Meet Jimmy. Jimmy smoked a joint two days ago. Today, he crashed his car into a pole while listening to a classic Clarence Carter tune. If the officer reasonably suspects drugged driving due to Jimmy's attire (a t-shirt stating "Legalize it") and red eyes, Jimmy can be required to submit to a drug test. That drug test will probably find the remnants of Wednesday's weed in his system and he can then be convicted of drugged driving under Illinois law.
The problem with these per se drugged driving laws is that they lead to inaccurate results. Jimmy was not necessarily. The crime is driving while impaired. Then again, at this point, we aren't even scientifically sure what impaired means, at least in a marijuana context.