The annual Urban Mobility Report, published by the Texas A&M Transportation Institute, said the nation's capital has the worst traffic congestion followed by Los Angeles, San Francisco-Oakland, New York and Boston.
Others in the top 10 are Houston, Atlanta, Chicago, Philadelphia and Seattle.
In this year's report, the institute said it has developed a multiplier called a Planning Time Index to calculate the extra time needed to arrive at important events.
For example, if the PTI for a particular trip is 3.00, the traveler would need to allow 60 minutes for a journey that normally takes 20 minutes when few cars are on the road.
"We all understand that trips take longer in rush hour, but for really important appointments, we have to allow increasingly more time to ensure on-time arrival," said head researcher Billy Eisele, co-author of the report.