This thesis comprises three essays on topics of environmental economics, focusing on the U.S. transportation sector. The first essay examines scrappage patterns of used vehicle in the U.S. market. We estimate vehicle life time and the elasticity of scrappage rate with respect to vehicle price. These two parameters are used in simulation models to evaluate public policies like CAFE standards and gasoline taxes. We find that not only are vehicles lasting longer but that scrap rates are less responsive to changes in vehicle price than previously estimated. The second essay examines the impact of removing minimum parking requirements by exploiting New York City's 1982 deregulation of parking in the Manhattan core. We use a Difference-InDifference model to estimate the impact of this policy change on housing density, vehicle density and population density. We find that eliminating the mandated parking spaces increases housing density by 23% and increases, rather than decrease vehicle density by 13%. The policy does not appear to impact population density, perhaps because household sizes shrink after its implementation. The last essay compares the energy intensity for public transit buses and private driving across 62 US cities. We find that there are only 11 cities where private driving outperforms public buses in terms of energy intensity. In addition, if there are other mode of public transportation, such as rail system, they are usually much less energy intense than private driving. To improve bus energy intensity, it might be very difficult to solely reply on transit agencies' effort of attracting more ridership.