At 1.5 miles, the human eye could probably not even discern movement, so it would be impossible to determine which vehicle is responsible for the reading at that distance, unless there was only one vehicle.
Also most smaller vehicles (read not a truck or SUV or Van) will not reflect a signal at that distance.
So the radar gun will transmit a signal that distance, but I doubt it would be useful.
"Speed is fine, but accuracy is final" --Bill Jordan
Remember those who died, remember those who killed them.