If you want to fight a speeding ticket, understand the different types of speed limits and how to defend yourself.
All 50 states use three basic types of speed limits, called "absolute," "presumed," and "basic." Because each type of speed limit violation often requires a unique defense, it is key to understand which you are charged with violating.

"Absolute" Speed Limits:
Most states have an "absolute" speed law. There is no trick to how this works: If the sign says 40 mph and you drive 41 mph or more, you have violated the law. There are not many defenses available for violating absolute speed limit laws, but some include:
1) Attacking the officer's determination of your speed. To do this you must discover what method the officer used to determine your speed -- like radar, laser, or pacing -- and then attack that particular method.
2) Claiming an emergency forced you to exceed the speed limit to avoid serious damage or injury to yourself or others. For instance, if you had to speed up to avoid a small plane making an emergency landing on a freeway, you would have a good case.
3) Claiming that the officer mistook your car for another car. With so many similar looking late-model cars, it is very possible that a cop could see a speeding car, lose sight of it around a corner, and then wrongly pick out your car further down the road.

"Presumed" Speed Limits:
Being charged with violating a "presumed" speed limit means you are accused of driving at an unsafe speed, considering the conditions at the time you were ticketed.If you're accused of violating a "presumed" speed limit, you have two possible defenses:
1) Claim you weren't exceeding the posted speed limit, just as you would if you were charged with violating an "absolute" speed law.
2) Claim that, even if you were exceeding the posted limit, you were driving safely given the specific road, weather, and traffic conditions at the time.

You have a good chance of prevailing if you can show you were just slightly over the limit and road, weather, and traffic conditions were good.For example, if you know you were driving 33-35 mph in a 25-mph zone, and the officer can probably prove it, you should concentrate your defense on showing that you were driving at a reasonable speed, considering the conditions at the time you were stopped.

In a "presumed" speed law case, you have the burden of proving your speed was safe and prudent. In other words, the speed law presumes the posted speed limit is the fastest safe speed. It is up to you to prove that going faster at the time you were ticketed is also safe.

While proving that it was safe to go 65 mph in a 35 mph zone will be close to impossible, there are many wide, straight roads designed for safe driving at 35 to 50 mph that have lower posted speed limits because of political pressure on public officials to crack down on speeding.

Here are important ways to build your case:
Go back to the scene and take photos at the same time and day of the week you were cited. Also, take a photo from the driver's viewpoint. It's obviously to your benefit if you can establish the road was straight, with good visibility.
Diagram the road, showing the location of your vehicle, the officer's vehicle, and any other traffic. It helps if you were not ticketed in a busy commercial district where cars enter and exit parking lots and businesses.

The presence of heavy traffic can sometimes be a plus. With lots of other cars on the road your argument could be that "everyone was exceeding the speed limit by about 10 mph and I would have endangered myself and others by driving slower than the flow of traffic."

"Presumed" speed limit laws work both ways. On a pleasant summer morning on a wide, uncrowded highway, it may be safe to drive above the posted speed limit. However, on a wet day when visibility is limited by fog, it may not even be safe to drive at the posted speed limit. An officer can still ticket you for driving at or below the posted limit, if it is unsafe to do so. This is true in all states.

The "Basic" Speed Law:
"Absolute" speed states set an upper limit, above which your speed is considered illegal. Drive one mile over the limit and you are a law breaker. But these states also have a way to ticket you when you are driving under the speed limit if an officer concludes your speed was unsafe. Called the "basic" speed law, it prohibits driving at an unsafe speed, even if that speed is below the posted limit.

In all states, tickets for driving under the speed limit, but too fast to be safe, are often referred to as "driving too fast for conditions." For example, driving exactly at the 65 mph posted limit on the freeway would not be smart amidst slower and heavy traffic, in a dense fog, or in a driving rainstorm or blizzard.

The difference between fighting one of these tickets and a speeding ticket for going over the speed limit is that here the prosecution has the burden of proving you were driving unsafely. (Again, that's because the posted speed limit is presumed to be safe). This means the officer must testify that, given the unusual road, weather, or traffic conditions, your below-the-limit speed was unsafe. This can be tough to do unless you were involved in an accident, since the cop may be hard put to come up with enough hard evidence to rebut the presumption established by the posted limit.

Police most often rely on the "basic" speed law after an accident. They reason that you were driving too fast, no matter how slow you were driving, because you were in an accident. However, you do not have to despair if you were in an accident and are charged with violating the "basic" law for driving at an unsafe below-the-limit speed. The fact that you've had an accident is not absolute proof that you were driving unsafely. Accidents, after all, are not always caused by your violating the law. Often they are caused when another driver screwed up.

If the police officer argues that the accident itself is evidence that you were driving at an unsafe speed, even though you were not technically speeding, you must be prepared to challenge him. Your best bets are normally to claim, and hopefully prove, that the accident could have occurred for a number of reasons. For example, it could have been:
a) entirely or partly another driver's fault;
b) the result of a freak act of nature, in the form of a sudden wind gust, a tree falling, or other natural occurrence, or
c) a defect in the highway, signs, or signals, which would happen if a stop sign is missing or a stoplight fails.