How would I make NPC's walk round a street, on a set path?

I need some NPC’s to walk around a street, on the pavement, on a loop.

How would i go abotu doing it?

Thanks in adance.

Its best to look that up in source sdk wiki: http://developer.valvesoftware.com/wiki/Path_track. Just make a path_track in the starting place give it a name make another one in another place name it no make the first on target the second and just make a chain.