A new study says the number of highway bottlenecks on the U.S. highway system has increased dramatically.

The study by the American Highway Users Alliance says the number of bottlenecks has grown from 167 in 1999 to 233 today – an increase of 40 percent.

However, the study did give drivers some cause for hope. Seven of the top bottlenecks from 1999 are no longer on the list because of highway improvements that have relieved congestion.

“While this update clearly shows that gridlock has grown over the past five years, motorists in cities that have moved aggressively to unclog bottlenecks are reaping the benefits of improved traffic flow,” Diane Steed, president and CEO of the highway users group, said. “However, federal highway funding that’s critically needed to finance these improvement projects will expire … and Congress must act to provide congestion relief nationwide.”

The advocacy group said eliminating bottlenecks would provide a number of benefits to drivers and the public at large, including:

• Preventing more than 449,500 crashes, including 1,750 fatalities and 220,500 injuries;
• Cutting pollution at the bottlenecks in half, reducing smog-causing volatile organic compounds by nearly 50 percent and carbon monoxide by 54 percent;
• Slashing emissions of carbon dioxide, a greenhouse gas, by 77 percent at those sites; and
• Saving more than 40 billion gallons of fuel.

The Top 10 bottlenecks in the United States, according to the study, are:

1. Los Angeles – U.S. 101 (the Ventura Freeway) at the I-405 interchange;
2. Houston - I-610 at the I-10 interchange west;
3. Chicago-Northwestern - I-90/I-94 at the I-290 interchange, also known as the “Circle Interchange";
4. Phoenix - I-10 at the state Route 51/state Rout 202 interchange, also known as the "Mini-Stack";
5. Los Angeles - I-405, the San Diego Freeway, at the I-10 interchange;
6. Atlanta - 1-75 at the I-85 interchange;
7. Washington, DC – I-495, the Capital Beltway, at I-270;
8. Los Angeles - I-10, the Santa Monica Freeway, at the I-5 interchange;
9. Los Angeles - I-405, the San Diego Freeway, at the I-605 interchange; and
10. Atlanta - I-285 at the I-85 interchange, also known as "Spaghetti Junction."

Although the study said most bottlenecks were a case of too many cars and too little highway, it also said many were caused by driver distractions, sudden curves or changes in highways, barriers such as toll booths and large-volume merges. However, the study did not include roadways such as bridges and toll roads in its results.

It pointed to increased capacity and highway redesign as a solution to many bottlenecks.