On March 26, 2015, I presented persuasive evidence to the Illinois Tollway Board that the mostly 55/60 speed limits being recommended would be less safe than a limit of 65 or 70 mph. One director was persuaded and voted against the proposal. In view of the evidence presented, I was astonished to learn that all of the others voted in favor of the 55/60 mph limits.
By Steve Doner, Life Member and Former Illinois Chapter Coordinator National Motorists Association
The directors were given four exhibits to support the case, with the most persuasive evidence coming from within the Tollway’s engineering studies and IDOT’s policy on establishing proper speed limits. To summarize the four exhibits:
A. USDOT SANCTIONED SOFTWARE CALLS FOR 70/75 MPH TOLLWAY LIMITS – Output reports from USLIMITS2, using Tollway data, including crash rates, recommends a speed limit of 70+ mph for all segments analyzed.
B.EXPERT TESTIMONY PRESENTED IN PAST BOARD MEETINGS - F/Lt Thad Peterson of the Michigan State Police, advocated posting limits at or near the 85th percentile speed of free flowing traffic under good conditions (70+ mph).
C. SEVENTY PERCENT OF STATES POST URBAN LIMITS OF 65+ – The vast majority of states have URBAN limits of 65, 70, or 75.
D. IDOT’S SPEED SETTING METHODOLOGY SUPPORTS 70 MPH LIMITS – The policy….
- Advocates setting limits at the “prevailing speed” (70 mph on nearly all segments)
- Discourages posting any speed limit below the 50th percentile speed which is 65+ on all segments
- Prohibits taking enforcement tolerances into effect in setting speed limits
- Requires use of sound judgment
- Permits, but does not require, use of certain adjustment factors for special circumstances
The 55/65 mph limits may not technically violate a mechanical application of the various adjustment factors that IDOT allows, but clearly do violate the intent of the policy which is to establish speed limits that are in line with the prevailing speed of traffic flow (70+) and in no case should they be less than the 50th percentile speed (65+).
The recommended limits are improper, may be illegal and certainly compromise the safety of Tollway users by comparison to a proper limit of 65 or 70. This counter-intuitive statement is true because most drivers travel at what they feel are safe and comfortable speeds (70-80 mph) on interstates, no matter what the speed limit is. This is proven by data within the speed studies which predict an 80% violation rate with a 60 mph speed limit vs only a 25-30% violation rate with a 70 mph limit.
The danger of under-posted limits is that a (very) few well-meaning drivers will travel at or near the limit. When they do this they endanger themselves and all other drivers because of the resulting speed variance and weaving. Also, under-posted speed limits cease to communicate any useful information to drivers as to what is a reasonable speed, plus when the limit is 55 or 60, the opportunity to post an advisory speed for a hidden hazard where people really should slow down from 75 to 60 is lost. There is no reason for any Tollway segment to be posted at a limit (for cars) below 70.