  Inter-vehicle communication (IVC) enables vehicles to exchange messages
within a limited broadcast range and thus self-organize into dynamical
vehicular ad hoc networks. For the foreseeable future, however, a direct
connectivity between equipped vehicles in one direction is rarely possible. We
therefore investigate an alternative mode in which messages are stored by relay
vehicles traveling in the opposite direction, and forwarded to vehicles in the
original direction at a later time. The wireless communication consists of two
`transversal' message hops across driving directions. Since direct connectivity
for transversal hops and a successful message transmission to vehicles in the
destination region is only a matter of time, the quality of this IVC strategy
can be described in terms of the distribution function for the total
transmission time. Assuming a Poissonian distance distribution between equipped
vehicles, we derive analytical probability distributions for message
transmission times and related propagation speeds for a deterministic and a
stochastic model of the maximum range of direct communication. By means of
integrated microscopic simulations of communication and bi-directional traffic
flows, we validated the theoretical expectation for multi-lane roadways. We
found little deviation of the analytical result for multi-lane scenarios, but
significant deviations for a single-lane. This can be explained by vehicle
platooning. We demonstrate the efficiency of the transverse hopping mechanism
for a congestion-warning application in a microscopic traffic simulation
scenario. Messages are created on an event-driven basis by equipped vehicles
entering and leaving a traffic jam. This application is operative for
penetration levels as low as 1%.
