An annual study of national driving patterns shows that Americans spent 5.5 billion additional hours sitting in traffic in 2011.

The Texas A&M Transportation Institute released a report Tuesday that found Americans are adapting to road congestion by allowing, on average, an hour to make a trip that would take 20 minutes without traffic. The Urban Mobility Report also says clogged roads cost Americans $121 billion in time and fuel in 2011.

It also determined that the 10 most congested cities are Washington, Los Angeles, San Francisco-Oakland, New York-Newark, Boston, Houston, Atlanta, Chicago, Philadelphia and Seattle.

The report is one of the key tools used by experts to solve traffic problems. But the institute advises that every community has unique challenges and require different, multi-faceted approaches to solving congestion.