My driving teacher taught me that driving a car is a matter of getting from one place to another as safely as possible without impeding the general flow of traffic on the way. Thus it amazes me how few drivers understand the simple concept of the flow of traffic.
On an empty road, you're approaching a side street. There's a car waiting to turn out. For the traffic to flow smoothly, they should either enter the road behind you where there are no other vehicles, or in front of you if they have enough time to speed up so they don't get in your way. Typically of course, the goon in the side street will pull out in front of you, then casually idle up to 5mph below the speed limit by which time you've already had to brake and slow down. The flow of traffic is destroyed.
The same is true at most junctions, on ramps, off ramps and roundabouts - people lazily going about their day in blissful ignorance of what they're doing to the general progress of everyone else.
Traffic flow - in physics it's called conservation of momentum.