Congress adopted a nationwide 55 mph speed limit law during the oil embargo of the 1970s and threatened to withhold highway funding for any state that didn't comply. It repealed the law 13 years ago, when oil was cheap and gas plentiful. But with prices going through the roof and everyone worried about global warming, there are increasing calls for Congress to bring back the double-nickel speed limit.

Advocacy groups like drive55.orgsay rolling the speed limit back to 55 will save fuel, reduce pollution and save lives. It seems logical, but not everyone is convinced slower speeds bring any real benefit, and the debate is heating up.

"Sheer physics tell you lower speeds equal better fuel economy, fewer injuries and lower emissions," said Justin McNaull, director of state relations for AAA. "But what happens when you change the speed limit is a little less clear."

It depends upon who you want to believe.

Connecticut adopted the nation's first vehicular speed limit 107 years ago today - setting the maximum speed at 12 in the city and 15 in the country - and people have been ignoring them ever since. Congress set the limit at 55 mph under the National Maximum Speed Law of 1974. The law was repealed 19 years later and the states allowed to set their own limits. Most of them bumped it up to 65 mph, although some went to 75 mph and there are stretches of highway in west Texas where you can cruise at 80.

The U.S. Department of Energy says gas mileage plummets above 60 mph and says every 5 mph above that speed is akin to paying an additional $0.20 per gallon for gas. But the American Heritage Foundation claims 12 years of 55 mph speed limits cut fuel consumption by just 1 percent. After Congress repealed the National Maximum Speed Law and 33 states raised their speed limits, the Cato Institute said traffic deaths dropped to a record low.

More than fuel economy and traffic deaths are at stake now. "Emissions increase pretty appreciably above

55," McNaull says. drive55.org claims Washington state would cut CO2 emissions by 10 percent if it cut its 70 mph speed limit by 15 mph. But even here, the evidence is debated. The Automobile Association in England claims reducing the speed limit from 30 mph to 20 on residential roads would increase CO2 emission by more than 10%.

drive55.org also says the average speed people travel on the highway has been rising for 20 years. McNaull notes that advances in vehicle deign, such as better windows and sound proofing have changed peoples' sense of how fast they're going, which is one reason people regularly exceed the posted speed limit. "Doing 60

mph in a 2008 vehicle feels a lot different than in the vehicles our grandparents drove."

Photo by Flickr user AccessWinnipeg.

While you're here, check out This Day in Tech: May 21, 1901: Connecticut Sets First Speed Limit at 12 MPH.