This paper describes the development of a compact and low cost millimeter wave doppler radar sensor (77 GHz band), which can measure the vehicle ground speed precisely. The sensor has three unique features: First, all the radio frequency components are integrated into a single chip, including a millimeter wave transceiver and an on-chip antenna. Then, the chip package is made of plastic resin without use of expensive ceramic. Finally, a tiny dome-shaped resin lens is attached to the chip to collimate waves. These technologies enable the sensor to measure 53 x 71 x 65 mm₃, to weigh 115 grams. Compared to a conventional optical measuring instrument, for example, the sensor weighs only about fifteenth and is one-fifth of the size, while the measurement accuracy is almost comparable. So this sensor seems to have a variety of potential applications.In this paper, we also considered the feasibility of some other applications than just measuring ground speed. One is the yaw rate detection by using two sensors mounted on both sides of the vehicle. It can be calculated from differential speed between sensors. From the experimental results, we found that the detection was possible and the limit of sensitivity depended on the speed measurement accuracy which is under 2% variation. The other possible application is detecting obstruction. Although it was difficult to cover from 0 meters to far side with one sensor because of its narrow beam shape, we confirmed from experiments that the sensor was able to detect vehicles in the next lane in the range from at least 10 to 60 meters.