You have to pick:

Wouldn't we all just eventually become dogs? The jam would just get worse and worse and then every single driver would just be a dog. Or do we still have our brains? or does leaving the car count as ending the jam?

Well, if the traffic in front of you clears you turn back, so the people in front can always move, thus allowing the traffic jam to ultimately end. It shouldn't really be that much different than a normal traffic jam ... except that while you're waiting you're a dog ... so who knows what you'll get up to during that time.

I guess the jam might clear a little more slowly because each person has to climb into the driver's seat from whatever chew toy they were playing with in the back.

Also, you would never want to drive with the windows down, because you don't want to escape.

@ DangerStevey
I assume you DO have the MIND of a dog during this time, not just the body ... otherwise it's kinda pointless. In that case the only issue is making seats that are comfortable both for humans and dogs, since in all other respects its not really different that waiting in traffic normally.

The room's not very big and the dolphin won't be actively avoiding me, in fact I bet it'll be curious about me. I suspect this won't be a very hard challenge to overcome. I hope the dolphin will teleport too otherwise it's at risk of drowning too.

You know what they say...never leave a dog in a car, they can't survive the heat. So no thanks.

Maybe. You are right the dolphin isn't actively opposing you. But I'm not so confident it will be curious and won't instead try to flee from fear.

I'm not saying FOR SURE either way. Maybe it'll be curious, maybe it won't. It's up to you to gauge that risk.

@andemon: Good point - make sure to always have the AC on

Once again, MERMAIDS CAN'T DROWN!