1. A microwave vehicle detecting apparatus having a receiving antenna array and a transmitting antenna array with signal-to-noise ratio varied from 28 dB to 34 dB for multi-lane vehicle detection at a respective distance ranged from 5 m to 30 m to the multiple lanes, comprising: a radio-frequency transceiving unit having a signal input and a signal output and generating a difference-frequency signal therefrom; a receiving antenna array and a transmitting antenna array, wherein said receiving antenna array and said transmitting antenna array are isolated by a value of at least 40 dB, and the receiving array receives a microwave signal having a signal-to-noise ratio reflecting from a target where said signal-to-noise ratio being varied from 28 dB to 34 dB corresponding to the multiple lanes, and the antenna arrays being configured at a position for the receive antenna array to receive said microwave signal at an elevation angle and being spaced from each other at a distance of less than 5 mm and coupled to said signal input and said signal output respectively; an adjusting device coupled to said receiving antenna array and said transmitting antenna array for adjusting one of said position and said elevation angle to achieve the signal-to-noise ratio; and an intermediate-frequency and digital signal processing unit for a real assessment of range measurement coupled to said radio-frequency transceiving unit and generating at least a traffic parameter corresponding to at least a lane in response to said difference-frequency signal, wherein said traffic parameter being selected from a group consisting of a vehicle speed, a vehicle classification, an occupancy, a density and a flow; thereby said microwave vehicle detecting apparatus being operable for multi-lane vehicle detection.