The high ground clearance of a semi-truck leaves enough room between the road and the bottom of the trailer that a car can slide underneath: an underride accident. There are a number of different scenarios on the Illinois roadways when this could happen, such as a sudden traffic stop or jackknife.

Transport Topics reports that although these accidents are not the most frequent, they tend to be the most serious because the top half of the car shears off when it slides under the trailer. Frequently, people in the front seats are decapitated.

Most of the time, underride accidents happen at the rear of the truck. So, in the U.S., tractor-trailers must have a metal guard below the end of the trailer to catch the front of a car that may slide underneath. The federal law that requires these also includes standards, but lawmakers have not updated these since the 1990s. Not only that, the regulations are not consistently and strictly enforced. In one five-day inspection sweep, about 900 guards examined by the Commercial Vehicle Safety Alliance had cracks or were otherwise structurally unsound.

According to Commercial Carrier Journal, a recent study found that in 2015, car-semi-truck accidents along the sides of trailers killed 301 people. Lawmakers want to prevent these types of crashes from happening by requiring side underride guards, as well as upgrading the standards for those in the rear.

Many trucking companies and truck manufacturers are proactively improving their rear underride guards, as well as installing side underride guards. However, not everyone views the proposed changes as improvements. Some trucking companies claim the extra weight of the side guards could weaken the structural integrity of the trailers. The pushback from these groups may cause further delays to a law that many people feel is long overdue.