I know for certain if I don't do even frequent stretching exercises, which include my neck, I do have problems doing my over the shoulder check when cycling and changing lanes.

http://www.vancouversun.com/health/S...585/story.html

A healthy neck should allow for smooth head movements in all directions, without pain or discomfort. Pinching or pulling sensations, or audible cracking or popping sounds as you move your head, can be early signs of faulty neck mechanics or degeneration. If everything is healthy and balanced, you should be able to rotate your head roughly 70 to 90 degrees to each side, and tilt your head to each side between 45 and 60 degrees.
If you stop challenging your neck through these ranges of motion, you'll gradually lose flexibility and begin to stiffen up. I've seen so many people in their 50s, 60s and 70s who have difficulty driving their cars because they can't turn their heads far enough to safely shoulder-check..