Who else goes to traffic court for a failure to reduce speed causing a accident?

You're not in court because of the accident. If you were, the other driver would be required to go. You're in court because of a traffic infraction. The other driver has nothing to do with that, other than being the victim. They may go if they want to but they are not required to go.

It depends what country you are in.

The person getting the ticket is the only one that goes for your first appearance. If you plea not guilty and request a trial, a date will be set and the officer and any witnesses will be given a subpoena and ordered to attend. You will not get a jury trial unless you request one.

depends on jurisdiction and whether the officer who cited you thinks s/he needs the witness [the other driver]

The accuser, the defendant and the District Magistrate are the only one who need attend. The defendant can subpoena the other driver or anyone else if they have anything to offer to the defense. There is no jury, no trial, just an informal hearing. If necessary, the defendant can hire an attorney.

Usually on a jury trial, the prosecution and the defense can call on anybody with evidence. If either of them thinks that the other driver has good evidence, then they can can question him or her. If they don't want to have the other driver testify, that's fine also.

Unless the at-fault driver pleads guilty, then somebody has to give some evidence. And if he pleads guilty, then why is there a jury?

In my very limited experience, traffic court doesn't use juries. You have to go up a level to get a jury trial.