No more fallout and no radiation sickness?

We see one small glimpse of the radioactive fallout when Megan and John are hugging, and that's it. I know we have already pointed out many things wrong with this movie, but that always bothered me. Then later on we learn that the radiation count is at about 600 rads an hour, a lethal dose in about 45 minutes. How on Earth could they have gotten out of a crowded city with clogged roadways in 45 minutes? They definitely would not have made it to the FEMA camp unscathed, that's for sure.