Interstate speed limits that are too low are dangerous, and the speed limits on suburban Chicago tollways fit this description. Transportation engineers advocate speed limits based on the 85th percentile method, the speed below which 85 percent of cars travel when not impeded by traffic or enforcement. An Institute of Transportation Engineers report explains how and why this method works and the dangers of not using it. The report states: "Drivers traveling significantly faster or slower than the 85th percentile speed are at a greater risk of being in a crash."
The National Motorists Association estimates that 85th percentile speeds on area interstates are at or above the new Illinois maximum of 70 mph. This is obvious to many area commuters. To test this estimate, I helped conduct speed studies last month on I-80, I-88 and I-294. We clocked 521 cars calculating an 85th percentile speed of approximately 72 mph in all locations. Only 4 percent of drivers were in compliance with the 55 mph speed limit. These motorists posed a greater risk than drivers cruising nearer the average pace. What kind of a law mandates dangerous behavior?
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Making matters worse, some of the drivers most likely to drive 55 are teens fresh from driver education classes that teach them to obey the letter of the law. State Sen. Jim Oberweis and I testified to this effect at an Illinois Tollway board meeting and in a later discussion with tollway engineers. We went so far as saying it would be negligent to keep limits low, but it's unclear whether the Tollway leadership's opposition to raising Chicagoland speed limits will change. Please write to the Illinois State Toll Highway Authority and IDOT and tell them that, for safety sake, the speed limit should be increased to 70 mph in suburban Chicago.
Former Illinois chapter coordinator
National Motorists Association