For 0<q<d fixed let be a (q,d)-Slepian-process defined as centered, stationary Gaussian process with continuous sample paths and covariance
Note that
where Bt is standard Brownian motion, is a (q,d)-Slepian-process. In this paper we prove an analytical formula for the boundary crossing probability , q<d≤2q, in the case g is a piecewise affine function. This formula can be used as approximation for the boundary crossing probability of an arbitrary boundary by approximating the boundary function by piecewise affine functions.