From rural covered passes to modern engineering marvels, bridges overcome natural obstacles and expedite transportation. Yet many of the bridges that people drive on every day are in rough shape. According to one transportation group, more than one in 10 of the country’s bridges are in need of serious repair or replacement.

In many states, the situation is more dire. Nearly a quarter of the bridges in Pennsylvania are structurally deficient, the highest of all states, according to Transportation for America, a grassroots organization advocating updated transportation infrastructure. Based on the group’s report, “The Fix We’re In For: The State of Our Nation’s Bridges 2013,” these are the 10 states with the most dangerous bridges.

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The states on this list tend to have older bridges — their bridges’ average age is older than the average age of all bridges nationwide of 43 years. In Pennsylvania, the states with the most dangerous bridges, the average age is 54, higher than all but four others.

David Goldberg, communications director with Transportation for America and a co-author of the report explained that a large number of U.S. bridges were part of the transportation projects initiated shortly after World War II. “A lot of these bridges were born with the baby boomers.” He said. “Like the baby boomers, they are nearing retirement age.” Transportation for America projects that in 10 years, one in four bridges in the country will be at least 65 years old.

For most states, gasoline taxes are one of the most important sources of funding for bridge construction and repair. Not surprisingly, many of the states with structurally deficient bridges have lower gas tax rates than other states. For instance, Oklahoma charges just 17 cents a gallon in taxes, lower than all but four other states. Meanwhile, Missouri charges 17.3 cents, lower than all but five other states. Both states are on this list.

But higher gas tax rates may not be sufficient to improve bridges in some states, Goldberg noted. Considering the number of bridges that need to be maintained, sparsely populated states simply do not have enough people filling up their tanks to bring in the revenue needed to rebuild or replace structurally unsound bridges. Such is the case in North Dakota and South Dakota, the fourth and fifth most sparsely populated states in the country, respectively.

Winter weather can also wreak havoc on infrastructure. Bridges tend to receive the most damage during the wintertime, Goldberg said, and states in the Northeast tend to have harsher winters. Of the 10 states on this list, four — Pennsylvania, Rhode Island, New Hampshire and Maine — are located in the Northeast. North and South Dakota also have very harsh winters.

Some of these states are heading in the right direction in terms of repairing bridges. The number of bridges in Missouri deemed structurally inadequate declined by more than 15% since 2011. The state has worked over the past several years to repair bridges through the $685 million Safe & Sound Bridge Improvement Program. In addition, both Pennsylvania and Maine have reduced the number of structurally deficient bridges by more than 8%.

Based on information from Transportation for America’s report, “The Fix We’re In For: The State of Our Nation’s Bridges 2013,” 24/7 Wall St. identified the 10 states with the highest percentage of bridges that are deemed structurally deficient. Transportation for America labels bridges structurally deficient if they “require significant maintenance, rehabilitation or replacement.” Most of the information is as of 2013, with the exception of data from Tennessee and New Hampshire, which is from 2012. Transportation for America also provided the data regarding the average age of bridges, traffic over bridges and bridge information by county. Data on current excise taxes by state came from the Tax Foundation, while population density information as of 2011 was from the U.S. Census Bureau.

These are the states with the most dangerous bridges.

10. Missouri
> Pct. of bridges structurally deficient: 14.5%
> Number of bridges structurally deficient: 3,502 (4th most)
> Average bridge age: 43 years (21st highest)
> State gas tax: 17.3 cents (6th lowest)

Since 2011, Missouri may have made more strides fixing its bridges than almost any other state in the nation. Although 3,502 of its 24,072 bridges are structurally deficient, this is actually 640 fewer than in 2011. In St. Louis County, just 3.9% of the 865 bridges received a subpar grade. Those 58 bridges in need of repair carry an average of 789,000 vehicles per day. However, the bridges in many other parts of Missouri remain in a serious state of disrepair. In five counties, more than 30% of the bridges are structurally deficient.

9. Maine
> Pct. of bridges structurally deficient: 14.8%
> Number of bridges structurally deficient: 356 (11th fewest)
> Average bridge age: 50 years (8th highest)
> State gas tax: 31.5 cents (16th highest)

Maine has 2,400 bridges, 356 of which are considered structurally unsound. The state is one of just eight where the average age of all its bridges is at least 50 years old. Despite still having among the highest percentage of problematic bridges, Maine has moved in the right direction in terms of repairs. As many as 33 bridges have been repaired in the state since 2011, reducing the number of structurally deficient bridges by 8.5%. This is among the better improvements of all states.

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8. New Hampshire
> Pct. of bridges structurally deficient: 14.9%
> Number of bridges structurally deficient: 362 (13th fewest)
> Average bridge age: 52 years (6th highest)
> State gas tax: 19.6 cents (10th lowest)

Although it has just 2,429 bridges, less than all but a half a dozen other states, New Hampshire may soon need to spend a considerable sum on their upgrade and repairs. Nearly 15% of its bridges have been judged to be in poor condition. The average age of all bridges in the state is 52 years, while the average age of its structurally deficient bridges is close to 74 years. By both measures, the state’s bridges are among the oldest in the nation. Of the bridges in need of maintenance, rehabilitation or replacement across New Hampshire, 50 were built before 1900.

7. North Dakota
> Pct. of bridges structurally deficient: 16.8%
> Number of bridges structurally deficient: 746 (23rd fewest)
> Average bridge age: 45 years (15th highest)
> State gas tax: 23.0 cents (20th lowest)

The number of structurally deficient bridges in North Dakota has increased by 2.6% since 2011, among the biggest increases in the country. While North Dakota has a high percentage of problematic bridges, just over 95,000 vehicles travel over them every day, lower traffic than any other state in the country. Although many states are able to fund repairs through state and federal gasoline taxes, Goldberg notes that states such as North Dakota have trouble repairing roads and bridges with this money due to the low volume of travelers in the state.

6. Nebraska
> Pct. of bridges structurally deficient: 18.0%
> Number of bridges structurally deficient: 2,778 (6th most)
> Average bridge age: 44 years (17h highest)
> State gas tax: 25.5 cents (24th highest)

Nebraska, the 14th least populous state in the country, has 2,778 defective bridges, more than all but five other states. By comparison, California, by far the most populous state in the nation, has just 2,978 deficient bridges. In recent years, very few bridges have been repaired, maintained or rehabilitated in Nebraska, with the number of structurally deficient bridges falling by just 42 since 2011. But not all parts of Nebraska suffer from this problem. Five counties in the state did not have a single defective bridge. In Douglas County, where Omaha is located, just 2% of daily bridge traffic travels over such bridges.

5. South Dakota
> Pct. of bridges structurally deficient: 20.6%
> Number of bridges structurally deficient: 1,208 (20th most)
> Average bridge age: 46 years (13th highest)
> State gas tax: 24.0 cents (23rd lowest)

South Dakota is one of just five states where more than 20% of the bridges are in need of maintenance. The average age of a bridge in the state is 46 years old. While this is far from the oldest of all states, it is higher than the 43 years average across the country. South Dakota’s less-than-stellar bridges are not used too often — the average traffic on South Dakota’s bridges is slightly more than 354,000 vehicles a day, lower than all but four other states.

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4. Rhode Island
> Pct. of bridges structurally deficient: 20.7%
> Number of bridges structurally deficient: 156 (6th fewest)
> Average bridge age: 55 years (4th highest)
> State gas tax: 33.0 cents (13th highest)

Rhode Island has just 754 bridges, the fewest of any state. Despite the limited amount, nearly 21% of the bridges are structurally deficient. And although Rhode Island is geographically small, each day more than 1.8 million vehicles use Providence County bridges that are in need of maintenance, rehabilitation or repair. In all, more than 2.6 million vehicles use potentially problematic bridges on an average day. This is more traffic on structurally deficient bridges than many, much more populous states, including Florida.

3. Iowa
> Pct. of bridges structurally deficient: 21.2%
> Number of bridges structurally deficient: 5,191 (3rd most)
> Average bridge age: 44 years (17th highest)
> State gas tax: 22.0 cents (18th lowest)

Iowa is one of just three states where more than 5,000 bridges are structurally deficient. In three of the state’s counties — Adams, Davis and Ringgold — more than 40% of the bridges are deficient, while in Lucas and Van Buren counties nearly 40% of the bridges are defective. The average age of all bridges and the average age of deficient bridges are both higher than the national averages. Members of the Iowa legislature attempted to raise the gasoline tax earlier this year to divert some of the revenue to maintenance projects through the state’s road fund. However, the legislation died because of Republican opposition.

2. Oklahoma
> Pct. of bridges structurally deficient: 22.6%
> Number of bridges structurally deficient: 5,382 (2nd most)
> Average bridge age: 46 years (13th highest)
> State gas tax: 17.0 cents (5th lowest)

None of the 10 states with a high percentage of deficient bridges has progressed less in repairing its bridges since 2011 than Oklahoma. In fact, the situation in Oklahoma further deteriorated. As many as 77 more bridges are deemed deficient now than were two years ago. More than 7.7 million vehicles use unsound bridges daily. In Tulsa County, 16.6% of daily bridge traffic is on bridges that need repairs or improvements. This amounts to an average of 7.2 million vehicles a day.

1. Pennsylvania
> Pct. of bridges structurally deficient: 24.5%
> Number of bridges structurally deficient: 5,543 (the most)
> Average bridge age: 54 years (5th highest)
> State gas tax: 32.3 cents (15th highest)

No state has a higher percentage of deficient bridges than Pennsylvania. Goldberg points out that the state has made a significant effort to improve its bridges in recent years. In three counties — McKean, Schuylkill and Potter — more than 40% of the bridges are structurally deficient. The average age of all bridges in Pennsylvania is 54 years, higher than all but four other states. Although Pennsylvania still has more bridges in need of serious maintenance or replacement than any other state in the country, it has reduced the number of these problematic bridges by 500 since 2011, more than any other state except for Missouri. The Pennsylvania legislature is currently debating legislation that would increase the state’s transportation funding by 50%.

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