Individualists in private cars competing against the clock for limited road space is a highly competitive environment. There are rules, but they are barely enforced in the boom time. As in unregulated capitalism, even the mild-mannered must develop thick skin and sharp elbows or risk being sidelined.  Humans are highly adaptable. 
Occasionally, there's a crash. Someone might be disciplined for an infraction, but not always. That's just some bad actors, they say. It's still a good system. If fact, it's the only way except for that collectivist, regulated business of riding together (even with strangers!) in large professionally-driven vehicles and predictably arriving all at the same time. 
The evidence suggests most people today prefer the costs of the lone "road warrior" battle to the compromises of collective cooperation. Can either psychologists or economists tell us why that is?