Do I plead not guilty? Cop told me to!

That doesn't sound like a defense.

Yes--it is a legitimate reason. Show up and explain the situation and see what the prosecutor/judge can/will do.

Emergency is a defense, but not in your situation. You may plead not guilty and fight that ticket. This violation carries three-points plus fines.

The cop told you the right thing but for the wrong reason. You see, cops are not the "heros" that everyone makes them out to be. Rather, they are just men and women doing a job for income. The cop told you to plead not guilty not for you but out of self interest - so that he would get a court date and overtime. That he was right was a mere coincidence. One should ALWAYS plead "not guilty" to a moving violation, whether or not you "did it," as a pleading of "guilty as charged" causes an insurance increase. It is their burden to prove the elements, NOT for you to "prove" your innocence. Further, the call you took was not an "emergency call" within the meaning of the law to get you found "not guilty," and if you gave such testimony you would be confessing. Rather, your interests would be best served if you retained counsel to fight this for you, as in the long run it will save you time and money.