Well, to me it looks like you have to get the enemy to actually stumble onto your ambushing tile. This would mean that roads don't help successful ambushing since they don't generally contain suitable terrain for performing ambushes and armies tend to move along roads as they are quicker. If you want to do ambushes then keep the terrain basic and then the AI will more likely stumble into your woodland tile. The same of course goes for the AI and your armies...

Nevertheless, I still think CA would do well to make ambushes happen more often. They're a cool aspect of the game and make a welcome change to repetitive battles. They just aren't very common... :(