Here's an official answer from Robert Kirkman:
- Severe brain trauma from car crash pileup.
- Another brain damage that killed them.
- Already zombies that got killed somehow.
From The Walking Dead's Glen Mazzara Addresses Frequently Asked Questions From the Fans:
Robert Kirkman answered this in Issue 95 of the comic. Robert says, "Any non zombie corpses you saw on the highway either suffered severe brain trauma in a car crash that resulted in that pileup, or had another brain damaging event that killed them. They could have even been zombies in the car for awhile -- until someone came along and killed them. We were well aware of the coming reveal when we wrote those scenes early in the season (Season 2)."