  Many-particle simulations of vehicle interactions have been quite successful
in the qualitative reproduction of observed traffic patterns. However, the
assumed interactions could not be measured, as human interactions are hard to
quantify compared to interactions in physical and chemical systems. We show
that progress can be made by generalizing a method from equilibrium statistical
physics we learned from random matrix theory. It allows one to determine the
interaction potential via distributions of the netto distances s of vehicles.
Assuming power-law interactions, we find that driver behavior can be
approximated by a forwardly directed 1/s potential in congested traffic, while
interactions in free traffic are characterized by an exponent of approximately
4. This is relevant for traffic simulations and the assessment of telematic
systems.
