Your not supposed to exceed the speed limit because its the Law. 
Sometimes, exceeding the speed limit isn't why you get stopped. 99% of cops are good guys. Dedicated to the community. Occasionally, you find a real jerk. One such jerk, opened up a huge can of worms, that cost him his career and still reverberates through local law enforcement
Several years ago, a man gets pulled over on the interstate highway and gets a ticket for 59 mph in 55 zone. Driver pleads not guilty. Argues that many cars were going faster than he was... Traffic Judge (typically, a lawyer who can't win in court, begs for county appointment. Now beholden to local politicians) could care less about argument. Convicted.
Driver hires a lawyer and appeals conviction.
Lawyer shows 40 minutes of video over a week long period where County police, including the one who wrote ticket are cruising in traffic on same highway at 70 mph with many other cars. None are stopped. Lawyer grills cop, who cannot explain why he doesn't stop drivers doing 70+, but stopped the defendant for doing 59 mph. Cop looks like a schmuck.
Lawyer claims that his client was singled out for reasons other than speeding.
State argues that this is nonsense. This opens door for lawyer to introduce evidence.
Lawyer says he will prove that defendant was stopped then and had been stopped again later, simply because he was driving a red late model Mustang GT. Lawyer presents video from a commercial in-car camera. Defendant installed this at lawyer's behest. He was stopped weeks later, same highway, by a different cop. As they stop, driver's wife calls lawyer on cell phone. Cop sees wife on cell phone, demands she hang up. Lawyer hears the conversation, tells her to hang up. Lawyer calls Deputy Police Chief. This time, driver was accused of doing 70 in a 55, but the in-car camera showed that he never exceeded 55 mph. Unaware of camera, cop writes ticket for 68 mph in 55 mph zone, claims to be giving driver a break. Cop asked to search car. Driver said no. Cop threatened to detain him and get a warrant. Driver said, "please do". Second cop stops. They try to browbeat driver into complying. He demands that they arrest him or let him go. Cops handcuff him and throw him into patrol car. Perform illegal search of driver's car. They find nothing illegal. They do find in-car camera. They demand driver erase video. Driver refuses. Cops decide to take camera, but Police supervisor arrives and takes charge. Driver is released. Ticket disappears. Bad news for two cops.... Lawyer is former county ADA, has excellent connections in DA and Police.
State argues video proves nothing, that speeding is speeding, and that is the only measure.
Judge disagrees. Judge admonishes police for obvious pattern of behavior and profiling. States that because the police seldom enforce the posted speed limit (stated he sees this every day himself), that the speed limit is, defacto, that speed which is actually enforced. The Judge stated that it appears that it must be something in excess of 70 mph. Reverses conviction.
That's not all of the story....
Both officers involved with recorded traffic stop are suspended, first one without pay. There is a pending lawsuit against county police and specific officers. First officer is fired. Union elects not intervene after video is shown. None of this ever makes local newspapers.
County Supervisor pulls County Police from highways. Hands over enforcement to County Sheriff. County agrees to 10 year deal with Sheriff's union on highway enforcement. Sheriff Department wins two national awards for highway safety and accident reduction by strictly enforcing speed limit, Texting and seatbelt laws. Last year, new county supervisor elected with police union support. Reinstates County Police on highway enforcement. Accidents go up, average speeds go up, number of fatalities go up accordingly. Sheriff Union had sued County. Judge says that County is bound by contract to put Sheriff back on highways. County appeals. Loses again. Sheriff expected to resume highway enforcement on June 1. Police having hissy fits... Will need to turn in riding trousers and jack boots for standard uniform.
Note that "County Police" only serves half of the county. Five towns combined police forces in 1960s to reduce costs. Four other unaffiliated "town" police departments serve balance. Sheriff has jurisdiction everywhere. There is a growing movement to combine all police forces under Sheriff Department. Police unions are strongly resisting. Why? Because of foolish arbitration agreement, County cop makes $102,000 after 2 years. Average Deputy Sheriff earns $88,000 after 5 years.