South Florida’s traffic conditions worsened slightly this past year, according to the latest survey of the cities with the worst road congestion in the country.

The 2012 Urban Mobility Report, being issued Tuesday by the Texas A&M Transportation Institute, ranks the Miami-Dade/Broward/Palm Beach region in 11th place of the 15 urban areas with the worst congestion.

The annual Urban Mobility Report is considered the nation’s most accurate measure of traffic conditions in large metropolitan areas. It is compiled by a university research institution that is considered the national authority on transportation issues.

While the report has come to be known chiefly as a measure of traffic in urban areas, the authors of the 2012 survey offered new ways to determine the impact of congestion on commuters.

The report for the first time includes a Planning Time Index (PTI), a measure of the amount of extra time drivers need to arrive on time for events such as appointments, airplane departures or cargo shipments at ports or airports.

“If the PTI for a particular trip is 3.00, a traveler would allow 60 minutes for a trip that typically takes 20 minutes when few cars are on the road,” the report said.

According to the report, PTIs on expressways vary widely across the nation, from 1.31 — about nine extra minutes for a trip that takes 30 minutes in light traffic — in Pensacola to 5.72, almost three hours for that same half-hour trip, in Washington, D.C.

The report assigned South Florida a PTI of 3.60. In the study’s PTI rankings, the region was also in 11th place, after Houston.

“We all understand that trips take longer in rush hour, but for really important appointments, we have to allow increasingly more time to ensure an on-time arrival,” said Bill Eisele, a co-author of the report. “As bad as traffic jams are, it’s even more frustrating that you can’t depend on traffic jams being consistent from day to day. This unreliable travel is costly for commuters and truck drivers moving goods.”

The worst PTI index city was Washington, D.C., which also was ranked as the area with the nation’s worst traffic congestion.

The nine urban areas with worse traffic conditions than South Florida were Los Angeles, San Francisco-Oakland, New York-Newark, Boston, Houston, Atlanta, Chicago, Philadelphia and Seattle.

The rankings were similar to those of a report issued in September 2011. “Traffic congestion in U.S. cities has remained relatively stable in recent years and continues to underscore the link between traffic and the economy,” the report said.

For the purposes of the report, an urban area includes major suburbs or municipalities around the “urban core” or downtown of a city. For example, the data for Miami/South Florida include congestion across Miami-Dade, Broward and Palm Beach counties.

Over the years, traffic congestion in the region has fluctuated compared to other cities. In 2000, Miami/South Florida placed 12th, and in 2008 it was 15th.

In another innovation, the 2012 report includes an estimate of the additional carbon dioxide emissions attributed to traffic congestion. The total, the report said, is 56 billion pounds — about 380 pounds per auto commuter.

“Including CO emissions . . . provides another dimension to the urban congestion problem,” said researcher and report co-author David Schrank. “It points to the importance of implementing transportation improvements to reduce congestion.”

The inclusion of emissions in the report might raise some controversy.

While the scientific consensus is that CO emissions are a major cause of global warming, some skeptics blame climate change on natural causes, such as an increase in cosmic rays or the sun entering a hotter phase.