Illinois has its posted highway speed limit and its populist highway speed limit — a fluid relationship based mostly on driver whim and the presence of the gentlemen and ladies in state police squad cars.

State Sen. Jim Oberweis, R-Sugar Grove, whose family business happens to be a dairy company with trucks that travel across highways, is proposing to alter that balance by goosing the official top speed.

A bill, sponsored by Oberweis, overwhelmingly approved by the Senate and on its way to the House, would raise Illinois' interstate highway and tollway speed limit to 70 mph. The state's speed is capped now at 65 mph.

Ivory Brown would support the higher speed ceiling, but she illustrates the complications of making such a change.

"I feel it should be increased," Brown, of Blue Island, said while enjoying oatmeal Wednesday at the Interstate 294 Hinsdale Oasis. "But I worry that people will then feel that it's acceptable to exceed the new speed limit."

Increasing speed limits stirs a fair amount of debate. Public sentiment supports raising them, and more states are doing so. But the Illinois Department of Transportation opposes higher limits, and transportation safety groups like the Governors Highway Safety Association and Insurance Institute for Highway Safety caution that faster speeds lead to more fatalities.

"We believe raising limits will raise average speeds further, and crashes/fatalities can be expected to increase," IDOT spokeswoman Jae Miller said in an email Wednesday.

Illinois has a significantly lower fatality rate than many other states, Miller added.

The proposed legislation, which Oberweis called "business-friendly," would allow Cook and the Chicago collar counties and two counties near St. Louis to opt out of the higher speed limits.

Cook County Board President Toni Preckwinkle's office said it was too early to discuss how the county might react to any change.

"The president will work with the board of commissioners on the best course of action for county residents should this bill ultimately become law," Preckwinkle spokesman Owen Kilmer said.

Although IDOT opposes increasing the speed limit, the Illinois Tollway, a separate but related agency whose board members are political appointees, will wait for the outcome of the legislative action before "evaluating its impact on our operations," a spokeswoman said.

With the debate certain to intensify in coming weeks, supporters and critics of the move point to research supporting their positions.

IDOT weighed in with studies of its own Wednesday showing that Illinois ranks seventh-lowest in the U.S. for overall fatality rates per 100 million vehicle miles. Of the 43 states with higher fatality rates, 70 percent allow speed limits of 70 mph or higher on rural highways, IDOT reported.

The transportation agency also noted that vehicle occupants are 26 percent more likely to be killed in crashes on rural interstate highways with high speed limits than on interstate highways with lower speed limits.

The National Motorists Association, a group established in 1982 to fight the 55 mph national speed limit, noted a 1996 Federal Highway Administration study. The analysis concluded that drivers' speeds change when posted limits do. But, the study noted, those speeds "are not sufficiently large to be of practical significance."

The association contends that states should set driving speeds near the rate that 85 percent of traffic is traveling. Communications Director John Bowman called that approach a "prevailing traffic engineering principle" that takes into account most drivers' tendencies and reduces speed differences between vehicles. That strategy also cuts into unpredictable driving and the crashes those erratic maneuvers cause.

"When you homogenize the traffic flow in that way," Bowman said, "you cut down on accidents."

State lawmakers, bowing to constituents' support for faster speeds, seem to agree.

The move toward raising speed limits began in 1995, when Congress gave that power to states.

A total of 35 states have speed limits of 75 mph or higher on some roads, the Insurance Institute for Highway Safety reports. A 2012 study by the Governors Highway Safety Association noted that seven states had increased limits to as high as 85 mph since 2005.

"In the case of speed limits, legislators are ignoring the research, which shows that there is a clear trade-off," Insurance Institute spokesman Russ Rader said.

Drivers get where they're going faster, but it's a much riskier trip, he said. Higher speeds also run counter to reducing fuel consumption, he said.

Bowman contends that the Insurance Institute advocates for lower speed limits because more citations translate to higher revenue for insurance companies.

But Rader pointed to studies in 1989, 1999, 2002 and 2009 showing higher speed limits brought more fatalities. That 2009 analysis of the 1995 repeal of the national limit "found a 3 percent increase in road fatalities attributable to higher speed limits on all road types," the Insurance Institute stated. The largest jump, 9 percent, occurred on rural interstates.

Increases in speed limits across the U.S. led to an estimated 12,500 traffic deaths from 1995 to 2005, according to the study, published in the American Journal of Public Health.

Rader called the 85th percentile approach "a moving target" and disputed the National Motorists Association's claims that motorists' speed remains relatively static when limits are raised.

As speed limits rise, studies show that drivers tend to exceed that limit by 5 to 10 mph, he said.

"People drive the speed at which they don't think they're likely to be stopped and ticketed," Rader said.

At the tollway oasis, Brown said she typically drives 60 to 65 mph on the highway but often is passed by cars traveling at 80 mph.

"I think driving 75 is dangerous," Brown added. "Sixty to 65 miles per hour is fast enough."

Greg Harrison said his comfort zone is about 70 mph. Harrison, a Chicago native who resides in Havana in central Illinois, was also at the oasis. The proposed speed limit boost is "a good idea," he said.

Truck driver Mark VanTholen, of Cicero, also said 55 mph is too slow.