  We study mixed site-bond directed percolation on 2D and 3D lattices by using
time-dependent simulations. Our results are compared with rigorous bounds
recently obtained by Liggett and by Katori and Tsukahara. The critical
fractions $p_{site}^c$ and $p_{bond}^c$ of sites and bonds are extremely well
approximated by a relationship reported earlier for isotropic percolation,
$(\log p_{site}^c/\log p_{site}^{c^*}+\log p_{bond}^c/\log p_{bond}^{c^*} = 1)
$, where $p_{site}^{c^*}$ and $p_{bond}^{c^*}$ are the critical fractions in
pure site and bond directed percolation.
