Will jams get worse when slow cars move over ?

– 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 ≤ b ≤ 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. Driven diffusive systems [1,2] have been widely studied since they are amongst the simplest models which settle into nontrivial nonequilibrium steady states. These are of fundamental interest to physicists in their quest to formulate a theoretical framework for nonequilibrium behavior, on a par with Gibbs ensemble theory. Moreover, such models serve to describe numerous scientific or engineering situations involving net currents of mass and/or energy. Examples include colloids sedimenting under gravity [3], molecular motors moving along a microtubule [4], or traffic flowing on a highway [5]. The behavior of such systems becomes especially interesting when two (or more) different components ('species') move preferentially in different directions, such as, e.g., positive and negative charges in a uniform electric field. In the traffic analogy, the two species of particles can be interpreted as 'fast cars' and 'slow trucks', viewed from a co-moving frame. In the simplest model, cars and trucks may pass each other with a small rate and change lanes randomly. For ring roads with two or more lanes and no exits, two distinct phases are observed: on one side of the phase boundary, typical configurations are disordered and particles move freely; on the other side, configurations are spatially inhomogeneous and jammed up [6, 7]. In particular, Monte Carlo simulations for two-lane (L × 2) roads show one large macroscopic jam, of size O(L), containing almost all vehicles. Only a few particles ('travellers') are found outside the jam, having just escaped by repeated passing. We should note that recent analytic [8] and simulational [9] studies …