The difference of car usage between USA and Europe should be much larger than the difference in car ownership. A car is useful for many things other than the daily commute, someone who mostly bikes to work is still rather likely to own a car but contribute much less to the traffic and pollution.

[edit] looking at random stats such as http://internationalcomparisons.org/environment/transportati... seems to confirm that - the vehicle ownership rates are just a bit lower, but annual distance driven per capita in European countries is twice less than USA.