When it's time to cross the road it's a good habit to make your puppy sit. You can train your puppy to do this by pressing down on it's bottom. If it lies down bring it back up again and try again. You want to be able to train your puppy in the future to lie down so never confuse the two by telling your puppy to sit down. The command is just sit. Practice makes perfect and you will need to be patient and keep persevering. Just think about the long walks you will be having in the future and it will make all the effort you put into training your puppy worth while.