or commercial use. Solar Photovoltaic or PV devices exploit various materials (principally Silicon) that experience sub-atomic variations when exposed to solar energy in order to induce an electric current. Both solar PV and thermal technologies provide a useful source of energy with little to no moving parts, no pollution and very little embodied energy.
Transport. According to the Appropriate transport manual, sustainable transportation is a strategy for the flow of people and goods across the Earth that meets the needs of the present without compromising the ability of future generations to meet their own needs. Transportation accounted for 32.6% of US green house gas emissions in 2005. In addition to the widely publicized environmental consequences of driving automobiles, it is also socially and economically costly:
Land use: Parking and roads use valuable land resources.
Transportation equity: Driving, with all of its expenses, costs the average U.S. household $7,000 per year per vehicle.
Economics: Most of the money spent on driving leaves a local economy, weakening it.
Community: Travelers outside of their cars interact more with their physical environment and each other.
Safety: The presence of pedestrians and cyclists make our neighborhoods safer from crime. Conversely, 42,000 Americans are killed in car accidents every year.
Health: Increasingly, Americans are suffering from weight-related illnesses. This is partly attributable to the decline in active transportation use and availability.
Building and encouraging alternatives to the single-occupant vehicle, or, for short "alternative transportation," is imperative. Some alternative transportation advocates have taken to using the term, sustainable transportation, instead of the previous, widely-used "alternative transportation" term to avoid sidelining their interest from the mainstream.
Many efficient, practical, and inexpensive sustainable transportation technologies already exist, meaning activism, policy work, and planning research is most often more crucial to developing sustainable transportation than technology development.