As a means to an end, it is important that the transportation system be accessible and usable by different types of people aiming to get to different places. Federal rules for accessibility and nondiscrimination are behind most funding streams, and local rules also augment requirements to make the transportation system usable for everyone. Transportation is the largest contributor of greenhouse gas emissions in the US. While automobiles have become more efficient over time due to fuel efficiency and emissions standards, these improvements are offset by an overall increase in vehicle miles traveled. Increased demand for trucking and air travel have contributed toward emissions. Proposals for achieving transportation emissions reduction goals at all levels of government include electrification, increased investment in active and shared transportation, and improved land use.