If you were an evil genius and wanted to develop a situation that made people angry, it would look a lot like driving.
Here are four reasons why:
Tension. Quite simply, driving is dangerous. Because it is dangerous, it makes us nervous. This is true whether we have been doing it for days, years, or decades. Even if we are so used to it that we don't notice it anymore, we still feel some tension when we drive. Our heart rate increases, our muscles tense up, etc. What this means for anger is that we are primed for feeling strong emotions. That tension state makes us more likely to get angry when faced with a provocation.
Goal-Blocking. Just about every time we are behind the wheel, we have a destination in mind. Unless we are truly just out sightseeing, there is someplace we are trying to get to and there is probably a specific time we are supposed to be there. In other words, we have a goal in mind and it's long been know that when our goals are blocked, we get angry. Every red-light that stops us, every driver going too slow, and every poorly marked intersection is one more thing keeping us from our goal. In fact, you've probably noticed that when you are running late for something (or when being on time is extra important), you get even angrier about these sorts of slow-downs. This is because the more important the goal, the worse those instances of goal-blocking become, and the more angry we get.
Unwritten Rules. Of course, there are all sorts of written rules for the road. We have speed limits, rules governing intersections, and a host of other laws that dictate how we should drive. However, few people follow those written rules to the letter of the law. Most people have their own set of rules that loosely follow the written ones. For example, most people don't follow the speed-limit but use it as a guideline for how fast they are willing to drive on a particular road. What this means is that we all have our own, slightly different, set of rules. When someone violates our rules, we get angry. Imagine, for a moment, that you think driving 65 mph in a 55 mph zone is appropriate. If you come up behind someone driving 60 mph and can't pass, you might get angry because he or she isn't driving fast enough. If someone comes up and rides your bumper because you aren't going fast enough, you might get angry at him or her for going too fast. In this scenario, you are all violating the written rule (i.e., the speed limit) but your anger is the result of the other drivers having broken your personal, unwritten rule.
Anonymous Offenders. Finally, part of what lends itself to the anger is that, most of the time, the other drivers are unknown to us. This makes it really easy for us to label them negatively or make assumptions about why they did what they did. So, when another driver cuts us off, we can call him or her a "complete idiot" with no evidence to the contrary. We can even make assumptions about his or her motivation (e.g., "he saw us and just didn't care", "he must be on his cell phone"). For example, imagine you were driving on the interstate and come up on a car driving just below the speed limit. You might label that person a hazard or overly cautious and get angry. But, imagine that you found out that the person had recently been in a terrible car accident and that this was his first time back on the road. Your perception of that person's driving might change greatly but, because other drivers are anonymous to us, we rarely get the opportunity for that sort of understanding.