Cut windshield from lightweight cardboard and cover with foil. Set aside.
Ice sides and background areas smooth with spatula. Outline car with tip 3. Pipe in tires with tip 12, hubcaps with tip 5, headlights, side mirror and door handle with tip 3 (smooth all with finger dipped in cornstarch). Pipe car interior with tip 10 and smooth with angled spatula. Pipe tip 5 vertical upholstery lines on car seat; pipe in tip 12 convertible hood.
Using buttercream icing, tips 2, 2A, 3, 5, 10 and 12, figure pipe driver and policeman directly on cake.
Attach lollipop sticks to each end of windshield frame with icing; cover frame with tip 14 stars and position in cake. Print tip 2 message. Add tip 17 rosette bottom border.