Interesting. I didn't realize Illinois had a 55 law for trucks, too. Does that apply to cars pulling trailers as well?
California is also 55 mph for 3+ axels, and although I know the intent is safety, it really seems to make things less safe. Even in most urban areas, the speed limit for regular vehicles is 65, and in other areas is 70. Keep in mind that most people drive 70-80, that's a huge differential compared to trucks/trailers. When you're tooling along in the slow lane at 55 and need to move over to let someone merge, or because there's a hazard, or someone is pulled over, etc., it's really tough when the people next to you are going 75.
Ditto when you're in a regular car and a slow truck suddenly needs into your lane. Just too much of a difference to be safe, IMO.
Now that IL is changing, maybe CA will, too.