We prove that for any finite real hyperplane arrangement the average
projection volumes of the maximal cones is given by the coefficients of the
characteristic polynomial of the arrangement. This settles the conjecture of
Drton and Klivans that this held for all finite real reflection arrangements.
The methods used are geometric and combinatorial. As a consequence we determine
that the angle sums of a zonotope are given by the characteristic polynomial of
the order dual of the intersection lattice of the arrangement.