Tollway speed limits: too little, too late, unsafe
A 70 mph speed limit on metro Chicago tollways would be saferthan the mostly 55/60 speed limits recommended. This counterintuitive statement is true because most drivers travel at what they feel (and which are) safe and comfortable speeds, no matter what the limit is.
Changing the limit does not change the speed. This has been proven and is even confirmed by data within the tollway's speed studies which predict an 80 percent violation rate with a 60 mph speed limit vs only a 25-30 percent violation rate with a 70 mph limit.
The danger from under-posted limits is that a few well-meaning drivers will travel at or near the limit. This increases risk for everyone because of the resulting speed variance, but there are additional problems.
Unrealistic limits cease to communicate useful information to drivers as to what a reasonable speed is. Also, the opportunity to post advisory speeds for hidden hazards, where people really should slow down from 75 to 60, is lost.
Some of the most persuasive evidence that all tollway speed limits should be 70 comes from within the tollway's engineering studies and IDOT's policy on establishing proper speed limits. For example, USDOT software, using tollway data, including crash rates, recommends a speed limit of 70-plus mph for all segments analyzed.
In addition, IDOT's speed setting policy advocates limits at or near the "prevailing speed," which is 70 for all segments and discourages limits below the 50th percentile speed which is 65 for all segments.
The tollway's recommended limits are improper, may be illegal and certainly compromise the safety of tollway users by comparison to an appropriate limit of 70. For unknown reasons, tollway officials have disregarded expert witness testimony, the actions of over two-thirds of other states and crucial data from within their own engineering studies.