You will want to add invisible blocks that are hit in place of the wall because of its shape.
Set these up along the wall as if they were the wall and set them to invisible.
Apply physics behaviour to all objects on the screen except the blue wall.
Set the physics properties of the invisible walls to immaovable and prevent rotations.
You will need the following conditions
Condition: On car collision with invisible wall
Car angle =0
Car angle =90
Car angle =180
Car angle =360
Event: Car impulse at car.angle
So if when the car collides with the wall and it is one of the 4 angles perpendicular to the blue wall it will casue an impulse.
Im at work so can't test it let me know of that works.