I've been a runner for about eight or nine years now. I started running in a very rural area, so I never had a problem with harassment from drivers, or fellow pedestrians. However, since moving out east, I've seen an entirely different environment. Now, the area I run in is largely urban, and every driver has an opinion.
Now, some will argue that responding to harassment is a waste of time. It only eggs them on, and reduces you to their level. Fuck that, I say. They're not expecting a response most of the time, and giving a good one tends to shut them up. And as for being reduced to their level, I could care less. I'm running. I'm obviously on a higher level.
Case 1: Drive-by (Aggressive)
Now, for a little bit of background, it is common practice for runners to run on the left side of the road in the USA, against traffic (on the right in other countries, I assume). We run against traffic so that if a car doesn't see you, you see it, and have time to get out of the road. If traffic were coming from behind you, you would have a lot less reaction time. Runners have been doing this for decades.
However, some motorists feel that the road is created expressly for them. And while this is true in some cases (interstate highways, etc.), on back roads and areas without sidewalks, it isn't. So, when one of these drivers comes by, and sees this skinny person taking up 2 feet of their precious road, they feel the need to honk, and/or yell "Get out of the road!" occasionally adding an expletive or two.
Since the car is moving too quickly to respond directly to the driver, there is little, if any proper response to this. If you are not alone, snidely comment to your companion that you are so glad this kind person alerted you to your location, as you had no idea. Granted, this is immature, and serves no purpose, but it will usually get a laugh out of your companion, and that makes any run nicer.
Case 2: Drive-by (Mockery)
This occurs often also. Usually in a more populated area, or other places where traffic is slow enough to get the driver or passengers' full witty commentary. The person who actually says the commentary (hereafter referred to as "asshole") usually has quite a bit of insight for the unsuspecting runner. Their call is usually something that makes most runners wish the movie Forrest Gump had never been made (Seriously, I think this movie did more damage than good). Occasionally, the more enlightened driver will make a Prefontaine reference, although that movie has a cult following of mostly high school and college-aged runners. If a runner is truly lucky, a quick judgment will be made towards their sexuality.
The response to this type of cat calling is usually better if it is simple, and to the point, since the "asshole" is in a car, and has a quick getaway. A quick "Fuck you," or merely flipping the bird will usually suffice. However, if the "asshole" comes up to a red light, then you have a unique opportunity. I have found it best to try to ask them in a calm and rational manner why they did this while they are stopped, or more simply just embarrass the shit out of them. Entire busloads of cocky football players have been reduced to stuttering apologetics due to this method.
Case 3: Pedestrian
The most common "asshole" that does this is usually a younger person, say 14 and under. This is probably because they don't realize that they are being such jerks yet. It usually occurs near elementary and middle schools, where the demographic is most commonly found. Keep in mind that the "asshole" who does this is rarely alone, and harasses mostly to impress their companion. Their lines are usually along the same line of the second case, although slightly less well thought through. Again, Forrest Gump is a prized reference for these "assholes", even though most are young enough to have never seen the movie. "Where you going?" is also a popular taunt.
The runner's response to this should be subtle, yet not overly harmful. Obviously, the "Fuck you" or middle finger response is a little harsh for the younger crowd. So, a strong "Go home," is a good reproach, in my book. If the taunt is a facetious question, an honest answer is usually enough to silence them. Telling a child you are going home is blindingly obvious, and usually keeps them from asking unnecessary questions in the future. I hope.
It should be noted that not all vocal interaction with motorists and pedestrians is negative. On occasion, a friendly honk and wave, or words of encouragement are offered. In these cases, it's best to encourage such actions, and respond in a positive manner. Waving, and saying thank-you are never under-appreciated.