Script:TokenController
From Roll20 Wiki
TokenController breathes life into your Roll20 Campaign with modular pathing.
As a completely menu and macro based mod, you don't need to worry about much. Just install it, build some paths, and set your tokens to patrolling.
Includes follow, lock, and other little control features, as well.
Paths detect loops automatically and will continue to patrol the loop, otherwise it reverses course once the end is reached.