I think that this only applies to California, however, it may also be true in other states (I've heard that Nevada is the same.) It's so useful that I couldn't resist noding it.
When you get a speeding ticket, you have 3 options: pay the fine and accept a guilty plea (give up), plead innocent and go to traffic court, and plead innocent and contest the charge by snail mail. If you go to traffic court, the police officer typically gets overtime pay to attend so he is highly motiviated. On the other hand, there is no such incentive to respond to trial-by mail claims. As a result, less than half of the trial-by-mail claims get up getting prosecuted. In addition it's likely that you can remember what happened quite a bit better than the police officer (you only got one ticket, they gave 20-30 that day) and without your face to remind them, they may be unable to come up with enough information to counter your claims.
The option to contest through the mail (trial de novo or something like that) is usually not prominently displayed to the offender and so most people with busy lives think the only way to avoid a 4 hour trial is to pay the fine and live with it. If you look carefully, you can usually find it. There is a form to fill out (I downloaded it from the Court Website). I have heard of people just putting something like "I'm Not Guilty!" on their form and sending it back, hoping for a no show from their officer.
The best thing about contesting through the mail is that you can change your mind AFTER getting the judgement and decide that a full-fledged trial would be better for you. This means that you can choose to try for the trial in person if the trial-by-mail doesn't turn out like you like! I actually ended up doing this with one ticket that I felt was unfair and got off because the officer wasn't there (I saw him going up in the elevator after I was going down but you snooze, you lose!)
There are some other things that you can do to make sure that you were not treated unfairly. One thing is that there must be a traffic survey of the road every 2 years or so to verify that the posted speed limit is correct for the road conditions. If it hasn't been done, it's an illegal speed trap.
If the day is good (weather and road condition wise) you can argue that the speed limit is artificially low given the conditions. In my case, I actually commented to the officer on what a beautiful day it was and he agreed.
Finally I think it's best to comment on what I believe the purpose of such information should be. This isn't some kind of disclaimer, it's really how I feel about it. Speed laws are there to protect people from killing each other with reckless abandon. In many cases, however, speed limits are set by arbitrary laws having to do with whether or not there is a driveway connected to the road or how many lanes the road has rather than what the conditions that are actually present on a road. In additon, speed limits need to assume the worse case (you have never been on this road and it's driving rain or sleet or something) and on most days, this is not the case and so the limit is lower than can be safely driven. There are also artificial pressures on police officers in the form of quotas and the like that prove that the officers have been busy.