Oswald in the doorway: the blog of the Oswald Innocence Campaign, by Ralph Cinque

Here is a link to Google Images showing people riding motorcycles. You will see that in no instance did the photographer capture just the distant arm of the rider. Not once did it look anything like bpete's image. 

https://www.google.com/search?hl=en&site=imghp&tbm=isch&source=hp&biw=1600&bih=775&q=riding+a+motorcycle&oq=riding+a+motorcycle&gs_l=img.3..0l10.2016.7177.0.7293.27.20.5.0.0.0.225.1892.3j10j1.14.0.ccynfh...0...1.1.64.img..8.19.1904.UXFF0c0Ze2s#

And in Mary Moorman's case, we know she didn't capture just the distant arm of any motorcyclist either.