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It is a Doppler radar, it only detects moving objects. The transceiver module outputs a signal when, due to movement, the frequency of the reflected pulse differs from the transmitted pulse. Because it does not output the trigger pulse itself, you can't measure the time between the TX pulse and the arrival at the antenna of the reflection. Distance measurements are therefore not possible with this radar. Using the received signal level is also not possible because that depends on the radar cross section of the object, i.e. its capacity to reflect the TX pulse. A truck does not have the same refelctivity as a human being. Only if the radar cross section is always exactly the same (identical objects, identical angle of incidence, etc.) can you try to do an estimate of the distance based on signal level (if other conditions don't change).