  Motivated by an analogy with traffic, we simulate two species of particles
(`vehicles'), moving stochastically in opposite directions on a two-lane ring
road. Each species prefers one lane over the other, controlled by a parameter
$0 \leq b \leq 1$ such that $b=0$ corresponds to random lane choice and $b=1$
to perfect `laning'. We find that the system displays one large cluster (`jam')
whose size increases with $b$, contrary to intuition. Even more remarkably, the
lane `charge' (a measure for the number of particles in their preferred lane)
exhibits a region of negative response: even though vehicles experience a
stronger preference for the `right' lane, more of them find themselves in the
`wrong' one! For $b$ very close to 1, a sharp transition restores a homogeneous
state. Various characteristics of the system are computed analytically, in good
agreement with simulation data.
