This paper reports research on three different paradigms for the real-time measurement of tire-road friction coefficient on moving vehicles. The three friction estimation systems studied are; a) Vehicle-based system: that utilizes measurement of vehicle lateral and longitudinal motion; b) Wheel-based system: that utilizes sensors on a special redundant wheel mounted on the vehicle; c) Tire-based system: that utilizes measurement of tire deflections from piezoelectric strips embedded in the regular tires of the vehicle. The advantages and limitations of each of the above three measurement systems are analyzed. The vehicle-based system utilizes differential GPS to accurately measure lateral slip angle and longitudinal slip. The novelty of the vehicle-based system in this paper is that it is designed to work under both acceleration and braking maneuvers for a wide range of slip values. Experimental results are presented for all three types of measurement systems. Compared to the vehicle based system, the wheel and tire based systems have significantly better ability to estimate friction reliably in the absence of significant cornering or acceleration/deceleration in the vehicle.