Armed with speed-measuring guns, traffic engineers stood atop an overpass across Interstate 94 in Lake County to document just how fast Chicago-area motorists are driving.
The first minute of testing on that stretch of the Tri-State Tollway that Wednesday morning found 10 of 14 vehicles exceeded 70 mph. One reached 91 mph — all in a zone where the posted speed limit is 55 mph.
Speeding has become rampant on the Illinois tollways around Chicago, a fact well established by the Tollway in several studies obtained by the Tribune. The data, like that collected on the Everett Road overpass in April, showed that along seven tollway stretches with 55 mph limits, most drivers are pushing the speedometer above 66 mph.
Only a few are obeying the law. In those stretches, an average of 1 out of 20 motorists drives at or below that limit.
As the state prepares to increase the maximum speed to 70 mph on some interstates starting Jan. 1, the Tollway studies and other research suggest that those large black numbers posted on roadside signs don't dictate how fast people really go. Instead, drivers are influenced by other factors, from road conditions to traffic flow and even some basic traits of human psychology.
National research shows a marked leap in highway speeds in recent years. How serious is it?
Even as research demonstrates that speed is dangerous and a contributor to at least one-third of fatal crashes, some drivers feel they need to speed to stay safe.
One is Jan Allen, a retiree from southwest suburban Wilmington. Her priority is to "go with the flow."
"I don't think it's a good idea to reduce your speed too much when everybody else is whizzing past you," said Allen. "I think that has the potential for causing more trouble."
Box salesman Lew Monckton, who travels the Illinois Tollway frequently, sets the speed on his Chrysler at 9 mph above the 55 mph limit, except on I-294.
"On 294, I go 20 over," said Monckton, of Naperville, "because everyone's going that fast. I normally can't even stay in the left lane if I'm going 75 because people want to go 90 or 100."
The data, gathered in April, May and September, showed that, depending on which tollway stretch was tested, 91 to 98 percent of drivers exceeded the 55 mph speed limit. In those stretches, the average speed ranged from 66 to 70 mph.
The studies followed a 2012 National Highway Traffic Safety Administration report that showed that average highway speeds increased to almost 71 mph in 2009 from 65 mph two years earlier. At the same time, traffic fatalities — 33,561 last year — are dropping, except for a slight increase in 2012. The report concluded that the higher speeds might have been the product of less speed enforcement in 2009 and fewer cars on the road that year, leading to less congestion.
Illinois State Police spokeswoman Monique Bond said she was reluctant to draw conclusions from the Tollway data. Vehicles would have been more likely to slow down if their drivers had seen a state police squad car than engineers atop overpasses, she said.
Bond acknowledged that speeding citations have decreased statewide but added that officers assigned to the tollways are among the most productive. A Tribune analysis of the agency's ticket data showed troopers assigned to the tollways write an average of 72 citations a day in metro Chicago.
Setting a limit
Those 72 citations a day pale in comparison to the volume of traffic on the tollway. Nearly 100,000 vehicles cross its busiest spots every day, and the job of deciding how fast they should be allowed to go falls to the Illinois Department of Transportation. The agency is required to set limits that are "reasonable and safe," IDOT spokeswoman Jae Miller said.
And to figure that out, engineers study crash history, ramp locations, traffic volume and traffic flow, among other factors.
But perhaps the most fundamental metric in deciding where to set a speed limit is a concept known as the 85th percentile, or the speed at which 85 percent of drivers are either traveling at, or below. In essence, it measures the limit that most drivers place on themselves, regardless of posted speed limits.
Tollway data showed that the 85th percentile speed ranges from 71 to 75 mph.