This paper studies the optimal sensor placement problem for Doppler-only target tracking. A ID target motion is considered on a road/line segment and the optimization criterion for sensor placement is selected to be the total position CramerRao Lower Bound (CRLB) over the road/line segment. The results obtained using numerical optimization tools are utilized to propose a simple sub-optimal sensor placement strategy with explicit formulae for the sensor positions. The proposed suboptimal strategy is shown to obtain very close sensor positions and very similar cost values to the optimal strategy. The merits of the proposed sensor placement strategy are illustrated on a simple target tracking example.