I just take it off the Met Office website, combined with observation, e.g. "Gusty SW" or "Light N". I merely do that so if I get a particularly good or bad time for a ride I can compare it with others to see if the wind might have affected it.How do you measure wind direction/speed?
There's a setting (in my Garmin at least, which is the Edge 705) where you can turn on 'Autopause'. When the bike is not moving, it pauses the timer, and then restarts it when you move (this being determined by the spoke magnet passing the sensor). So your time is not altered by being sat at a red light for 3 minutes for example.
You can guarantee that any post quoting average speeds will subjected to scrutiny by the ASP (Average Speed Police) to ensure absolute accuracy. Things that are likely to lead to inflated 'averages' are one-way routes (tailwinds or downhill), miscalculation of distance, or cherry-picking of recorded rides. These are the tell-tale clues that the ASP look for.
Is now a good time to mention that my fastest recorded speed (on my cat eye) is 88mph!