You do not have to have an attorney to contest a traffic citation. Instructions for pleading not guilty and requesting a hearing are probably on the reverse side of the citation. Follow the instructions for requesting a hearing. There are very specific and strict time limits on doing so, and you will have to deposit some amount of $$$, sometimes the full amount of the fine. If you are successful you will receive a refund. Having an attorney with experience in handling traffic matters can be helpful.
You can appear in court and set the case for trial. You will explain your contentions to the judge and the police officer will tell his story. The judge will decide wither you are guilty or not. An attorney may be able to make the argument clearer, but it still comes down to your testimony vs. the police officer's testimony.
You can reach Harkess & Salter LLC at (303) 531-5380 or info@Harkess-Salter.com. Stephen Harkess is an attorney licensed in the state and federal courts of Colorado. This answer is for general information only and does not create an attorney client relationship between Stephen Harkess or Harkess & Salter LLC and any person. You should schedule a consultation with an attorney to discuss the specifics of your legal issues.