You can use the 1, 2 or 3 arch bridge from Riverview to give the illusion of a walking bridge. Found here about half way down the page.
http://www.modthesims.info/showthread.php?t=427515 Not sure if this can be used outside of CAW.
You can try this -
https://www.thesimsresource.com/art...cks/id/1114562/ - I've used this across small ponds on lots and sims will actually walk over it.
In general, sims hate walking over the little home made bridges as shown in this tutorial
http://www.modthesims.info/showthread.php?t=358933 They mostly will find a way to just go around, from what I've seen anyway. You can trick them into walking over it with routing paint if working in Create a World. There are probably some objects that will have the same effect, that is to block certain areas so they walk on the ones you want. Not sure tho...maybe I dreamed it?