If you want to go to Florida by car,

In our example, the comma after "car" signals to the brain that the subordinate clause has probably ended, and that the brain should probably start looking for another subject / verb pattern. (Scroll down for comment.)

If you want to go to Florida by car, you must plan to drive awhile.

When to use a comma is an interesting problem. The theory of chunking proposed in this model suggests that commas primarily function to keep "chunks" separate. Note what happens, for example, in the following sentence:

When Stan was hitting the first baseman played back.

Without a comma after "hitting," most readers will think that Stan was hitting the first baseman. Then they will stumble over "played back" and have to reinterpret the sentence.