Buses run from Dharan to Biratnagar and Kakarbhitta every half hour during the day, less frequently to Birganj. To reach other Tarai destinations, change at Itahari or Biratnagar. If you want to go straight to Kathmandu, you'll have to book ahead on one of the night buses that leave in mid-afternoon. Buses to Dhankuta and points north are basically local and chronically overcrowded, and it can be a real ordeal getting a seat. Faster and more comfortable Land Rovers depart from the eastern side of Bhannu Chowk - they usually operate on a "reserve" basis (ie, you have to hire the whole vehicle), but it may be possible to share a ride for not much more than the cost of a bus ticket.