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In this paper, we obtain an ergodic dichotomy for {\it directional} flows,more generally, subspace flows, for a class of discrete subgroups of aconnected semisimple real algebraic group $G$, called transverse subgroups. Theclass of transverse subgroups of $G$ includes all discrete subgroups of rankone Lie groups, Anosov subgroups and their relative versions. Let $\Gamma$ be aZariski dense $\theta$-transverse subgroup for a subset $\theta$ of simpleroots. Let $L_\theta=A_\theta S_\theta$ be the Levi subgroup associated with$\theta$ where $A_\theta$ is the central maximal real split torus and$S_\theta$ is the product of a semisimple subgroup and a compact torus. Thereis a canonical $\Gamma$-invariant subspace $\tilde \Omega_{\theta}$ of $G/S_\theta$ on which $\Gamma$ acts properly discontinuously. Setting$\Omega_\theta=\Gamma \backslash \tilde \Omega_\theta$, we consider thesubspace flow given by $A_W=\exp W$ for any linear subspace $W< \fraka_\theta$. Our main theorem is a Hopf-Tsuji-Sullivan type dichotomy for theergodicity of $(\Omega_\theta, A_W, \mathsf m)$ with respect to aBowen-Margulis-Sullivan measure $\mathsf m$ satisfying a certain hypothesis. Asan application, we obtain the codimension dichotomy for a $\theta$-Anosovsubgroup $\Gamma <G$: for any subspace $W<\mathfrak{a}_\theta$ containing avector $u$ in the interior of the $\theta$-limit cone of $\Gamma$, we have$\operatorname{codim} W \le 2$ if and only if the $A_W$-action on$(\Omega_\theta, \mathsf{m}_u)$ is ergodic where $\mathsf{m}_u$ is theBowen-Margulis-Sullivan measure associated with $u$.

Ergodic dichotomy for subspace flows in higher rank