Here’s a break down of our Carbon Dioxide Emissions

Per-Capita Emissions:
(i.e., each of us: you, me and each of  “them” )

19.1 metric tons of carbon dioxide is the total per-capita emissions for the United States. (Source: wikipedia)

One third of the total per-capita carbon dioxide emissions are due to transportation. (Source www.eia.doe.gov)

“In the U.S., the transportation sector accounts for approximately 33 percent of total carbon dioxide emissions from fossil fuel combustion, the largest share of any end-use economic sector in 2007. Emissions from this sector increased by 29 percent from 1990 to 2007, representing an average annual increase of 1.7 percent. Nearly 60 percent of the emissions resulted from gasoline consumption for personal vehicle use. The remaining emissions came from other transportation activities, including the combustion of diesel fuel in heavy-duty vehicles and jet fuel in aircrafts. Current details on emissions from the transportation and other sectors are available in the U.S. Greenhouse Gas Inventory.”
(Source: http://epa.gov/climatechange/fq/emissions.html)

Based on these sources, per capita emissions for personal transportation (personal vehcile use) is calculated as: 19.1 * .33 *.60 (i.e., 33 percent of total emissions of which 60% is for personal vehicle use) for a total of 3.782 metric tons.  Essentially, if everyone stopped personal vehicle use, there would be 19.9% reduction in per-capita emissions:  3.782/19.1 = .199 = 19.9%

Countries by their carbon dioxide emissions

Metric Ton:
A tonne (unit symbol t) or metric ton (U.S.), also referred to as a metric tonne, is a measurement of mass equal to 1,000 kg or 2,204.62262 lb, or approximately the mass of one cubic metre of water at four degrees Celsius. It is not a unit in the International System of Units (SI), but is accepted for use with the SI.  In SI units and prefixes, the tonne is a megagram (Mg), but this usage is rare. The spelling tonne pre-dates the introduction of the SI system in 1960; it has been used with this meaning in France since 1842, and is now used as the standard spelling for the metric mass measurement in most English-speaking countries.  In the U.S., the units were originally referred to using the French words millier or tonneau, but these terms are now obsolete.  The Imperial and US customary units comparable to the tonne are both spelled ton in English.
(Source http://en.wikipedia.org/wiki/Tonne)

Carbon Dioxide:
One litre of gasoline produces about 2.325 kg of carbon dioxide (CO2).
(Source: http://www.epa.gov/otaq/climate/420f05001.htm)