Grab a cardboard box, and cut it open so that it lays flat. Then you can either draw or paint roads over your flattened cardboard.
You might like to print out these road signs and add them. An easy way to use them once printed and cut out, is to use a peg to hold the sign, then secure the peg onto your road using blue tak.
Add blocks, cars, animal figurines, duplo etc for added fun.