In a collision simulation, such as a two-car event using EDSMAC4, what is the value of the Inter-Vehicle Friction value set for the interaction of the Vehicle Meshes represent in real-world behavior?

Inter-Vehicle Friction is essentially the same as Coulomb friction. In other words, the two vehicles do not actually lock together during the collision phase of the simulation, but rather, Inter-Vehicle Friction behaves more like slide friction between the two vehicle geometries.