Now replace the roads with wires, the parking spaces with stationary positive charges, and the cars with negative charges flowing through the wires. Both wires appear to have roughly equal densities of positive and negative charges, and should therefore be electrically neutral; an observer would measure the Coulomb force between the wires to be zero. However, an observer moving alongside an electron (negative charge) in the lower wire will perceive a different scenario, in which the upper wire has a net negative charge and exerts a repulsive force on the lower-wire electrons. In a moving frame, the electrons in the wires therefore experience a repulsive electric force; how can this be reconciled with the lack of any electric force in the rest frame?