Highways consume enormous electric power and therefore contribute to heavy economic costs due to the operation of auxiliary road facilities including lighting, displays, and health-monitoring systems for tunnels and bridges, etc. We here propose a new strategy of electric power supply for highways by harvesting mechanical energy from the reciprocating deformation of road pavements. A series of wheel tracking tests are performed to demonstrate the possibility of using piezoelectric elements to transform the mechanical energy stored in pavements due to vehicular load into electricity. An analytical electromechanical model is developed to predict the correlations between electric outputs and loading conditions in the wheel tracking test. A simple scaling law is derived to show that the normalized output power depends on the normalized loading period, location, and size of the piezoelectric device. The scaling law is further extended to a practical highway application according to the analogy between the wheel tracking test and a highway in an idealized condition of periodic vehicular load. It suggests that the output power may be maximized by tuning the material and geometry of the piezoelectric device under various conditions of speed limit and vehicle spacing. The present results may provide a useful guideline for designing mechanical energy-harvesting systems in various road pavements.