Speed Traps measure your current speed as you cross them. It doesn't matter which side you come at them from. Depending on what speed you reach, you will be rated on a scale of 1 to 3 stars, and you will also receive a certain amount of cash ($250 / $750 / $1500). There is also an additional level of difficulty, above the third star, the so-called "Rival" which guarantees the biggest cash grab ($3000).
There are 50 speed traps in total, and once you score 3 stars on each of them ("rival" is not required) you will unlock the "Caught on Camera" achievement / trophy.
