Internal Combustion Engine (ICE) cars, trucks, trains, buses, planes all have to go. No single approach can do it. This page explores some combinations that might.

Urban density alone won’t get Americans out of their cars
This article is based on the study below. Increased housing density in combination with expanded public transit, regional links that enable people living in one part of the city to get to jobs in another, and good neighborhood design will be necessary to get residents of American cities to reduce the number of miles they travel by car. Read article.
by Sarah DeWeerdt, Anthropocene, Dec 26, 2017

Intersecting Residential and Transportation CO2 Emissions: Metropolitan Climate Change Programs in the Age of Trump
John D. Landis, David Hsu, Erick Guerra, Journal of Planning Education and Research
Sept 27, 2017
Researchers use a series of fixed-ratio projections and scenarios to determine that mandating residential energy conservation standards in cities could reduce residential CO2 emissions in 2030 by an average of 30 percent over and above 2010 levels. With aggressive efforts to get drivers out of their cars, local compact growth programs could reduce auto-based CO2 emissions in 2030 by as much as 25%, over and above reductions due to higher fuel economy standards. Without those efforts, the local compact growth programs reduce auto-based CO2 emissions by only 0 to 7%, depending on the metropolitan area. Local compact growth programs are also likely to have problems in terms of their goal- and scale-efficiency, and to incur political pushback.
Abstract here; you have to sign up or email the authors to get the report.