I was hoping the expression around here was unique, but I see I was wrong. Our version is:
Why did the chicken cross the road?
To prove to a possum it could be done.
One thing is for sure: the joke is good as long as you plug in the local varmint.
It’s possums by a long shot in this area. After that it’s maybe squirrels or house pets. Believe it or not we saw an armadillo dead on the Natchez Trace Parkway in Tennessee a while back. Dead or alive I had no idea they got this far from Texas.
Here in Florida the occasional bear becomes road kill, yes. Also, unfortunately, the occasional Florida bobcat.
Squirrel here in Central Indiana.
Skunks, opossums, cats. Suburban northern California.
In that case, unless the offending vehicle is at least the size of a dump truck, I presume that the road kill is the car and passengers. Moose usually beat cars, so elephants probably are barely diverted.
It’s weird, I don’t think I’ve ever seen a roadkill elk. There’s 1-2 freeway moose kills per year, just saw a calf this weekend. 
Can’t believe the OP left skunks off the list. I’ll bet that’s why there are so many ‘other’ votes.
More skunks than anything else around here, but we get enough deer that they’d be #1 by mass if not quantity.
ETA: Sheesh, roadkill skunks even have their own theme song.
I’ve heard a variation: “Why did the chicken cross the road? To prove to the deer that it could be done.”
Funny no one has ever run over a Sasquatch.
Or hit a ‘rod’. Think about the rods, we have video of them, so the substance they are made of interacts with photons. So if a car made of matter should ever hit one, bingo, we would have a squished rod to study.
But it has never happened . . .
I see squirrels most often.
Suburban Cleveland area (far west side)
Northern Virginia. I almost never see anything but deer. That may be due to the highly efficient hawk population though. Logically, one would think there would be more squirrels.
ETA: My car threw a rod once, does that count? Into the fuel line . . . it was fiery . . . at 395 and Glebe . . . on a Monday morning. Fun was had by all.
Birds and squirrels run (broken) neck and neck around my neck of the woods. Followed by possums and skunks.
I smell them too, on the way to work in the morning. I’m not sure they’re all dead. I think some may have just been disturbed in their regularly scheduled skunk business. But I do see a fair number of dead ones, along with raccoon. But I voted deer.
I voted raccoons Number One in the Chicago area. Often they die with little visible external damage. They look like they’re sleeping on the shoulder of the road.
Coyotes are up-and-coming on the local roadkill charts. The only animal I’ve ever killed personally, locally, was a mallard drake. The damn thing walked under my car, raised his head, and got de-crap-itated.
Squirrel, by far. Houston.
Ditto, squirrels by far. Dallas.
I’m pretty confident in my theory.
Your theory doesn’t explain all the similarly sized roadkill that lays in the traffic lanes until it is totally obliterated. Or if just out of the traffic lane, like right on the white line it’ll stay there 'til it rots away. Can hardly imagine police officers around here moving roadkill unless it was a deer or something large enough to cause a car to lose control if struck. Even deer lie along the highway for days or weeks if they’re outside the lines. If they’re in the grass they may skeletonize where they land. Can easily imagine redneck locals swerving out of traffic lanes to squish a turtle.
In my earlier posts I forgot groundhogs! They may be tied with raccoons for first place. Skunks are common too. See the occasional coyote and I think a badger once.
In MN, it’s bird, deer, bird, bird, skunk, raccoon, deer, MOOSE, bird.
In WI, it’s deer, deer, deer, deer, raccoon, deer, deer, deer, skunk, deer, deer.
I’ve hit two in my time-a tree swallow in Maryland (on the windshield), and a rabbit in my old yard that I didn’t know about until I went to get the mail the next day-he was right in the tire rut in our old gravel driveway, flat as a pancake.