Here is our article about driving in Yucatan: http://yucatantoday.com/en/topics/driving-tips

You do not need exact change, the toll booths make change. I have just updated the toll amounts in the posting above. You can take the free roads instead, they are perfectly safe, but take much longer because of all the villages and speed bumps. At night it can be a bit tricky because of pedestrians on the road, so don't drive too fast...not to mention hitting a speed bump you cannot see!!!