If you're thinking of fighting a traffic ticket in court, it's sometimes advisable to obtain evidence from the government (typically, the police) through a process called "discovery." In some cases, requesting discovery can mean the difference between winning and losing.
In criminal and traffic cases, the person who's accused of breaking the law (the defendant) has a right to evidence the government has in its possession that could be exculpatory. In other words, the government must turn over evidence that could aid the defendant in fighting the case.
In traffic cases, discovery evidence tends to be minimal. In many traffic cases, there's no evidence other than what the officer wrote on the ticket, which the defendant should already have a copy of. However, occasionally officers write notes somewhere other than on the ticket. And in certain types of traffic cases, government records can be relevant.
Officer notes. In pretty much all traffic cases, the government is going to rely—at least to some extent—on the officer's testimony to prove the case. So, any personal notes the officer made related to the citation can come in handy when cross-examining (questioning) the officer in court. For example, if the officer gives testimony in court that contradicts his or her notes, confronting the officer with the contradictions can be an effective tactic. However, in many cases, the officer will not have made any notes.
Video recordings. It's becoming more common for patrol vehicles to be equipped with cameras (sometimes called "dashcams"). In some cases, the footage these cameras capture can aid the defendant in fighting a ticket. For instance, a dashcam video might show the officer had a poor vantage point to view whether not the defendant ran a stop sign.
Radar maintenance records. Police often use radar or LIDAR devices when ticketing drivers for speeding. These devices are generally reliable only if properly serviced. So, requesting the maintenance records for the radar or LIDAR police used to clock your speed is typically advisable when fighting a speeding ticket.
The process for requesting discovery varies by jurisdiction. A local traffic attorney would have the best idea of how things work in your area. But making a discovery request generally involves writing a letter to the law enforcement agency or the prosecution. A discovery letter should detail what evidence you're requesting and include as much detail as possible, including the case or citation number, the date and location of the incident, and the name of the officer who wrote the citation.
If the government doesn't respond to a discovery request, but the evidence you requested does exist (for example, maintenance records for a radar unit), you can file a written "motion to compel discovery" with the court, or at least raise the issue in court and explain to the judge what happened.