Family transportation needs depend on four factors: no. Of people in the family requiring transportation, How often in a day or week and how many people I the family can use public transportation. The fourth factor: how many I the family can car pool for their transportation.
Once these data are compiled, then you can use following strategy ti minimize transportation cost.
Figure out how many definitely need their own transportation. This cost would include the cost of car, gasoline, insurance etc. So you can budget amount say 150 dollars per moth per person.
Then if someone is going to use public transportation, then figure about 50 dollars per person.
If someone is going to use car pool figure about 30 dollars per month.
This way for a family of four, two needing own transportation, one car pooling and one using public transportation, monthly cost would b e approximately 380 dollars.