Measurement of moving objects using computer vision
how difficult is it to stitch multiple images based on speed to get a complete object?


Trivial. If you know the speed and the time at which an image was taken, it's just a linear translation...


  My first instinct is to use line detection algorithms for detecting object edges and calculate distance between them, based on speed and time.


Excellent approach!

Just: reduce your image to a single row, maybe the average of the 10 center rows from your picture.

That reduces your problem from 2D to 1D.

Then, the question you have is "when does that start and stop being black?", which is trivial to answer if you just convert that color row to its brightness, and use a threshold.