move enemies over road

hello
I want to make this movement by code (as3)

http://board.flashkit.com/board/

only by code , I mean how to make the mc move only over the green area

thanks in advence

If the green area is predrawn, probably easiest way to move around, is to use checkpoints. Put invisible points on it, for example at the curves, then use simple "move toward point" code. Once close enough, start to move toward next point.