At the start of the second season of The Walking Dead, the group is driving down the interstate with a destination of Ft. Benning. They then have to stop because of a grid lock of parked cars blocking the RV from continuing through. This sets a scene for the next several episodes.
All the cars are either abandoned or have a corpse sitting in the seats. And I think it was either Andrea or Carol making a solemn comment that that it's a graveyard.
But the passengers are not zombies. And many of the cars with corpses are still intact.
So what did everyone on the road die from while sitting in their cars?