The major roads over minor roads is due mostly to expected driver behavior. In slower traffic/residential areas, you tend to have more sidewalks, traffic devices, driveways, side streets, and other forms of merging traffic. On a major road, I know most cars are going straight, most cars joining traffic are coming from well defined intersections or parking lot entrances, and if I'm on the major road I have a lot less traffic lights and stop signs. If I'm on the side streets, there are more stop signs, and lights that aren't timed for the side streets, plus a lot of driveways and other side streets where I need to watch for right hooks and people pulling out a lot more. Then you also have random pedestrians doing pedestrian things because they're less worried about crossing a side street than a 2 lane each way road. And, cars turning onto the side streets are used to less and slower traffic and seem to do more rolling stops, while when turning onto the busier streets they tend to re-verify before merging in due to the higher speed of cars and the heaver traffic.