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][1]:

> 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)."


  [1]: http://blogs.amctv.com/the-walking-dead/2012/03/glen-mazzara-faqs.php