Have someone help you cut the egg carton crosswise, so there are four little wells and a hinge on one side that lets the top swing open and shut. Cut out the side panels and back panels to make windows. You might want to glue clear plastic wrap over the inside of the windows and across the opening in front.
Poke slits in the sides of the passenger wells and thread garbage-bag twists to make seat belts.
You could paint the car and use magic markers to make headlights on the front, brake lights on the back, and wheels with hub caps on the side. For a road, tape two strips of masking tape on the carpet or tabletop. You might also want to add handmade stop signs along the road. That's something your child needs for learning to use fingers carefully for writing. That kind of outer control can help your child develop inner self-control, too.