Although it's far more common to be ticketed for speeding, it's also possible to get a citation for driving too slowly. Generally, it's illegal to drive so slowly that you blook or impede the normal flow of traffic. Here are some of the circumstances where this can come up.
Most drivers are aware that when there are multiple lanes of traffic going in the same direction, the far left lane is for passing and faster traffic and that slower traffic should generally stay in the right lanes. But this isn't just a general understanding—it's required by law.
The laws of most states say something like this:
"Any vehicle proceeding upon a highway at a speed less than the normal speed of traffic moving in the same direction at such time shall be driven in the right-hand lane for traffic or as close as practicable to the right-hand edge or curb, except when overtaking and passing another vehicle proceeding in the same direction."
Most state laws also contain an exception for vehicles that are preparing to turn left off the highway.
Defenses to this type of driving-too-ticket might include:
Basically, it comes down to whether you were driving unreasonably slow in the left lane or you had a legitimate reason for doing so.
"Impeding traffic" is similar to driving too slowly in the left lane. The difference is that you can be ticketed for impeding traffic even if you were driving in the right-hand lane or on a single-lane road.
Impeding traffic laws are normally worded something like this:
"No person shall drive upon a highway at such a slow speed as to impede or block the normal and reasonable movement of traffic, except when reduced speed is necessary for safe operation, because of a grade, or compliance with the law."
The defenses that might work for an impeding-traffic ticket are similar to those that apply to driving too slow in the fast lane. Basically, you'll want to show that you were either driving the speed limit or you were driving at a slower speed because it was reasonably necessary for safety reasons.
Another variety of impeding traffic is based on a driver's failure to use roadside turnouts when a line of cars has accumulated behind.
Most states have a law that reads something like this:
"On a two-lane highway where passing is unsafe because of traffic in the opposite direction or other conditions, a slow-moving vehicle, behind which five or more vehicles are formed in line, shall turn off the roadway at the nearest place designated as a turnout by signs erected by the authority having jurisdiction over the highway, or wherever sufficient area for a safe turnout exists, in order to permit the vehicles following it to proceed. As used in this section, a slow-moving vehicle is one that is proceeding at a rate of speed less than the normal flow of traffic at the particular time and place."
Again, if you're ticketed for violating a turnout law, you're best defenses—assuming there were five more cars lined-up behind you—are going to be based on safety or that you were driving the speed limit.