Position information, as a fundamental element for many of the modern vehicle-based logistic applications, is comprehensively provided by Global Navigation Satellite Systems (GNSS) such as the Global Positioning System (GPS). A variety of applications, including navigation and intelligent transpiration systems, require position data with certain accuracy. However, the shortcomings of GNSS, such as limited accuracy and availability, have been a motivation for recently emerging Cooperative Positioning (CP) methods based on vehicle-vehicle and vehicle-infrastructure communications. The majority of earlier CP methods assume the availability of distances between the participating nodes as a main parameter, using common techniques of radio ranging such as the Received Signal Strength (RSS), Time of Arrival (TOA), and Time Difference of Arrival (TDOA). However, the feasibility of these radio-ranging methods in the harsh environment of vehicular networks is questionable. Avoiding the radio-ranging challenges, in this paper, a new CP method is presented to improve the GPS estimates using internode range-rates based on the Doppler shift of the carrier of Dedicated Short-Range Communications (DSRC) signals, which is the nominated medium for vehicular communication. Depending on the speed of the participating vehicles and traffic intensity, improvement of up to 48% over the GPS accuracy is achieved. Because the Doppler effect is used, relative mobility of the nodes, which is generally a challenge for radio-ranging techniques, is a requirement for the proposed method, making it a more suitable solution for vehicular applications. Addressing the viability of the proposed technique, Doppler-based range-rating is verified in practice using DSRC transceivers. One section of highway is surveyed and modeled for simulation. In addition, GPS estimates are provided by feeding GPS signals, generated by a GPS signal generator, to a real GPS receiver.