Transportation refers to systems which are designed to move people and goods. It provides people with access to what they value and need, to live their lives, build economies, and interact with the environment. Transportation is the backbone of a global network that trades over 18 trillion worth of goods and moves the world's population over 81 trillion kilometers every year. The system also impacts the climate. As of 201 5, transportation accounted for 23% of the world's greenhouse gas emissions. One way to understand this complex system is by breaking it down into three different components: mode, fuel type, and technology. Modes of transportation allow people and goods to move across air, land, and sea. People often move by human power, cars, and planes, while goods are transported by truck, rail, or ship. Fuel is the crucial element that propels the transportation system. The primary fuel types are derived from human power, renewable fuels and fossil fuels. Different fuels and how they are consumed have distinct impacts on the climate and our air quality. When petroleum products are burned in vehicle engines they release energy, but they also release greenhouse gases and other pollutants. Currently 95 percent of transportation is fueled by petroleum. However, other fuels like electricity and biofuels are becoming increasingly prevalent as we adopt new technologies. Technology refers to what the vehicle uses to move, like an internal combustion engine in a gas car or an electric motor in an electric vehicle. Technology is very greatly in efficiency. Efficiency determines how much of the fuel is actually used to propel a vehicle versus energy lost in the process.