Revealing mistake: At the end scene when the Sheriff is escaping in the truck away from the nuclear blast, the truck begins to flip through the air. While it is flipping we see under the vehicle and can see the hole they use to launch a post to help flip the vehicle for the stunt. A brief puff of white smoke also emanates from that hole briefly during the flip.
Revealing mistake: When the deputy throws the spike strip across the road in front of the black suburban, as it's flipping through the air, you can see the hole they launch the post from underneath the vehicle that helps to it into a roll.
Answer: Possibly because the Government didn't want word about the virus getting out to the general public so they ordered the solders to shoot the non infected people in order to silence them all, and then created a cover story about a chemical plant exploding in the town.
Trainman