We have examined the relationship between the radial mean Doppler velocities observed with coherent radars, such as in the STARE (Scandinavian Twin Auroral Radar Experiment) system, and the electron drift velocities observed with an incoherent scatter facility. The two sets of measurements were made simultaneously and can with high accuracy be related to a common magnetic flux tube. We find that the Doppler velocities of electron density irregularities produced by the two-stream instability in the E region are limited to a value near the ion acoustic velocity. Moreover, we find that this value is a strong function of the electron drift velocity, in agreement with theoretical predictions. An algorithm has been determined which enables us to use the STARE data to derive estimates of the electron drift velocity which are in excellent agreement with values deduced from EISCAT (European Incoherent Scatter Facility) data.