brass_9: your game is working fine! Non-Industrious workers need a base 3 turns to build a road in Grassland or Plains. As Tao stated, this can vary for Anarchy or Democracy, but in Despotism, Monarchy, Republic (or Communism), it should take 3 Worker Turns to build a road in Grasslands/Plains.
To only use 2 Worker Turns to build the road, you'd have to be an Industrious civ (like Txurce said), and it would be 2 Worker Turns in Despotism AND Monarchy (and Rep., and Comm.)
BTW, the Tech Replaceable Parts also speeds up the Worker by 50% once learned.