The Lake County Division of Transportation (LCDOT) is responsible for setting the speed limit for county highways under our jurisdiction, and township roads. Safety on the roadways is always our top priority and guides everything we do, including the determination of speed limits. One main tool we use to determine the appropriate speed limit is a spot speed study (PDF).
A speed study begins with data collection. We measure and record the real-time speeds of 100 vehicles in each direction, between 9 a.m. – 3 p.m., using a radar gun and speed study software. The software then calculates a speed for the zone that 85% of the drivers were comfortable driving at or below, through free-flowing conditions.
This number is then applied against three other main criteria – the number of access points within the zone being studied, pedestrian usage and needs in the corridor, and the crash history for the zone being studied.
At LCDOT, we have a program in place to make sure all speed studies are less than five years old. In some cases, changing conditions of an area, or requests from local authorities, residents, or businesses can prompt a review of the current speed in advance of when the road is scheduled for review.
The Illinois Vehicle Code gives LCDOT the authority to set speed limits and requires that an Engineering study be completed to alter a speed limit. Once the study and analysis is complete, the results are reviewed by the Engineer of Traffic and the County Engineer. If it is determined a change is needed, the proposed speed limit change is presented to the Lake County Board for a vote.
The general rule of thumb is that 85% of driver’s will drive the speed at which they feel comfortable and safe, not necessarily at a speed of 5 mph over or under the speed limit. If the speed limit is raised or lowered without proper reasoning, the overall speed of traffic for those 85% of drivers is not likely to change. But as other drivers adjust their speed to follow the newly posted speed limit, this causes disparity in the range of speeds cars are traveling. This may lead to indecision, reduce the level of safety, and increase the amount of crashes along the roadway.