When I became radar certified, I was taken out in a patrol car with a radar unit and I had to know the speed of the car within 3mph without looking at the radar screen. Atleast that's how NJ works. After awhile of doing radar, you know how fast a car is traveling without looking at the radar screen.
How close are the vehicles before you got the 3 MPH error consistanly better than 50% and at what speed? Come on you are asking us to believe somehting I don't believe anyone can do. I have been building and racing vehicles for 30 years, drag racing, road racing and roundy-round racing and have over 2 million miles of professional driving experience. I judge distance very well also. I don't believe for one minute you can "guess" within 3 MPH the speed of a vehicle coming directly at you. Until that vehicle is within a 100 yards you have no idea if that vehicle is doing 45 or 65. From an angle yes I would believe someone with experience can pretty accurately judge the speed but from straight on, no way and the difference between 63 and 60, not.
Mikey