Let us all not forget the fact that Clem's neighbourhood had at least 1 Roamer before Lee's car accident happened. Namely: the one he hit.
For a zombie to become a roamer, he first needs to have nothing left in the near area to eat. That means that from all the sounds he hears in the near vaccinity, the highway must have been the one most ressembling food at the time. So no more people nearby in houses to see or smell, only other dead ones.
I think the neighbourhood was already hit relatively bad in the morning of Lee's drive. The city just had priority for the police.
And, as I said before: The driver of Lee's car might be all calm because, basicly, he came into work and started driving Lee before morning rush hour. The one moment when a few zombies stalking in the night turn into a huge problem out on the streets.
Concerning Clem's parents. They obviously called after the babysitter died which was, as Clem herself said: two nights ago. My theory that she might have been the victim of a violent robbery instead of a zombie, seeing the state of the house, is still solid as well.