Other witnesses can provide the needed testimony to prove such a charge.
However, witnesses can be cross examined and their testimony/recall can be challenged.
You should consult with a criminal defense attorney to discuss your options.
Best of luck to you.
Denver Criminal Defense
No answer here should be considered to form an attorney-client relationship. You should consult with a licensed attorney in your jurisdiction so that a full evaluation of the facts of your case can be conducted.
John is correct. Understand that most violations or tickets are assessed for incidents that the police do not observe themselves. Almost all careless and reckless driving incidents are not observed by the police. And of course, much more serious criminal offenses such as rapes, child sexual assault, and murders rarely take place in front of other people, much less police officers. So, if they have other witnesses and evidence to support the charges, there is no obligation for them to have observed the incident.
The information provided in this answer does not create an attorney-client relationship and is not considered to be legal advice.
You can receive a ticket for failure to yield to an emergency vehicle even if the police did not witness the violation. You could also plead not guilty to the violation and contest the citation in court.