Graphic: Speeding on the Illinois Tollway

[Brandon]

Graphic: Speeding on the Illinois Tollway

Methinks the problem is the 55 mph limit, not the speeds at which people are going.  They do not change much between the 55 and 65 mph zones, if at all.

The Tribune also has this article behind a paywall (free if you already subscribe to the paper):

Speed limits largely ignored, Tollway report finds

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.

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.

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.

Did it ever occur to her that the limit might be too low?  The Michigan State Police did the same study and concluded that limits around Metro Detroit should be raised to 70 mph.  Maybe the ISP needs to ask the MSP for their study results.

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.

Then the limit along the entire Tollway should be 70 mph.  Makes sense, eh?  Good geometry, good distance between exits, etc.

Citing the safety argument, Tollway engineers say the interstates near Chicago are too congested to raise 55 mph limits, with some exceptions.

I call bullshit.  People are already going 70-75 mph even with the congestion.

Not everyone agrees. Supporters of higher limits note research showing that most drivers reach a comfortable maximum speed of 70-75 mph, regardless of how high the limit is set.

And Tollway research suggests higher limits may have very little effect on drivers' speed.

Consider two stretches of highway – I-94 through Lake County and I-88 west of Aurora. The I-88 stretch has a 65 mph limit – 10 mph higher than the I-94 stretch – but drivers are traveling about the same speed on each road. In fact, the 85th percentile speed is just 3 mph faster where the speed limit is 10 mph faster.

Quite in contrast to the IIHS's claims:

In an April 2013 report, the Insurance Institute for Highway Safety pointed to research showing that raising limits that high would lead drivers to push down on gas pedals even more, creating a cycle of ever-increasing speeds.

"The 85th percentile is not a stationary point," the IIHS states in the report. "It is, rather, a moving target that increases when speed limits are raised."

As shown by the Tollway's own data, the IIHS's claims are baseless junk.

One of the experts who advocate higher limits is Leon James, a psychology professor at University of Hawaii who has been studying aggressive driving for 20 years.

"The way they set speed limit is somewhat subjective and arbitrary," he said. "It's not an exact science."

He maintains that research supports the 85th percentile approach; that "if most of the people travel at the same rate, that's the safest way of going, regardless of the speed limit."

To add to that in the article,

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."

Good advice.  Go with the flow, and then set the limit somewhere near the flow speed.

[JREwing78]

Heh - learn something new every day. I just assumed the limits around Chicago were posted at 55 for Clean Air Act reasons. The absurdity of the existing limits is obvious whenever you drive these highways. 70mph is perfectly appropriate on these stretches of highway.

[ET21]

Little late on the report, been like this for 5-10 years

[Henry]

I agree, 55 is too low for this region. At least raise it to 65 in metro areas, and 70 in rural parts.

[Brandon]

I agree, 55 is too low for this region. At least raise it to 65 in metro areas, and 70 in rural parts.

Illinois (DOT, ISTHA, etc) is supposed to use the results of a speed study to help determine speed limits.  Since ISTHA did a speed study and discovered the median and 95th percentile, then the limits should be 70 on the tollways, all tollways.