Build America Act of 2018 This bill directs the Department of Transportation (DOT) to carry out a national infrastructure investment grant program for capital investments in surface transportation infrastructure. Projects eligible for funding under the program include, at a minimum, highway and bridge projects, public transportation projects, passenger and freight rail transportation projects, and port infrastructure investments. In distributing grants under the program, DOT shall ensure an equitable geographic distribution of funds, an appropriate balance in addressing the needs of urban and rural areas, and investment in a variety of transportation modes. At least 20% of grant funds must be set aside for projects in rural areas. The bill amends the Internal Revenue Code to: (1) establish a National Infrastructure Investment Trust Fund and a Fixed Guideway Capital Investment Trust Fund, and (2) increase the tax on small cigars and cigarettes and on gasoline other than aviation gasoline and on diesel fuel or kerosene.