Fork in the road

You are ill and travelling down a road to the hospital. You reach a fork
in the road and find a pair of identical twin boys standing there. One
of the twins always tells the truth and the other twin always lies. You
are allowed to direct only one question to one of the twins, and as such
you will be assured of the correct road to the hospital. What is your
question and to whom?


Ask the question,

What will the other person say if this this road goes to the hospital?

Person |On Road|  Answer
------------------------
True   |  yes |  False
       |  no  |  True
False  |  yes |  False
       |  no  |  True


Explanation, treat the two persons as Functions:

T(a) = a
F(a) = !a
so we make use of the following property,
T(F(a)) = F(a) = !a
F(T(a)) = F(a) = !a