In other words, is it as simple as a car attracting attention (for pretty much any reason) being sufficient for an "observation"?
Typically the requirements for tracking history are spelled out in the operatorÃƒÂ¢Ã‚â‚¬Ã‚â„¢s guide for the various speed measuring devices. For instance, many radar units emit an audible tone where the pitch varies with the speed. Some radar manuals state that part of the tracking history is ensuring the audio tone matches the display/visual observation. This isnÃƒÂ¢Ã‚â‚¬Ã‚â„¢t a requirement for all radar units however and IÃƒÂ¢Ã‚â‚¬Ã‚â„¢ve seen requirements gets added and removed over time as manuals are revised.
Not sure what exactly you mean by your last question. For a speeding offence, the officer would have to articulate that the observation was sufficient to estimate the vehicleÃƒÂ¢Ã‚â‚¬Ã‚â„¢s speed. So factors like distance, traffic, visibility would all come into play.