I noticed that when Papyrus turns the wheel, you have the pivot set at the top-middle of the wheel rather than at the center; the wheel should not be jarring back & forth like that. I noticed that San's jacket just changes shape at random as he rotates his head. I don't get why Fritz's head is morphing as he bobs it.
I highly recommend using the onion tool. It is your friend.
I also noticed that you gave Papyrus three fingers on his right hand but four on his left. I also noticed this with Fritz as well.
I don't know anybody who drives with their thumbs pressed against the wheel like that, typically you grab the wheel with your thumbs clamped underneath your fingers to supply a better grip.
Also not a fan of how the lane marks change length as they drift away, nor do I dig they way they combine for a single frame of the loop.
Also not a fan of the street lamps that literally just restart & don't complete a real loop, which is especially noticeable since a new street lamp never appears from the far right; nothing in this loop ever enters from ahead of the car or exits from sight over the horizon.
Speaking of horizons, the only part you really got right were the support beams of the railing to the right. Too bad the horizontal line on the concrete part of the railing keeps bobbing up & down.
Since this is a loop, the buildings shifting over to the right makes no sense because they just end up restarting to where they first appeared anyway.
Lastly & this is the part that bothers me the most, I really do not understand the way Papyrus is thrashing his head. The other characters are "dancing" rhythmically but Papyrus is just whipping his head to his right. It looks like it was meant to be a circular motion like what Sans is doing but you didn't complete it. Oh and that motion blur where you fused two frames together is quite atrocious, I'm sorry to say. Especially since his head comes to clean halt just after it. It's so jarring & distracting.
I know loops are hard to complete, especially when you have so many that need to work symbiotically but this comes off as lazy rather than shaving off the rust.
Anyways, I hope you take these criticisms to heart because as it stands, this loop is essentially unfinished.