My guess (stab in the dark)
1) huge traffic jam attacked by zombies
2) People defend themselves and return to their cars
3) Husband/Wife (or other passenger) notice one of them was bitten in attack.
4) As Husband/Wife/Passenger shuffles off this mortal coil, the other persons in the car delivers the necessary blunt force trauma to the head, leaving the dead body in the car.
5) The living person is then grossed out/horrified that they're in the car with their now dead husband/wife and then leave, where they get attacked more more zombies.
or else, after #2 above, the people that were bitten then stay in their cars and allow themselves to be killed via headshots.