Increases in speed limits over two decades have cost 33,000 lives in the U.S, according to a new IIHS study.

In 2013 alone, the increases resulted in 1,900 additional deaths, essentially canceling out the number of lives saved by frontal airbags that year.

“Although fatality rates fell during the study period, they would have been much lower if not for states’ decisions to raise speed limits,” said Charles Farmer, IIHS vice president for research and statistical services and the author of the study.

Maximum speed limits are set by the states, and they have been on the rise since 1995. However, during most of the 1970s and 1980s, the threat of financial penalties held state speed limits to 55 mph.

In 1973, Congress required that states adopt 55 mph as their maximum speed limit in order to receive their share of highway funds. Concerns over fuel availability, rather than safety, had prompted Congress to pass the measure, known as the National Maximum Speed Limit, but the most dramatic result was a decrease in fatalities.

In 1987, with energy concerns fading, Congress relaxed the restriction, allowing states to increase speed limits to 65 mph on rural interstates. The law was completely repealed in 1995.

Proponents of raising the speed limit often argue that such increases simply bring the law in line with reality, since most drivers exceed the limit. Once the limit is raised, however, drivers go even faster.

Institute researchers found that travel speeds increased following the repeal of the National Maximum Speed Limit. They also found that fatalities went up, first on rural interstates with the law’s partial repeal and later on all interstates after the full repeal.

The increases have continued apace. Today, six states have 80 mph limits, and drivers in Texas can legally drive 85 mph on some roads.

The new study looked at the effect of all speed limit increases from 1993 to 2013 in 41 states. Nine states and the District of Columbia were excluded because they had relatively few vehicle miles traveled each year, leading to wide fluctuations in their annual fatality rates.

Farmer looked at deaths per billion miles traveled by state and roadway type. Taking into account other factors that affected the fatality rate — including changes in unemployment, the number of potential young drivers (ages 16-24) and per capita alcohol consumption — he found that each 5 mph increase in the maximum speed limit resulted in a 4 percent increase in fatalities. The increase on interstates and freeways, the roads most affected by state maximums, was 8 percent.

Comparing the annual number of fatalities in the 41 states with the number that would have been expected if each state’s maximum speed limit had remained unchanged since 1993, Farmer arrived at the estimate of 33,000 additional fatalities over the 20-year period. That number is approximately equal to the nationwide annual tally of fatalities during recent years.