-Chasm or river with bridge: Travelers might have to divert miles off their path to be able to cross their obstacle. This should slow the players down and keep them on the road you want them.
-Ferry: As with bridges, a ferry might be the only safe way to cross a river for miles around.
TOO LATE, WHAT CAN I DO NOW? -Have some encounters prepared in advance for adjacent regions to slow them down so you can prepare the next session.
-Put an encounter or trap that will cripple them forcing them back into safe and known territory.
-Try to find a limit that could fit well into place even if the region have been mapped out. For example, if they are on a mountain path, an avalanche or rockslide may prevent them from going further.
-Have an NPC show up and beg for the help of the players.
-Have the PC captured by monsters.
-Have the PC stopped by local authorities for any reason (maybe they are wanted, maybe they want to enter forbidden ground, maybe they must be interrogated…)
I hope this article will help you to get some ideas on how you can keep control of your game and even spice up your campaign. And remember, the importance for your player is not to have total freedom in your world but have the illusion of it. As long as they think they can move freely and that their characters face obstacles when going out of the way instead of being the players that faces the limit, they will enjoy their game. Have fun!