(USA TODAY) -- Last year, the average American driver wasted 38 hours sitting in traffic. While the occasional traffic jam inconveniences most drivers, some unlucky people live in the nation's most congested cities. In addition to spending an average of 42 hours a year in traffic, drivers in these cities face peak hour congestion that can increase travel time during rush hour by 15% or more.
INRIX, a traffic information and services group, collects data for individual road segments. In its 2012 Traffic Scorecard, INRIX calculated the amount of time that congestion added to drivers' peak hour commute for each road. After aggregating these segments for each metropolitan area, it ranked the Los Angeles area as the city with the worst congestion in 2012. At peak hours, traffic on Interstate 405 in Los Angeles moved at just 14 miles per hour, adding 26 minutes to what should be an eight minute drive.
Based on the INRIX Traffic Scorecard, these are the 10 cities with the worst traffic.
According to INRIX, congestion is the difference between the drivers' actual speed on the road during peak hours and their speed when congestion is not an issue, as distinguished from how fast people are traveling during peak hours. Of course, this problem also extends travel time during both morning and evening rush hours. In 2011, six of the 20 cities with the longest average commute time were among the 10 cities with biggest difference between peak and nonpeak travel time. Two of these metro areas, Washington D.C., and New York, had average commutes of more than 34 minutes - the longest in the nation.
Not surprisingly, population density is a contributing factor to traffic congestion. Of the 10 metro areas that had the highest population density as of 2010, six were also among the nation's 10 most congested. This group includes New York, Los Angeles and San Francisco, the three most-densely populated metro areas in the United States.
In many of the worst cities for traffic congestion, a relatively high percentage of commuters use public transportation. In 2011, more than 31% of New York area residents took public transportation to get to work, more than double any other metro area. Despite this, New York's peak hour congestion was among the worst in the nation. While a relatively high percentage of residents in these cities use public transportation, highways, bridges and tunnels remain overwhelmed by the millions of commuters that continue to drive to work.
To determine the 10 cities with the worst traffic, 24/7 Wall St. reviewed figures published by the INRIX Traffic Scorecard for 2012. The scorecard assigns an index score for the 100 largest metropolitan statistical areas, and individual road segments within those areas. Scores are functions of the percentage difference between road segments' uncongested (or "free flow") travel time and the calculated travel time on the roads during peak hours (6 a.m. to 10 a.m. and 3 p.m. to 7 p.m., Monday through Friday). 24/7 Wall St. also reviewed population density from the 2010 Census, as well as travel time and commuting methods for each metro area from the Census Bureau's 2011 American Community Survey. All data used were the most recent available.
These are the 10 cities with the worst traffic:
> Congestion score: 14.7
> Population density: 1,305.4 people per sq. mile (9th highest)
> Average commute time: 29.2 minutes (tied for 10th highest)
> Pct. driving to work: 76.6% (8th lowest)
It took commuters in Boston 14.7% longer to travel during peak hours than it would without traffic in 2012. This is more than double the nationwide congestion level. Among the major reasons for this was Interstate 93, three stretches of which ranked among the 50 most congested corridors in the nation. The interstate was rerouted from an elevated highway running above Boston to a tunnel running below the city as part of the area's famous "Big Dig" project, which ran billions of dollars over budget and took decades to finish.
9. Washington, D.C.
> Congestion score: 16.4
> Population density: 997.1 people per sq. mile (18th highest)
> Average commute time: 34.5 minutes (2nd highest)
> Pct. driving to work: 76% (6th lowest)
Just 76% of all Washington D.C. area commuters used a private vehicle to get to work in 2011, less than nearly all other large metropolitan areas. And as many as 14.8% of commuters used public transit - among the most in the nation. But with the Washington area among the nation's most congested, the average commute time to work was 34.5 minutes - behind only the New York metro area. Traffic congestion did improve in 2012, when the city received a congestion score of 16.4 - down from 19.9 the year before. The lower score indicates that drivers are traveling closer to "free flow" speed during peak hours, even as large stretches of Interstate 95 and the Capital Beltway ranked among the worst congestion corridors in the nation.
> Congestion score: 17.6 (tied for 7th highest)
> Population density: 585.8 people per sq. mile (35th highest)
> Average commute time: 27.6 minutes (22nd highest)
> Pct. driving to work: 81% (20th lowest)
Congestion in Seattle actually improved in 2012, with the INRIX index score declining from 19.6 in 2011 to 17.6 last year. Despite this improvement, Seattle remains one of the most congested metro areas in the nation and had some of the most congested individual roads in the country in 2012. Among these was a nine-mile, southbound stretch of Interstate 5, which ranked as the 11th most congested corridor in the nation in 2012. Last March, The Seattle Times noted that new tolls on the nearby Highway 520 had led to increased congestion on Interstate 5.
7. San Jose
> Congestion score: 17.6 (tied for 7th highest)
> Population density: 685.7 people per sq. mile (26th highest)
> Average commute time: 24.8 minutes (65th highest)
> Pct. driving to work: 86.7% (61st lowest)
Only one stretch of road in the San Jose area ranked in the top 100 most congested in the nation in 2012. Despite this, overall congestion in the area was among the nation's worst last year, receiving an INRIX index a score of 17.6. Additionally, the San Jose area's score actually worsened in 2012 compared to 2011, even as the nationwide congestion score improved from 8.4 to 6.6. San Jose residents were roughly as likely as most Americans to drive to work, according to Census figures.
> Congestion score: 19.1
> Population density: 1,467.2 people per sq. mile (6th highest)
> Average commute time: 28.3 minutes (18th highest)
> Pct. driving to work: 82.2% (29th lowest)
The worst traffic in the Bridgeport, Conn., metropolitan area - which serves for many of the state's commuters as a gateway to the New York metro area -
5. New York
> Congestion score: 19.9
> Population density: 2,826 people per sq. mile (the highest)
> Average commute time: 34.9 minutes (the highest)
> Pct. driving to work: 56.6% (the lowest)
With 2,896 people per square mile in 2010, New York had the highest population density of any metro area in America. That year, nearly 19 million people lived either in New York or the surrounding towns and cities, and many of them made long commutes to work daily. The average travel time to work in 2011 was nearly 35 minutes, the most of any metro area in the nation. New York ranked among the most congested metro areas despite more residents using public transit to get to work and fewer residents using cars than anywhere else in the nation. Four of the 10 worst congested corridors in America last year were located in New York, including the nation's worst: an 11.3 mile stretch of the Cross Bronx Expressway.
> Congestion score: 20.7
> Population density: 406.7 people per sq. mile (70th highest)
> Average commute time: 25.8 minutes (45th highest)
> Pct. driving to work: 85.8% (47th lowest)
No metro area with more than a million residents had a greater percentage increase in population from July 1, 2011, and July 1, 2012, than Austin's 3% growth, according to the Austin Statesman. This is hardly news for the area, which has expanded rapidly for more than a decade and, like much of the state, has been unable to expand transportation infrastructure to handle this growth. In 2012, Austin was one of four metro areas with an INRIX index score higher than 20, well above the 6.6 score for the U.S. overall. It was also one of just six large metro areas in which the INRIX index score worsened compared to the year before.
3. San Francisco
> Congestion score: 23.5
> Population density: 1,754.8 people per sq. mile (3rd highest)
> Average commute time: 29.2 minutes (tied for 10th highest)
> Pct. driving to work: 71.4% (3rd lowest)
As many as 14.6% of workers in San Francisco took public transit to work in 2011, the third-highest rate of any metro area in the nation. Additionally, just 71.4% used a car, truck or van to get to work, the third-lowest percentage in the U.S. But despite the high public transit use, the area remained highly congested. Likely contributing to its high congestion is the area's high density. The San Francisco area had the nation's third-highest population density in 2010, with just under 1,755 people per square mile - behind only the New York and Los Angeles metro areas. In late 2010, the San Francisco County Transportation Authority decided to study whether charges would discourage drivers from traveling by car through highly congested areas during peak hours. A decision on implementing such charges has yet to be reached.
> Congestion score: 26.0
> Population density: 1,586.7 people per sq. mile (5th highest)
> Average commute time: 27 minutes (27th highest)
> Pct. driving to work: 79.5% (14th lowest)
Honolulu is one of the densest metro areas in the nation, with more than 1,586 people per square mile, as of 2010. Commuters were also considerably less likely to get to work by car, truck or van than most Americans, and were far more likely to walk or use public transit. Although just two Honolulu road segments, barely totaling 11 miles, were among the nation's 100 most congested corridors in 2012, the area still ranked exceptionally poorly. However, the city's congestion score improved from 2011, when it had the worst levels in the country. After years of planning and delays, Honolulu broke ground on a massive public rail transit project in 2011. The project has long been controversial due to its environmental impact, cost and the possibility of disturbing the burial sites of Native Hawaiians' ancestors.
1. Los Angeles
> Congestion score: 28.8
> Population density: 2,646.0 people per sq. mile (2nd highest)
> Average commute time: 28.6 minutes (15th highest)
> Pct. driving to work: 84.1% (38th lowest)
After being replaced by Honolulu for a year, Los Angeles once again earned the title of the most congested metro area in the country. In 2012, on a Friday at 5:00 p.m., the average driver wasted more than 28 minutes in traffic. Four of the 10 most congested corridors last year were in the Los Angeles metropolitan area. The worst is an eight-mile stretch on Interstate 405. Los Angeles also had the second highest population density of any metro area in 2010, behind only New York, at 2,646 people per square mile. Only these two metro areas exceeded 2,000 people per square mile that year. However, in Los Angeles, commuters were far less likely to get to work via public transportation. In 2011, just 6.2% of area workers took public transit to work, versus 31.1% in the New York area.
24/7 Wall St. is a USA TODAY content partner offering financial news and commentary. Its content is produced independently of USA TODAY.