Two Roads Diverged in a Yellow Wood, and I Took the One With Traffic

While I am driving, I see a lot of folks running alongside major roads -- roads with heavy traffic and no sidewalks -- and usually these roads are adjacent to suburban developments: neighborhoods riddled with winding, low volume 25 mph. streets that would be perfectly safe and pleasant to jog on, and I always wonder why people choose to run on the highway instead of the alternative . . . and so if you are one of these people who run on the shoulder of a busy road, instead of opting for something more serene, can you please explain why?