Signal far enough ahead so other drivers in your vicinity can make adjustments to your change in speed and

DIRECTION

Explanation:

the answer of the blank will be DIRECTION

As the signal is far ahead so driver can make the decision of what he have to do either he can change direction or he can be on his same path.

But for that  driver has to change his or her speed.

so, from the statement given the blank will be filled with DIRECTION

.) the observation by semmelweis that more mothers died of childbed fever when their doctors also worked in the morgue